BrickFielder

Fairly tricky to forecast what will happen storm wise today as there is a mixture of scenarios going on and its all a bit marginal. First off we have a marked long wave trough stretching down to the south. Fairly tricky to forecast what will happen storm wise today as there is a mixture of scenarios going on and its all a bit marginal. First off we have a marked long wave trough stretching down to the south. Satellite shows some short convective lines moving into the west, while met office fax charts suggest a surface trough of moisture moving across the UK. Aloft there is still a strong jet until late on although cloud tops may not tap really tap into this. It might however make clouds tops stream off to the north making any storms in the south of a line a little stronger. In addition we have some forecast convergence zones developing over the UK particularly in the east later on in the day. These are more likely to create pulse like storms. There is a slight problem though in that despite moderate mid level lapse rates there is very little surface based Instability (Cape). Which means the moisture is not really up high enough (dew points) for strong storm development. Temperatures boosted by urban effects might help a little but it still looks marginal. Wind Sheer what there is of it is not forecast to be at the right level for rotational storms, however there is a risk of weak spout like funnels developing near convergence zones and possibly where storms overlap an outflow boundary. Higher cloud bases might limit this. Key areas I think will be parts of the midlands and south mid afternoon moving to the east particularly London and East Anglia late afternoon where there might just be a hint of a low level jet. (925hpa wind). Overall there is some marked risk today mainly from localised flooding and hail, but forecasts today are not really suggesting a classic strong storm potential setup.

Hints of a northerly regime developing next week.

Yes isentropic level380K is close to the 100 hPa isobaric surface which is generally just above the tropopause (150hpa tropics down to 400hpa further north) and isentropic 475K level is close to the 60hpa level. So yes 380K very close to the boundary of the Stratosphere and Troposphere and 475K is a little above that. In looking at these charts I am thinking about synoptic eddy feedback as a mechanism for lower stratosphere potential vorticity to have impacts on the troposphere. Discussion on the role of synoptic eddies in the tropospheric response to stratospheric variability. http://www.cgd.ucar.edu/staff/lantao/publications/2013_DSC_GRL.pdf Paper which discusses stratospheric vortexes and the different ideas about how the stratosphere influences the troposphere including PV inversion, changes in refractive properties / Rossby wave propagation and synoptic eddy feedback. http://www.columbia.edu/~lmp/paps/waugh%2Bpolvani-PlumbFestVolume-2010.pdf

The royal Society publishing document on the relationship between stratospheric structure and tropospheric blocking patterns is interesting. I would agree that it makes sense that tropospheric conditions would affect the lower stratosphere. Where conditions in both the stratosphere and troposphere are acting to raise or lower the tropopause then they are likely to affect each other. What I think is not really explained well in the report is the concept of decoupling between the stratosphere and the troposphere and the importance of the scale of disturbances. Lets look at two current charts. Firstly the 380K Potential Vorticity Chart which I think has some correlation to tropospheric conditions (Not exactly but could be used for an element of prediction). Here I assume (always dangerous) that tropospheric influence on the stratosphere has been taken into account in the modelling. Secondly the 475K Potentail Vorticity Chart which correlates much better with the upper stratospheric vortex conditions rather than the lower stratosphere or troposphere. What I think is happening is that the lower stratosphere and upper troposphere tend to be reasonably well coupled, but when you get a major disturbance in the upper stratosphere you get some weak coupling between the upper stratosphere and the very lower stratosphere. Depending on the size of the upper stratosphere disturbance (like a vortex split) you get an element of decoupling somewhere in the mid to lower stratosphere initially (I think some of the graphs in the paper show this). Over time the stratosphere will slowly begin to strengthen the coupling to all levels of the stratosphere (barring another disturbance due to upward effects) having effects on the troposphere (blocking). Equally when the upper stratosphere disturbance goes away you get a similar weakening of coupling somewhere in the mid to lower stratosphere initially. In the absence of a significant stratospheric vortex or change in that vortex then it seems reasonable that tropospheric conditions would tend to drive stratospheric conditions (Ignoring mesosphere and ionosphere conditions which is unwise). When it comes to the question of time between an upper level stratospheric disturbance affecting the troposphere then I think the answer is complicated. It may have no effect, or if conditions in the troposphere are conducive to a change it may be relatively quick. Either way you are quite right to point out that the lower stratosphere is not a major driver of winter weather conditions and ice cover along with sea surface temperatures amongst others have a big influence (even reaching up into the stratosphere). The paper does sort of imply the stratosphere is important though. Here is a quick quote from the paper (It is out of context so reading the full paper is advised). An accurate representation of the stratospheric vortex structure (and thus the vertical wind shear in the lower stratosphere) is necessary for accurate predictions of blocking. On the Charlton et al paper then I think the key here is that they are looking at seasonal timescales. They do imply the statistically the effect of the stratosphere is limited on the troposphere . A later paper suggests that a stratospheric vortex displacement and stratospheric vortex split affects on the troposphere are statistically more significant than 5 percent (Slightly different to arguing that the lower stratosphere influences the troposphere I agree) http://centaur.reading.ac.uk/34207/1/mitchell.pdf A quick quote from this paper (out of context ). It is further found that the stratospheric contribution to tropospheric development can be larger in magnitude than the tropospheric contribution and can even overwhelm an opposing tropospheric effect. Following on from the Colucci paper. http://www.see.leeds.ac.uk/misc/ejournal/Issue%209%20articles/9;%2064-121.pdf Quote Most layers of the stratosphere and indeed possibly even the mesosphere can have a direct and immediate impact on the whole column of atmosphere below through thickness changes altering the surface pressure of possible mid latitude cyclones below. I think my head hurts now from reading all this stuff. Essentially I think we are agreeing that both the stratosphere and troposphere affect each other, that significant stratospheric disturbances can affect weather, 380K potential vorticity forecast charts can be useful as a hint to possibilities.

