BrickFielder

Early on today I think the South west could see a few storms as a lobe of vroticity passes over. From Mid day Onwards I would expect the pre frontal trough currently over the midlands to start to destabilize as it moves north into the north midlands and north of England. There is a good reason why the north of England and into Scotland should be favoured for storms today. Low level moisture is being swept north to pool over the north of England. That might not be the whole story though. We still have the high level jetstreak sticking its nose into the UK over the afternoon. Storms appear to have developed over France and may head towards the UK. A couple of forecast SkewT's show some of the differences between the north and south. Think looking at the radar and satelite pictures will tell us more than the models now.

Today Its a little bit complicated to forecast whats going to happen over the next few days. Looking at a forecast SkewT for today a couple of things show up. Firstly that low level moisture is not that deep which means instability (CAPE) is probably over stated on the charts. Secondly that top level lapse rates (no chart for this) are not that great, which means cloud tops may be a little limited. So not that impressed with storm opportunities today. Overnight we have a cold front approaching ( looking at UKMO Fax charts) swinging into the south of the UK. Ahead of this is just a suggestion of a trough. This might suggest some elevated storms ahead of this overnight. Forecast SkewT's suggest rather meagre instability which will be elevated rather than surface based. There is also the possibility that the front becomes convective itself especially into tomorrow where it would tend to effect areas further north. Tomorrow I rather like the look of the storm potential tomorrow, but I think cloud cover may play a part. Looking at a forecast SkewT then we can note that instability is not so much reliant on shallow low level moisture. Upper level lapse rates are high meaning cloud tops can reach quite high. The Uk comes under the nose of a high level jet streak, with a little divergence aloft. There seems to me just enough wind speed shear through the mid levels for some updraft downdraft seperation. Low level winds are light and although there is not much wind speed shear between the surface and cloud bases winds might be light enough for any storm to develop its own winds. Weak directional shear and low level shear could limit severity. Over All I am still not convinced about tomorrow and am worried about cloud cover. Its still too early to really forecast at the moment so its just initial thoughts at the moment.

Think I will be watching for vorticity lobes spinning round the low just off Spain. Some suggestion of approaching vorticity in the early hours of Monday. WInd's will steer storms from France and Spain towards the UK, so there will always be the chance of just getting the cloudy remnants of their storms. Low level winds will begin to blow from a south of France direction. These types of wind can be a little low on moisture some times. Perhaps some chance for mammatus cloud photography , or even a short lived tornado in any lead storm. Later elevated storms can produce some good lightning. There is still a chance of course that nothing will occur storm wise.

500mb relative vorticity is not easy to explain and comes down to how small parcels of air act under certin shear conditions as far as I can tell. In my view there are some simple ideas that you can take away from the relative vorticity charts. These are that approaching vorticity should act to increase instability and are often associated with fronts or troughs. They also indicate a curvature in the winds which would lead to divergence (ahead) or convergence (behind) of winds at that level. Divergence of winds may cause air beneath to be lifted up to fill in the voids and rising air will tend to help cloud formation depending on air moisture content.For today then models dont quite agree. Low level moisture pooling across the center of the UK should cause dewpoints to rise. This is why a litle surfaced based Cape is forecast.Low level wind convergence could be a spark for shower formation.Looking at forecast skewT's then good daytime temperatures should just about spark showers up to a height where they become more storm like. The trouble is that the overnight soundings showed more marked caps which we are unable to tell whether they still exist.So not convinced about todays storm threat. Over the weekend steering winds should shift to come from firstly a southerly maritime direction and then from across Spain and France.No real hint of a trigger until Monday and by then any storms could be rather elevated (less rain)