My understanding of the potential vorticity charts at 380k is that they represent low and high pressure areas within the lowest level of the stratosphere. These areas of low and high pressure at this height (low stratosphere) will have a forcing effect on the troposphere through trying to raise or lower the tropopause height (tropopause height varies with low and high pressure weather conditions). This forcing effect must contend with other forcing effects in the troposphere of which Sea Surface Temperatures (SST's) can modify the signal. This is particularly noticeable over the Atlantic and Pacific Oceans, which means a pinch of salt needs to be used when extrapolating potential tropospheric weather conditions for the UK. Looking at the current 380K potential vorticity charts you might extrapolate that there is a stratospheric forcing to create high pressure towards Scandinavia. If we compare this to the forecast potential vorticity in about a week and a half then it looks very different. For me this would tend to suggest a forcing to remove high pressure in the near continent. The potential here is for a significant winter storm to cross the UK perhaps leading to some flood conditions for the southern part of the UK. There is always the risk when looking at charts this far out that they will not match up to reality, so its something to look out for rather than any sort of forecast. Source for charts is at the following link. http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/index.htm I am still trying to decide whether current QBO conditions are conducive to stratospheric warmings and am starting to think maybe it is. When considering whether easterly or westerly QBO conditions prevail then it is not so simple because QBO changes tend to start at the top of the stratosphere and work their way downwards. This means that you can have different conditions at the top of the stratosphere to further down. Source for this chart is http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/qbo/ This suggests to me that we have a Westerley phase moving down giving way to easterlies at the topmost level. Assuming that the top most level is the most relevant to stratospheric warmings then we should probably take this as a sign that a stratospheric warming might be more viable. Again it is not so simple because you actually need to combine the ENSO (El Niño) signal with the QBO state to better understand Stratospheric warming viability. Generally the stratospheric vortex is significantly more disturbed in El Niño years as compared to La Niña years due to increased propagation of ultralong Rossby waves toward high latitudes. However when you combine ENSO signals then SSWs (Sudden Stratospheric Warmings) might be favored by the east phase of the QBO and warm El Niño events or by the west phase of the QBO and La Niña events. There is then a third variable which can affect SSW viability namely the solar cycle. More SSWs occur during QBO east, solar minimum and QBO west,solar maximum conditions. You can read some of the thinking behind these ideas at the following link. http://eprints.ucm.es/25520/1/calvofernandez07libre.pdf Finally I wanted to try to explore whether geomagnetic conditions in the Mesosphere play a part in whether stratospheric warmings occur. The reasoning here may be due to variable propagation and reflectivity of layers above the stratosphere when geomagnetically excited due to changes in ozone. I have struggled to find discussions that really back this up but I did find a workshop summary from 2013 which suggested that not all SSW periods occur during geomagnetic quiet times. Which kind of suggests a relationship to me. https://cedarweb.vsp.ucar.edu/wiki/index.php/2013_Workshop:Stratospheric_Sudden_Warmings There are some interesting concepts talked about in the summary which has peaked my interest to perhaps examine some other aspects. Selected Quotes below. SSW signals are present in the EEJ which are stronger during solar maximum than minimum, and correlated with the lunar time. There is either a direct influence on the stratosphere by production in the stratosphere or an indirect effect by the descent of EPP NOx and HOx from the mesosphere. Several workshop presentations have identified gravity waves as a significant source of SSW variability in both hemispheres up to the thermosphere, and therefore highlighted its importance.

Many expect a weather regime change during January or February due to stratospheric warmings, but perhaps an interesting question for me is what happens if it does not change. What happens exactly if the North Atlantic Oscillation remains positive right the way through Winter. For those who have not yet mastered what a positive NAO means in terms of weather I am talking about a strong Icelandic low and Azores high with wet and windy weather for the UK. What I think we can expect is that the cool blast of air coming down the western part of Greenland would continue. This I think should push the ice extent to the west of Greenland unusually far south. We should expect even colder polar air meeting warm air off the coast of the United States. Here lies a possibility for a record low pressure system in the Atlantic. There are risks here that the jet stream could be pushed a little further south with flooding and high winds affecting the majority of the UK not just the north. Equally we should expect snow cover to continue to build over northern Russia. This should be a signal for high pressure to build here, but is no guarantee. It does give the opportunity for cold air to spill unusually south across the continent. There is also a risk that the Azores will spread eastward spreading cold across southern Europe. So not only could we face the prospect of a record low pressure system and record high winds , we might hear that places like Spain, Greece, Turkey, Morocco and Egypt see snow before the UK gets even a hint of it. Our unseasonably high temperatures during December have relied on a feed of warm air from northern Africa and I wonder what happens if an extending Azores high cuts of that flow. You should certainly expect weaker low pressure systems, but also the cold air coming down from the west of Greenland becomes less warmed. The risk would be that wet and windy turns to snowy and windy. OK so even with the arctic flow down the west of Greenland you still have a lot of Atlantic ocean to cross. Deep cold has not really had a chance to build up in northern Russia and ice extents to the north of us are still way below normal. So I don't expect any of this to happen, but as a bit of an experiment in forecasting and thought I wondered what other people thought a persistent NAO through out winter may through up in the way of surprises.

Thanks to Dennis for pointing out that I should have put the source for the WAF diagram. So just to reminds us. Credits to AER for the WAF plot : http://www.aer.com/science-research/climate-weather/arctic-oscillation I think that 10hpa plot from Dennis's post shows an elongated stratospheric vortex due to the transfer of energy from the troposphere. Think ECMWF plots are a little different. http://www.geo.fu-berlin.de/en/met/ag/strat/produkte/winterdiagnostics/index.html This elongated pattern I think should set up some opportunities for high pressure blocking in the near future which may set us up for a warming later. Wave 3 Pattern possibly coming up. Wave 2 Patern to follow. Ideally they should be further north though. Plots are from the winter diagnostics link mentioned above. From the temperature plots in Lorenzo's post I notice the stratospheric temperatures are lower than normal at both 10hpa and 30hpa which makes me think of ozone destruction and possible intensification of the stratospheric vortex later. It also usually means that at 1hpa to 5hpa there may have been a warming. In reply to recretos when he says The wave number does not specify directly which warming will occur or if at all. Yes I absolutely agree that the wave number does not determine the type of warming, but I think a major warming is more likely from a wave 2 or 3 than from a wave 1 disturbance. Yes final warmings will happen without a troposphere disturbance but can at times be precipitated by instigated by a disturbance. If I get anything slightly wrong or over simplify then remember I am still learning as well. So I welcome all clarifications.

Stratosphere forecasting is probably just as complicated as traditional weather forecasting and when you break things down there are both positive and negative factors which will affect whether a stratospheric warming will occur. Before I get into these factors lets just recap a few basics about stratospheric warmings. Warmings tend to come in three flavours. The first is a minor warming (wave 1) which tends to push the stratospheric vortex away from the pole and tend to be common. These warmings can often push the vortex towards the UK resulting in wet and windy weather while setting up blocked weather patterns elsewhere. This blocked pattern can often provide the conditions for a major warming to follow in two to three weeks time. A major warming (wave 2) will tend to split the vortex in two and tends to be a little less common than minor warmings. The result for the UK depends on where the two lobes of the vortex are at a particular point, but often you can expect a slowly moving blocked pattern giving a cold spell for the UK. Finally there is the final warming which indicates the break up of the stratospheric vortex and ultimately the polar vortex which can lead to a variable pattern and some cold out breaks over the UK. These warmings are caused by upward propagating planetary waves (Rossby Waves). OK so let me explain what I think these planetary waves are and how they work. Wave 1 represents a blocking pattern where there is a single major blocking high pressure in northern latitudes. Typically this blocking high is connected to the Siberian High or norther European high pressure in Winter. Wave 2 represents a blocking pattern where there are two major blocking high pressure areas in northern latitudes. Typically these might be North western Canada and Siberia. Wave 3 would represent three blocking highs in northern latitudes. Placement of the blocking high also seems to play a key role with a suggestion that Blocking Highs over the Euro-Atlantic sector tend to enhance the upward propagation of Planetary waves, whereas blocking highs over the western Pacific lead to a strengthening of the polar vortex. OK if you want to know more and see how these work follow the link below. http://www.sparc-climate.org/fileadmin/customer/5_Meetings/GA5_PDF/KazuakiNishii_SPARC_GeneralAssembly2014_14Jan.pdf So any old high pressure in winter will cause a stratospheric warming? No it is never quite that simple and there are precursor and conditions for these waves to propagate upwards into the stratosphere and affect the polar vortex. One source of precursors is possibly gravity waves in key areas with these smaller scale waves being able to penetrate above the stratosphere (mesosphere and ionosphere) to affect conditions there which will slow the winds slightly at the top levels of the stratosphere. These lowered winds then become susceptable to planetary waves disturbing them. Another link which sort of skirts around this. http://www.sparc-climate.org/fileadmin/customer/5_Meetings/GA5_PDF/PosterSessionB.pdf Now your head is probably totally confused about different waves I have better give a brief explanation of what gravity waves are as opposed to planetary waves. Gravity waves tend to result from either thunderstorms or low pressure systems crossing over mountain ranges. It turns out that particular mountain ranges and storms systems tend to have a large effect on whether planetary waves affect the stratospheric vortex. These gravity wave hot spots in the northern hemisphere winter include the mountains of Norway, Greenland and the UK. This might suggest that wet and windy weather in the UK and Norway might be a precursor to a stratospheric warming. Another link with some nice maps of where gravity wave hot spots occur. http://onlinelibrary.wiley.com/doi/10.1029/2012JD018658/full So we are starting to get a bit of a picture about how stratospheric warmings might come about. I have of course missed out on Kelvin Waves (particularly relevant to Tibetan plateau in winter) and solar tides. I am sure somebody here will go away and check up on these things to give an even clearer picture. Link explaining how all the different wave types combine together. http://arxiv.org/pdf/1412.0077.pdf Waves are part of the story but there is perhaps one more dimension that needs exploring a little and this is around ozone amounts over the pole. Ozone concentrations over the pole at high levels in the stratosphere alter the temperatures at those levels. Ozone depletion at the top of the stratosphere results in warming at this level and cooling through lower layers in the stratosphere. When temperatures go below -78 C Polar Stratospheric Clouds form which deplete the ozone further. This causes increased wind speeds in the stratospheric vortex which become ever harder to displace (i.e. No amount of planetary waves could shift it). One cause of ozone depletion in the top layer of the stratosphere is the pulling down of Nox from layers above due to Energetic Particles. What I think this means is that space weather or more particularly solar activity can play a part in whether stratospheric warmings occur (Note that EPP activity while related to the solar cycle can be slightly different). Gravity waves can off course play a part in excitation of the thermosphere and mesosphere changing ozone concentrations as well. A little bit of discussion about this at the following link. http://www.atmos-chem-phys-discuss.net/15/33283/2015/acpd-15-33283-2015.pdf Seems to me that Geomagnetic activity might play a part as well. http://www.swpc.noaa.gov/products/notifications-timeline So having said all that and knowing a little about the drivers are we any closer to understanding whether a stratospheric warming is likely this winter ? The answer I think is that there are some possibilities as energy is transferred from the troposphere into the stratosphere, but that we have a very strong stratospheric vortex which is going to take some shifting. After the current wave which we should not expect to shift the vortex much there is another wave forecast for around the 5th of January. I don't expect much from that one either but it might set up the conditions ready for a further wave. So I think we are looking at late January and into February as realistic windows of opportunity. Much will depend on ozone depletion due to cold stratospheric temperatures against ozone transport from lower latitudes.