Over the last few days I have been looking at the overnight soundings and thinking there is a bit of a cap between 700hpa and 500hpa.I have also looked at the NMM detailed Mixed Layer and Surface based Instability charts (Cape) and thought instability is a bit meagre yet one or two mild storms have developed. So the question is whats going on. Personally I think models tend to struggle in a number of ways with current conditions. The first is that as good as satelite analysis is , it will not pick up the detail that a sounding can and there are no soundings out in the ocean (where tomorrows weather generally comes from). Secondly dewpoints or depth of surface moisture can be slightly over estimated at times. Thirdly under slack conditions daytime heating temperatures can be slightly under estimated.So bearing this in mind we have another day where low level wind convergence can spark off some showers and possibly thunder storms in some areas. As the day goes on the steep level lapse rates will begin to decline from the west. Cloud tops today look a little limited for storms and possibly better the further north you are. As temperatures are not that impressive especially to the far north this must be taken into account as well.In terms of low level convergence we have an area around the pennines which will move eastwards from Mid day onwards. We have some convergence areas in Scotland and perhaps a suggestion of convergence towards south wales and possibly mid ways into the midlands. Some mid level approaching vorticity from the west and wind divergence could enhance storm formation. There is also a bit of low level jet coming in from the west which could enhance speed sheer in the lowest levels. I dont think updrafts will be strong enough to lift any low level vorticity though. Over all the greatest risk is slow moving or in situ showers giving heavy downpours with a risk of flooding mainly towards the East.Convergence zones give rise to a risk of very weak short lived funnels.Limited cloud tops may prevent severe storm formation. Slight risk of storms in the midlands perhaps towards nottingham, but mid level laspe rates declining from the west as the day goes on. Do go and read Jo Farrow's blog and the discussion about nowcasting.Using the radar at this point will be much better than any model anaylsis. See if you can work out where the storm which developed over merseyside is heading. Keep an eye on the showers towards essex.

The simple explanation is the jet stream across the atlantic has been very strong and further south than normal this winter. Looking further afield then it is down to abnormal jet stream patterns across the Pacific. To my way of thinking this should correspond to a la nina event where thunderstorm activity shifts from of the coast of south america towards Indonesia. Floods in Jakarta this year would tend to back this up, yet sea temperature anomalies across the pacific dont look that exceptional, neither does the SOI (Southern Ocean Index) which tends to drive el nino and la nina (Heat waves in Australia do show some abnormal activity though). Monsoon variability is driven by more than one factor and since the 1970's the dominant factor has tended to be el nino. Secondary factors have tend to be the southern indian ocean and the north west pacific. Looking at the 500mb height anomalies for the last 180 days something significant shows up. http://www.esrl.noaa.gov/psd/map/images/fnl/500z_180b.fnl.htmlhttp://www.esrl.noaa.gov/psd/map/images/fnl/250wnd_180b.fnl.html So maybe this year the monsoon was influenced by none el nino factors but it is not exactly a convincing argument. How about the strong westerly QBO which should give rise to a stronger polar vortex, but may allow more upward propagating waves to affect it. The vortex does seem strong and we have had a split vortex although displaced in a way to enhance current patterns. To me it looks like the vortex has been displaced towards the north atlantic, much as it does towards late autumn, but has never moved closer to the pole as it tends to do in the winter months as it gets stronger. Not convinced by the QBO argument (me neither). Well how about a completely off the wall explanation where I get to add 2 and 2 and come up with 10. It seems to me that the jet stream across the UK has been further south than normal for a couple of years, its just this year events have combined to make it more noticeable. How about the phase transition of the solar magnetic field (no not sun spots) around 2007 being the begining of the change. http://www.cdejager.com/wp-content/uploads/2013/10/2013-CdeJ-HN-Sun-climate-NS-5-1112.pdfReading the article in the link above would tend to confirm some sort of change occured. It might be a bit of a reach to conclude it affects weather patterns and the jet stream though. http://iopscience.iop.org/1748-9326/8/4/045001/articleOK so that article confirms there is a link between the solar magnetic field and weather patterns. May be I should build an Ark. The answer of course is that I am still confused about why the weather has been fixed in the pattern and no doubt it is a combination of climate change ,QBO and other factors , but just maybe there are some other forces in effect as well. Food for discussion anyway?