About this time of year I trawl through all the winter forecasts looking for indicators of what weather the winter will bring. Will this be the ever so rare year when we have a snowy Christmas, or will we be taking to our boats for the winter months. Usually I can glean some sort of impressions from the various weather drivers to at least have a punt at what the weather will bring. This year however the drivers present some fairly unusual conditions which makes me very wary of even having a guess. Unfortunately a number of forecasts I have seen make rather broad sweeping assumptions about the drivers of winter weather which I think would be good to discuss here. The Typical El nino. El Niño 2015 is turning into a fairly rare one. Whereas a typical El Niño develops throughout the spring, summer and fall, peaks in early winter and then dissipates early the following year, this one developed much earlier.In the 60-year record, only one El Niño event, in 1986-1987, had similar behavior. The evolution and strength of this event might be a little easier to predict if it were starting at a more typical time of year.Even though this year seems to be developing along the same lines as conditions in 1987, looking back to that year really doesn't help. The world is simply a different place than it was 28 years ago. There are also different types of el nino depending on where the warmest waters are in the pacific. When the warmest water relative to normal is found close to the coast of South America (Regions 1 and 2) (The classic el nino) then you tend to see a very different weather pattern across the united states. However, when the warmest water is shifted west towards the Central Pacific (Regions 3 and 4) (The Nino Modoki), the winters over the eastern half of the US typically turn significantly colder compared to what we expect with a classic El Niño. Eastern Pacific el ninos (classic) tend to give rise to a positive NAO and Central Pacific el ninos (modoki) tending to give rise to a negative NAO. My best guess would be that we have a hybrid el nino modoki during the winter. Bearing in mind that warm waters have spread up the western seaboard of the United states and there is a blob of warmer water in the north pacific then forecasting the response to the el nino conditions becomes extremely difficult. El nino in theory will tend to trump any effects of the north pacific blob, but this may have the effect of twisting the Arctic oscillation a little (shifting the high pressure westwards). North Atlantic Sea surface Temperatures It is generally acknowledged that spring sea surface temperatures in the Atlantic tend to re-emerge in winter. This would suggest the cold pool of water in the Atlantic will remain through winter. We also hear hints that the Atlantic Multidecadal Oscillation (AMO) has turned negative which would also suggest that cooler waters will remain in the Atlantic. I should note here that the AMO has been attributed to a number of things from a slowing gulf stream to less salty water, but my take is that it is due to up welling of cooler water due to consistently lower pressure resulting in higher water levels. This cooler water in the Atlantic is often associated with a positive NAO but there have been occasions where it has been trumped by conditions in the Pacific. There is also the possibility that cool water spreading south in the Atlantic shifts the Azores high southwards. This could suggest that the jet stream blasting across the Atlantic during positive NAO environments takes a more southerly path. The wet and windy weather normally heading the UK's way then tends to head for southern France allowing high pressure to build out of Siberia into northern Europe. The point here is that for this winter it might be unwise to associate cold or warmth with traditional weather patterns. QBO These are high level winds which blow first easterly and then westerly on a roughly 30 month cycle. Typically a easterly QBO will allow for more stratospheric warmings and blocking whilst a westerly QBO will relate to a colder arctic and strong polar vortex. From current readings it looks like we have a QBO transitioning from Easterly to Westerly at the moment.This westerly QBO's tend to relate to a Positive NAO with very little blocking and arctic out breaks. I should note that the transition time this year for the QBO is unique over the last 30 years as far as I can tell. Equally the traditional positive NAO leads to wave breaking across the mountains of Norway and Greenland which with a time lapse can force a stratospheric vortex split even with the strongest vortex (think late winter cold). Winter 1982/83 seems like the closest analogy based on QBO. Exiting a Solar Maximum We currently have low (ish) solar output. Influences of solar activity on our climate are still not fully understood but in combination with our other background signals (mainly the e QBO), low solar output will increase our chances of seeing a SSW this winter. This would equate to more blocking and a split/shifted stratospheric vortex. Putting it together. From this analysis, it has become clear that in a number of ways, we are in uncharted territory this coming season. Perhaps this winter we might be better off listening to the chap who is forecasting a cold winter based on squirrels burying their nuts early. Best get the canoe ready then. Sorry if I have not covered your favorite teleconnection pattern (PDO) or other patterns (MJO Cycle) Wave type forcing from mountain torque and stratospheric warming types, climate change effects but hopefully this gives a flavour of why I think forecasting this winters weather could be difficult.

Not quite as I envisaged but definitely interesting to watch. The low should I think lift out to the north sea and the delayed movement of the front eastwards will most likely start to happen. Quite a strong jet pushing in across the Bay of Biscay which I think might mess up the modelling a bit for tomorrow. Possible storm development towards Bristol and Welsh marches. Not much electrical activity showing up so instability must be limited.

So we have a slight risk of storms today mainly towards East Anglia according to the models. There is however differences in the different modelling solutions, so again there has to be a large caveat on any forecast. I am again not entirely convinced the model output has totally got a handle on environment. Looking at the EuMetsat pictures I can see a lobe of vorticity over northern France which does not clearly show up on the model output. I don't think this lobe will come the UK's way but I think it may delay the eastward movement of the Front across the UK. My best guess at the moment would be that the low over France will weaken with perhaps a secondary weaker low center developing towards the UK. This I base on the movement of cloud to the north west of the low center. Any weak low development towards the UK could perhaps lead rise to a few surprise thunderstorms. I think because this is all hunch and guess work the probability of this should be close to zero percent. I personally find it interesting for a change not to be totally reliant on the modelling and to try to second guess what I think might be happening from the Satellite and rainfall radar images. This is when you get to test your understanding of the atmosphere and to learn new things (second guessing sophisticated modelling invites failure). Getting things totally wrong is the best way to learn, so nobody should be ashamed of that and provided you are honest and admit your failings , then you will resist the temptation to be complacent in your knowledge. So I invite all budding forecasters to have a go. We will probably all be wrong today, so there is no pressure.

Looking at the models there seems to be a lot of similarities with the modelling from yesterday. Once difference is that the storms coming up from the south over night have mostly not materialized and much of the southern half of the UK is covered with cloudy murk which is not conducive to instability. From the models I am still picking up potential for the south east with storms developing over France perhaps making it across to the UK. The vorticity disturbance showing up on this lightning wizard chart suggests a potential for a severe storm developing its own dynamic environment (south east). There is still the suggestion of convergence and moisture pooling for the south west later on in the afternoon. The problem is that rainfall modelling does not really match that well with current rainfall radar. It looks to me like the low over the Bay of Biscay has moved further east than predicted and is not lifting out along the forecast timeline. Satellite pictures suggest to me than the south coast of the UK is under a warm front shield with cloud stretching up into the midlands. This shield looks to be moving westwards around the Biscay low with thunderstorm development limited to the tail of the warm front over France. That tail could also move westwards meaning there is the potential for storms to develop over the south east. The cold front stretching into France looks fairly vigorous but there are hints of a new front developing closer to the Biscay Low. Over all I am still confused about the potential today and feel models just have the timing and placements today a little off. Areas to watch are the South East/ East Anglia from mid day onwards and perhaps the southwest later in the afternoon. One final chart which suggests that there is an ability to produce high rain fall amounts from this scenario. Happy radar watching.

Some initial thoughts on tomorrow. Pretty tricky to pin point tomorrow down as the models are still not in alignment, but I think we can talk about a few things the models are showing up. The first thing I noticed was that instability in the form of mixed layer cape and mid level lapse rates are not really that special on first look. That might be a little misleading though since the initial storms coming in from the south are likely to be elevated. I think these storms form as a result of trough development behind the warm front with a strong possibility of elevated storms developing to the south of the UK and then reaching the south coast in the early hours. As those storms move inland then I think there is a chance that they will weaken as they move north. 700hpa vertical velocities seem to drop off as the storm move north. Interestingly NMM suggests a bent trough with some interesting attributes to the bend down across the London area around mid day. GFS via lightning wizard also picks up on the possibility of an over shooting top and a storm with its own dynamics. ( Link http://www.lightningwizard.com/) Later on across the south and perhaps into the midlands we might get some surface based storms, if the cloud clears a little. Since the air profile looks fairly moist all the way up apart from some areas towards West Wales this might be messy convection. Still there is some surface based instability and low level wind convergence. Since there is very little strength to the winds at 500 hpa (mid level) I dont see this really being utilised. In summary its confusing picture which is not nailed down by the models yet and could still amount to no storms. Suggestions are at the moment that there will be some overnight elevated storms which will tend to become outflow dominate (heavy rain rather than stormy) as they move north and less electrical. Just a chance of a more intense storm mid day to early afternoon south eastern counties with a chance of some surface based storms late afternoon across the south coast moving northwards. Key areas are Brighton to Exeter overnight, perhaps Kent and London area mid day and south west stretching up into Wales late afternoon (perhaps getting as far north as the midlands). Tomorrows models may off course move everything northwards into the north of the UK, with neither todays or tomorrows early models being correct, as plumes are notoriously hard for models to get right.