Its too early to tell where the path of the storm is going to be, but there is enough of a concern at the Met Office to issue an amber warning. I think that tells me there are aspects to the storm other than how deep it is that might be a concern. Perhaps this might be a Shapiro-Keyser cyclone as perhaps hinted at by skyraker.(the cold front moves roughly perpendicular to the warm front such that the fronts never meet, the so-called 'T-bone'. Also, a weakness appears along the poleward portion of the cold front near the low center, the so-called 'frontal fracture' and a back-bent front forms behind the low center. Cyclones embedded within diffluent flow (e.g. jet-exit regions) tend to evolve like the Norwegian cyclone model, whereas cyclones embedded within confluent flow (e.g. jet-entrance regions) tend to evolve like the Shapiro-Keyser cyclone model.) The key might be the back bent front forming behind the low center. At the tip of this cold front air rushes downward from a warm front to a cold front. This is called a sting jet, which can increase low level winds speeds which models will find hard to pick up.Here is what storm jannette looked like on the fax charts. Note that the sting jet really did not descend until the it had almost cleared the UK in this case.Compare this with a forecast fax chart for Monday (Red circle could be a potential sting jet area). The key thing to remember is the rapid cyclogenisis of this storm with the existing storm to the north makes it slightly different to your average atlantic storm. There are some potentials for high winds which most likely will not show up on model charts. Having said that quite often that potential never materialises. Even without a sting jet reaching the surface the winds will be strong enough with trees still having some folliage to cause damage. I think the Met Office has made a good call here (not getting too technical but a broad warning and better to act on the side of caution). Go read the links posted by Coast on the previous page. Eumetsat also has a good review on sting jets.

Estofex----------------At mid-levels, steep lapse rates have started to spread northward and will overlap with the moisture. Current thinking is that elevated storms will develop over a broad region. During the day, storms will have a greater potential to root to the boundary-layer. Storms that form may be capable of producing large hail and tornadoes given the moderate low-level vertical wind shear. Main potential will be the British Isles, but a weak threat cannot be excluded for northern Germany and Denmark. Limiting factor will be the increasing capping inversion that will suppress deep moist convection further to the south.----------------Yet most of the models do not agree. Certainly there appears to be some instability forecast for today.For surface bound storms though you need some sort of trigger and there really is not much of a trigger. Perhaps late afternoon early evening in some parts of Northern England.As for the elevated storms that Estofex is thinking about then humidty through the mid levels is rather low.Despite that forecast SkewT's do suggest posibilities for elevated and surface based storms today.Overnight soundings do hint at a CAP though which is not clear on the forecast skewT's.Overall the greatest risk today is in Ireland, but there is just a hint of risk (20%) for parts of Northern England. Wind speed shear and drier air aloft could indicate a slight tornadic risk as well. Its one of those situations where some models are not so keen and 8 times out of 10 they will be right. Still worth a quick discussion though.

Mid Level lapse rates are not that impressive again today, although slightly better in the north and east (mid level lapse rates of 26C + best for storm development). Low level convergence could again be a focus for development. Eastern coasts, parts of the north and areas in scotland. Instability (Cape) looks moderate to very weak with northern areas where the mid level lapse rates are best. Surface based cape is the same except for a toungue down into the south west (Seems to aligned with surface convergence). Perhaps the only surprise is the possibility of some heavy showers developing in the south west. As for Saturday then its a bit far out for guessing at the moment.

Some disagreement in the models today again. First thing to notice is the mid level lapse rates which look to be around 24 C which would normally not be considered enough for storm initiation. Other factors include the low level convergence across parts of the north of England which could be a focus for storms. Next up is a low level trough forecast by NMM to sweep across the north of the UK. At upper levels some divergence is forecast for parts of the UK. The problem is that these charts have a low confidence at the moment. GFS forecast SkewT's for London show preety much no instability (Cape), but there is a chance that a temperature of 23C and Dewpoint of 12C might be on the low side. In other words the risk of storms is there, mainly for parts of the north of the UK, but I would not rule out a storm virtually anywhere. So perhaps a 25% risk in the north, 10% elsewhere particularly in the west or near large cities and a 80% chance over Ireland.

I think the models are a little behind actual events at the moment so its hard to predict much for the rest of the day. Storms for the North East and eastern parts look likely to arrive late afternoon rather than early evening. Elsewhere and storms over northern France have complicated matters somewhat. Best guess of places to watch over the next few hours based on Eumetsat images would be the following. There are no guarantees on this and its a changing situation, but ahead of vorticity advection can be a good place for convection to initiate providing temperatures and dew points are high enough.