Modelling is not that much different overnight from Yesterday and there is still the risk of no storms at all today. An extract from Estofex had the following to say. There are two factor that may limit the overall severity of the situation: A/ Degree of initiation. With no strong synoptic-scale upward motion and in the capped airmass, it may be difficult to initiate storms. Initiation will be more likely in the evening hours as 850 and 700 hPa temperatures begin to drop from the west and cold front affects the area. B/ Stable boundary layer. Due to the very hot airmass around 850 hPa, it will require a lot of heating to destabilise the boundary layer and allow storms to root in it. Thus, especially in the early period, most storms will stay elevated (with primarily hail threat), as they move from France towards England. Surface based development will be most likely in the late afternoon and early evening hours. Level 2 is introduced for the area, where best CAPE/shear overlap is simulated, as well as with the best chances for the surface based storm initiation. Go to the web site to see more http://www.estofex.org/ Overnight soundings hint at the problem. Catch the mid day soundings at the following web site http://62.202.7.134/hpbo/sounding_europe.aspx That is a convective temperature of 37 Degrees. The cap however should begin to relax as the cold front approaches. Satellite water Vapour imagery suggests a curve of vorticity moving north and eastwards which should help development, but these might just initiate elevated storms. There is also a suggestion of some pre cold front troughing. To see more see Web Site http://oiswww.eumetsat.org/IPPS/html/MSG/RGB/ Surface wind convergence varies during the day which could be focuses for development. The drier air aloft still sneaks in late on. I am not posting up any more of the modelling charts for today as I think events will over take the models at this point. So in conclusion I would say there is a risk of severe storms today, particularly with regard to hail, although I would still not like to pin point where and if they will occur.

I still would not like to make a forecast for tomorrow. Diurnal heating has not really driven thunderstorms yet over the northern mountains of Iberia today (there is still time) which worry's me about the forecast. There are different predictions in the models for precipitation rates. So this update will have to be about broader scale developments again. First there is still the question of a triggering for storms. Although an elevated mixed layer will destabilize I tend to like a trigger for any surface based storms. Compare the areas of maximum instability and precipitation on the following two charts. The culprit is of course Convective inhibition. That does not appear to be the whole story though because if I look at the 500hpa vorticity charts then there are clear signs of storm development. These I think are elevated storms. That again is not the whole story because we have suggestions of low level wind convergence zones towards Wales and western parts of the UK. These I think might be the focus of surface based storms as moisture pools in these regions. There might even be more to the story tomorrow in that later on drier air comes in aloft. Notice how there is not a clear demarcation edge as it moves north which would tend to suggest potential storm development along that edge. Equally there is a weak area of vorticity advection at the 700hpa level moving northwards late afternoon. This may or may not be important, I am not sure yet. Forecast SkewT's suggest a relaxing of the cap from the north moving southwards during the afternoon. What confuses me though is how the cap it is modeled to almost disappear completely in the north of England. I am not convinced by this as a cap does not normally weaken that much over a short distance and is usually overcome by high temperatures and dew points. So a mixed and confusing picture and still a chance that nobody will see any storms at all. After all those Iberian storms have not appeared yet today.

Still way to early for any sort of detail forecast. One thing to remember is that all the instability in the world is no use if there is no triggering mechanism for storms. So plenty of instability mid day wednesday. There is not really any triggering mechanism forecast at this point of the day. Looking at the forecast Skewt's then there is a marked warm nose at 850hpa height which would suggest most of the instability would be elevated. These would give hazy sticky upper level storms with not much rain. I don't think these types of storms really need much of a trigger. A little later in the day and things change a little with some marked dry air coming in at a height of around 400hpa. This still overlaps somewhat with instability. The forecast Skewt shows the difference clearer. That is a lot closer to a surface based storm. Wind shear however has changed with less directional shear and a touch more speed shear. Most likely looking at the model output this early will not tell us much and we might expect timing to be inaccurate and it could all move eastwards. Interesting but lets wait until we get a little closer before we start predicting any thing.

Broadly speaking not a huge difference in the modelling from yesterday. The upper cool dry air still swings across the UK, just with a slightly different orientation. Vorticity charts suggest the leading edge of the trough is where developments should happen. EUMetsat satellite pictures suggest the broad synopsis is correct with a small window where the sun should break through. I am not entirely convinced by the timing suggested by the NMM 00z model output based on precipitation not quite happening where it should at the moment. I think NMM may have moved things through a little too fast. Considering the direction of the wind then I have suspicions that dewpoints may be a little over egged as well. Key areas suggested by the 00z nmm model are from somewhere just north of Worcester down to the south coast around mid day moving across to the south east by late afternoon. Some things to note are the increase in mid to upper wind speed shear from modelling yesterday and the small window of backed light surface winds. In theory there is some significant potential today, but I just get the feeling something is a little off with the modelling. Probably best to now cast from radar rather than take much notice of the models. With weak instability there is still a possibility that there will be no storms today, all it takes is for dewpoints to be a little lower than forecast or for cloud cover to limit temperatures a touch for nothing to happen. We will just have to wait a see.