More surface based convergence as pointed out by Estofex is likely to be a focus over the UK during the afternoon with storms developing tend to move northwards toward north eastern coasts. Eumetsat images suggest some marked vorticity approaching from the west. The Cambourne sounding from last night shows some drier air coming in at Mid levels with a bit of a Cap above that. My view having looked at the other soundings from the UK is that higher temperatures will be required for storm development in the south and west. Mid level lapse rates begin to dwindle from the west into the evening meaning storms will begin to be limited to the east as the evening goes on. Mid level winds look to be unidirectional (no veering as you go up), but there is some speed shear. Most importantly is the speed shear between the surface and 925hpa in the north east and perhaps to a lesser extent the high level winds over central areas in the afternoon. Forecast SkewT contrasts between London and Middlesborough show saturated air in the north whilst somewhat less so further south. Don't forget the Cambourne sounding Cap which is not showing up on the forecast SkewT's when assessing these. Some key things to watch for might be how messy the convection is in the north and whether it affects the ability of storms to be surface based or their intensity (don't think it matters much). Another might be whether temperatures in the south reach high enough for storm initiation. Beyond that and you are better off watching the radar.

Bit of a bust of the forecast today so some examination is required. Soundings show rather higher temperatures and humidity required for initiation early on than thought and the morning cloud cover played a part in keeping temperatures just below those needed. Combined with that was the convergence zone not really developing in the way expected and humidity arriving in the UK a little late. Radar shows some storms have developed central south which are trundling along towards Leicester and Northampton with further development towards Peterborough. There is a storm to the south west of London as well. I expect most of these storms will be elevated rather than surface based in many cases. Eumetsat images suggest things are not quite going to play out as forecast overnight and tomorrow either.

Today===== Complex convergence zones look to develop over the UK late afternoon, these will most likely produce the focus for storm development. Forecast SkewT's show light winds aloft but appear to be marginally turning, instability looks to be high for the UK. Drier air through the zero degree isotherm could enhance the hail risk. One key area to look at considering weakly turning storms and slight updraft and downdraft seperation are the winds at the lowest level. At convergence zones surface winds should be light, whilst in comparison 925hpa winds are around 20mph. This wind speed shear in the lowest level combined with some turning at convergence zones might be enough for tornado development. Moisture at upper levels and no real CAP might make things messy and probably limit the risk. With steering winds light one of the key risks will be localised flooding. Adding to that because of the high instability is the risk on hail, although for me this is a little late in the season for a hail risk, ideally for large hail you want the highest updraft velocities through the zero degree isotherm. Very high cloud tops and high instability mean the risk of lightning is enhanced, and I think this will be something to watch out for. As for tornado risk then I am hovering between Slight and Moderate and for me the 925hpa winds tip things slightly more towards moderate, on the other hand looking at the storms over northern france this morning I am wondering if storms will have long enough duration for tornado development. Tomorrow========Elevated storms with some good lightning coming in from the south overnight. Sometime during the morning (9 - 10am at a guess) some of these storms might become surface based.Through mid levels there is much more wind speed shear but no veering of the winds. Slight jet at 925hpa suggests the possibility of a weak tornado through the midlands with the lead storm moving north (not totally convinced at this point). Might be worth getting the camera out for some mammatus photos at a guess. The rest of the day forecast is as usual subject to significant changes. Storms firing again across the south especially towards London by mid morning while storms trundle north through the midlands into northern England. Those storms near London head north across the eastern half of the UK ending up with a particularly nasty storm mid afternoon around the Hull area. Meanwhile further storms trigger across the south and midlands as the sunshine comes out. Key charts for tomorrow.

Too early for a proper forecast for tomorrow yet, but low level convergence moving north could be a focus. Forecast skewT's suggest some veering winds at low level but upper level winds are weak. Moist upper layer could give some high cloud limiting things. Mid to upper level wind veering is limited but might just be enough for a weak super cell (depends on local conditions and I am not convinced). Low level wind convergence would be conducive for weak tornadoes enhance perhaps by decent lift from high instability. Drier slot through the mid level does help convection, but there does not appear to be much of a cap. You could end up with a messy MCS instead of individual storms. There is still time for things to change significantly in the forecast (i.e. no storms at all).

Yes late afternoon in the south west and perhaps Wales. Mid level lapse rates not as good as yesterday and steeper rates do not come in until late afternoon. Low level convergence zones are likely to be a focus. Interesting change of wind direction through the mid levels. Some diagreement between models on that though and lower level winds are not high enough to start thinking supercells. Forecast SkewT's show some instability but tops being slightly limited. Overall a risk of some storms in the south west late afternoon which are likely to be steered towards Wales. Possibility of some weak convergence zone type funnels, but main risk is localized flooding and hail.