Saturday afternoon early thoughts. Model agreement is still a little elusive and I think this might be due to the amount of cloud cover forecast. Looking at the fax charts then a warm sector trough is quite often a good sign of thunderstorm potential. For me a key signal is the drier slightly cooler air coming in aloft. Dry air aloft can often make for strong updrafts and downdrafts, with better separation.I think there are hints of this on the 700hpa height vertical velocity charts. Mid level lapse rates are not that impressive, so instability will be a little limited.So much so that cloud cover could limit temperatures and hence convection. Some low level wind convergence is forecast in a few places which could help to concentrate storm potential and provide some low level wind shear. Winds at a high level are quite strong, but might give rise to anvils reaching out ahead of storms with a risk that instability is shut off.On the other hand it can also help with updraft and downdraft separation. Forecast SkewT's suggest relatively low cloud bases, moderately high 925hpa winds, so any convergence zones (I am not convinced at the moment) could provide some lifted low level vorticity. Key areas from this modelling run look to be parts of the midlands moving eastwards into the early evening.If those convergence zones materialize then there is a slight risk of a severe storm. With model agreement missing we should expect this modelling to be significantly out, so it is hard to assess at the moment other than to point broad factors. There have also be a number of occasions in the past where similar scenarios to those forecast produced no storms at all (Cloud cover prevents temperatures rising and dewpoints not as high as forecast). Once in a while it does come together to produce a severe storm.

With this type of event now casting is usually the best option.However modelling can sometimes show up the broader details. What we should be looking for in our now casting is convergence zones setting up across the UK. This should help with moisture pooling which will drive higher dew points. I think this is more likely to be the trigger than actual temperatures. Another detail to look for today is a low level Jet. This Jet is in marked contrast to winds above it. There is also divergence at the mid level. 500hpa humidty charts sugest some potential for the south and east anglia later on as well. Oh well back to now casting.

There seems to have been a fair bit of consternation about why the models moved weather eastwards over the last few days. I honestly expected this to occur based on previous occasions with two root causes identified. The first is that it is very diffcult to predict a low pressure system before it has really developed with models tending to have a bias towards continental modelling accuracy as opposed to maritime modelling (UKMO and netweather NMM being the exceptions) . The second is that there are differences between accuracies in satellite monitoring of air conditions and actual air soundings done with a balloon. If previous modelling anomalies are anything to go by, then we should expect a downgrade in instability over the next few modelling runs, only for it to pick up slightly for the 00z tomorrow.We might also expect the forecast to move very slightly westwards from the 18z onwards (30 miles or so). On Tuesday when I looked at the forecast I was concerned a bit about timing, with an expectation that mid level instability would move in and exit eastwards earlier than modeled. Current Modelling shows mid level instability moving in from midnight instead of Tuesdays modelling of the early hours of the morning. Instability moves out of the UK by mid afternoon for all but the south east and East Anglia. The driving force for destabilization of the plume appears to be a jet streak moving up through the bay of Biscay (I think the absence of a forcing mechanism is why storms are not initiating over France yet). Models suggest the first development should be over Spain late today which trundles up to the channel islands. This development is unlikely to be surface based over the UK apart from as it crosses the coast and as daylight heating begins to affect it (Limits severity and generally prohibits tornado activity). Differences in surface based instability and mid level instability show this. This could be in the form of an MCS with a leading super cell. There is a slight risk of the lead storm developing a weak tornado as daylight catches up with it (Possibly towards Hull or northwards depending on timing and positioning - current models suggest it might be over the sea before 8am when it might start to become surface based). If it materializes then it should give a moderate lightning show and there is a chance precipitation will be elevated (not much reaching the ground). Small lobe of vorticity at 500hPa (mid level height) might suggest super cell development. 700hpa (height) Vorticity charts suggest a warm front moving north over the UK with a trough/ cold front trailing. Destabilization occurs just behind the warm front over night and in the warm sector under peak day light heating. I think the trough/front gets absorbed into the main cold front later on in the day. The overlap between mid level and low level shear tomorrow might be quite narrow and depend on the existence of a low level jet. Any storm developing where that low level jet is would have properties which in theory could create a tornado. The problem is that the NMM model does not develop storms there but rather just eastwards along a convergence zone. So all the ingredients are there in different places for a severe storm, they just don't quite come together on this modelling output. My general rule though is that if it comes together on the model it will not on the day and if it does not come together on the model it is more likely to on the day, except when the models are completely wrong and you get no storms at all. Unfortunately it is still too early for modelling to give much in the way of detail and the model outputs displayed in this post are mostly likely wrong in some aspects. Timing of this event means further model analysis of detail will be of little use compared to actually observations. I guess this post is aimed at those who would like to understand how different aspects need to come together for storms to develop rather than any serious forecast of what will happen tomorrow. Still it is nice to ponder and wonder sometimes.

Tricky scenario for Friday with lots of potential to change. Typically models tend to move convective potential eastwards two to three days out and then down grade a day before. On the day the models will often slightly upgrade potential and move it back westward a touch. Relying on historical performance bearing mind models are constantly being upgraded is always risky. A developing low system pass northwards just to the west of Ireland with a trailing cold front probably over Ireland. This puts the UK in the warm sector with a long pull of warm air coming from around Spain. Models suggest a trough develops over northern France and stretches northwards during Friday while moving slowly eastwards. The question will be how long the air feed from Spain will be in place and will it really have time to feed heat northwards. There is also a boundary between the warm feed from Spain and the cooler but moister maritime air from the Atlantic. Even at low level you can see a potential marked boundary between the hot feed of air and the maritime air potentially leading to low level wind convergence zones over the UK. Over Laying the 925hpa height winds would give areas of marked low level vorticity. With the jet stream powering overhead slightly out to the west there is potential for mid level vorticity as well. Based on this I would not be surprised to see some excitement based on lightning wizard tornado and super cell forecasting charts tomorrow. These charts might need to be taken with a little pinch of salt for the UK on Friday. This is a sort of Spanish Plume scenario which tends to mean that storms are elevated and not surfaced based (Tornadoes not possible). The plume at this point has not really got going for the UK, 850hpa temperatures are elevated, but mid level steep lapse rates are a little late arriving and surface temperatures may not get the chance to rise into the high twenties. A GFS forecast skewT for London shows the problem with unstable air above 800hpa but a marked CAP against surface based convection. I think it might be better to start thinking about two separate areas of convective potential over the UK for Friday. To the south east initially there is a risk of elevated storms producing Lightning but not much in the way of rain. The second area of interest is the boundary between the plume and maritime air which moves north and eastwards across the UK. The risk here is that an MCS develops to the south of the UK which keeps temperatures low and you just get lots of rain. Some more charts from Friday. Some rainfall predictions which are bound to be wrong as well (This is only one models output and none are in agreement). Some things perhaps to watch out for are elevated storms becoming surface based in the east. Lead storms moving north or storms in a convective line break. It is of course way to early for detail and the modelling is bound to change over the next two days. My hunch would be that the timing is not quite right (instability moves in over night, french storm outflow from previous day, surface temperatures not high enough) for significant convective potential based on current modelling. Key places for me at the moment might be London and somewhere like Hull, but I expect to change my mind over the next day or so. There are however some potential ingredients for a severe storm to develop. More perhaps as we get closer to Friday.

Slight risk of storms today mainly in Lincolnshire and Norfolk. Triggering for storms appears to be a convergence zone developing along the east coast. Instability is fairly weak but should be enough given daytime heating is not blocked by cloud (which is a risk). Low mid level vorticity charts show some development is possible. Forecast skewT's and mid level lapse rates suggest cloud tops may be limited, but there is a risk of spout like weak tornadoes along convergence zones (low level winds turning). Over all the limited cloud tops and warmer upper layers will limit development. The low zero degree isotherm presents a possible hail risk. I am not convinced from the forecast air profile that storms are really viable, so would suggest the risk is very limited. Sharp downpours in some eastern counties most likely.

I think I am still favoring North Wales , Manchester and the North West Midlands late afternoon personally for any storms to develop. Steep lapse rates slowly spread south during the day. Triggering of more than a sharp shower seems to be either vorticity moving in across North Wales late on or low level convergence zones along the east coasts. Models don't seem to entirely agree on either of these options. So not all that convinced about low level convergence zones developing. I am a little more convinced by the vorticity advection at mid levels into the north wales area based on Satelite imagery. There is not much directional wind sheer except for perhaps northern east coasts like Cumbria and wind speeds at 925hpa and 500hpa are not much different. Surface temperatures (which should be taken with a pinch of salt in the sun does consistently break through) along with dewpoints are fairly poor. What I am struck by is the level of free convection on lightning wizards GFS based chart and the weak instability. So over all I am a little confused about what will happen. Due to the low temperatures I think any storms are more likely to be similar to early spring heavy showers, being less surface based and more mid level based. The zero degree isotherm is reasonably low so there is a good chance of some hail, but weak updrafts will limit hail size. Moderately high cloud tops should allow for some lightning. Its not a classic thunder storm situation and for many I think messy mid level convection might be all we see.

Yes looks like a little bit of line convection embedded in the frontal system showing up on the rainfall radar. Tomorrow ------------ I am not convinced by the suggestion of wide spread activity tomorrow. Firstly current modelling suggests the triggering for storms would be mainly in the west towards North Wales, Manchester and perhaps the north east. Divergent upper level winds tend to be concentrated in the west. Forecast SkewT's suggest a little instability, but cloud bases are not that low due to lack of moisture. There is a hint of a low level jet but realistically with temperatures around 10 C this is unlikely to result in updrafts strong enough to utilise the jet. Squally pulse storms are more likely. There is still time for the modelling to change, so we will wait and see.

Very outside chance of a storm today mainly across southern counties. Storms would be embedded in the frontal system and messy. Isle of Wight around mid day would be a key area. I am not totally convinced about the risk here and models do not entirely agree. Looking at the Satelite pictures there appears to be a little development towards the core of the low which is off the south west of the UK. Forecasts would suggest this area will skirt the south of the UK with a slight chance of the tip of corwall being affected. Watching the satelite loops (sat24) I am uncertain about the exact trajectory, so would suggest it is worth monitoring to check. Most likely there will be no storms in the UK today, but the very slight risk is worth a few words at this point.