Not much to add from yesterday really. NMM is suggesting some low level convergence and an area of vorticity advection across the UK. Mid level lapse rates look OK (You dont get much better than this in the UK - 32 Max typicaly) Instability looks on the low side though, with surface temperatures tending to limit updraft strengths. There is a little turning of the winds in the lower levels in some areas, but its not concentrated in the lowest layer , with very little speed shear in the lowest level and temperatures might limit updraft strengths. However it could mean that moist air feeds into storms are unlikely to be shut off by any downdraft or clouding out.Just wanted to pick up on a few good points made in the comments. So potential for the upper region of the more vigorous updrafts to be pushed forward in relation to storm motion, increasing longevity and perhaps enhancing strength. Steering mid-level winds, however, will be light, so storm motion rather slow.Steep low to mid-level lapse rates supports generation of moderate-sized hail and gusty winds. (Weather09).Fairly weak CAPE will preclude severe weather developing, although slow storm motion could give rise to localised flooding as they are likely to last a while.Around 30-60knts of deep layer shear will allow separation of updrafts and downdrafts which brings the probability of more organised storm activity (Supacell)I know the TT index should't be taken as an indicator on it's own, but putting the Netweather Extra NMM ones in amongst the other GFS charts this morning, makes it pretty interesting this afternoon: (Coast) Key risks I think are from slower moving storms producing hail and significant downpours. So a good chance of some localised flooding if storms materialise (Triggers are a bit weak). Beyond that it is a wait and watch the radar.

Still not quite sure of what to make of tomorrow as in theory there there should be a modicum of instability. Mid level laspe rates look good and both surface and ML Cape show instability. The question as always is what is the trigger for storm formation. Here it is a more complicated story and current fax charts dont give much of a clue, neither does low level winds and convergence zones. Mid level vorticty charts do suggest approaching vorticity and humidity charts show some drier air slots through the mid level. So maybe hints of a mid to upper level trough. Steerage winds suggest source winds are over the bay of biscay, then through northern france and then from the south over the UK. Its not exactly a plume but does portend some moisture will be available in the lower levels. Low level wind shear looks meagre and mid level shear looks better over the eastern side of the UK comapred to the western. Low level temperatures are not that high either so surface convention effects could be limited. SkewT's show good instability but slightly limited cloud tops and cloud bases are a little high. Later on tops get a bit higher and bases a little lower. Still too early to make a proper prediction and there are some things that do not line up the way you would expect for storm initiation. Good lift through the zero degree isotherm means hail could be a player as well. Wind speed differential between west and east sides of storms through mid to upper levels could play a part in eastern areas as could anvils getting ahead of the storm. Too early to be convinced one way of the other and the triggering mechanisms for storms is not clear to me at the moment (although satellites do show a showery regime out in the area which is coming our way).

Not that impressed with mid level lapse rates today. Now compare that with overnight ones. Tropopause top looks limited today as well. Some chances of some sharp showers today. Forecast SkewT's show limited cloud tops though. Overnight we do appear to get some mid level instability, but it will not be surface based. Even here mixed level instability is limited (MLCape). Foecast SkewT's look more frontal than convective, but there is a good chance of some heavy possibly thundery showers overnight (not convinced myself). Tomorrow sees a jet streak approaching and crossing the southern part of the UK. The jet streak exit region over the north and central UK should see some divergent winds aloft and hence conditions for lift. There is also a suggestion of a surface moisture trough. Too early to say much about tomorrow at this point though, but northern and perhaps some central eastern areas late afternoon look best.

GFS is not impressed with today although NMM and the UkMet office suggest some slight possibilites. Low level wind convergence zone towards the south west and extending a little into the midlands is still showing up in the NMM charts. It also looks to be giving some moisture pooling. Up in the north east the frontal system breaks down and could give a few storms. Along the south coast we have vorticity anomaly (trough or front) which could spark some storms if there were any instability (NMM suggests not). Key area could be late afternoon around Cambridge and towards the Wash. It all looks a bit marginal with lapse rates only just high enough really for convection. Still there is an outside chance of some storms. Over all it looks a bit marginal for storms today.

Worth keeping an eye on the charts for tomorrow.UKMO synoptic fax charts suggest the rain band sweeping in over night is an occluded front with a further cold front to sweep in later in the day. This suggests we might still be in sort of warm sector conditions (not low 20s temperature but maybe 18C in places). Occluded front may well break up and become convective which is perhaps something to watch for in Eastern and Northern areas. Possible low level wind convergence zone around the bristol channel which could spark some convection and develop as it goes in land. Some directional wind sheer through the mid levels, dew points dont look overly cooked, but lapse rates are really only just good enough for storms. Forecast skewt's suggest a cap at 800hpa - 850hpa height being broken by temperatures in the region of 19C with drier air coming in aloft. Looks a bit touch and go at the moment with instability on the thin side (cape). Too dynamic and marginal to say much more at the moment, but certainly a chance of a storm.

It is too far out to be able to tell, but the outlook might be for a cooler spell. It seems possible that spain and portugal could have atlantic maritime air circulating via the UK or possibly scandinavia (see charts below). I guess temperatures might just dip below freezing over night, but charts are not at the moment suggesting anything exceptional for Portugal and Spain. Now if you were looking at Greece, Italy or North Africa then you might expect an unusually cold spell. See wind directions on the charts below. These charts are at the limit of model forecasting and will most likely be substantially wrong. At the moment the best we can honestly say is that there is potential for some cooler weather.

First the good news. It definately looks like we are shaping up for a split vortex. Initially we have the displaced vortex. Then the split. The a reformation by the looks of it over Canada. In terms of what the forcing from the stratosphere will look like then I guess it would be something like the following. For the displaced vortex it looks like high pressure over or to the north of the UK. For the split vortex it looks like high pressure towards the UK and north of the UK with a deep low over canada. finally potential for high pressure to our north but canadian polar vortex is shifting downstream towards the uk. Now perhaps the bad news, but this will most likely change as models catch up. Any stratospheric forcing for a high towards the UK will most likely be modified by tropospheric conditions to push the high further south than it would otherwise be. Instead of a high to our north we could get high pressure moving up from the south. This could give fog for the south and some snow for scotland. Any deep low upstream( like a polar vortex) from us will most likely create energy which will spread downstream to us. This means although the stratospheric conditions would support a high for the UK tropospheric conditions might well cause low pressure systems to spin off from the tropospheric polar vortex to the UK. For snow lovers the UK now becomes the battle ground between cold and warm air. From past experience I would suggest at this point that the models will not have the balance between stratospheric and tropospheric forcing correct. My guess is that the models will tip more in favour of the stratospheric forcing, however the troposphere will still modify the signals. What we should give consideration to is that fact that potentially the polar vortex is going to sit over Canada, which possibly means a great deal of snow for northern parts of the US. What happens after that will be curious to see as either the polar vortex will migrate back to the pole or it will shift towards scandinavia putting the UK in a northerly flow. Not a great outlook for an easterly at the moment but some significant potential for snow especially for the far north.

Some reading for those interested. http://www.atmos-chem-phys.net/12/3659/2012/acp-12-3659-2012.pdf Some selected quotes In sumamry, we conclude that the SSW end of January 2010 resembles the displacement type. The eventual vortex break up into two lobes occured in the second half of february. The upward propagation of the planetary waves into the stratosphere tends to be weakened by blocking highs over the Far East and Western North Pacific, but enhanced by blocking highs over the Euro Atlantic sector. Greenland, northern scandinavia, iceland and Novaya Zemlya can be identified as the mosts active locations for stratospheric mountain waves during the 2009-2010 winter. Reading through the paper I think there are some key things to take away about prediction of how the stratospheric warming plays out. Typically you get three distinct types of warming. 1) Stratospheric vortex displaced over Nothern Europe/ Russia. 2) Stratospheric vortex displaced over Canada. 3 Vortex split. Quite often one event will be followed by another about a month later. Which of the major sources of mountain torque is dominant can determine what type of warming occurs. As for weather types then it is impossible to predict apart from warming 1) tends to be shorter lived and can give northerly or easterly winds. warming 2) tends to last longer and sets up blocking over Greenland and Iceland (think easterly , but is rare) warming 3) tends to last the longest and gives alternating cold and warm (week at a time) as the dual vortex spin round each other.