BrickFielder

Estofex forecast for those who have not seen it http://www.estofex.org/ Couple of quotes. A surge of dry low-stratospheric air wraps cyclonically around the cyclone's center, probably ending the stage of a potential offshore sting jet event. Downdrafts may bring severe wind gusts to the BL. What I am pondering is the forward speed of the storm which I roughly calculate to be 50mph (6 hours to traverse Ireland) which makes me question highest wind speeds of 80mph. Looking at forecast storm surge levels which range from about 1m to 2m then there seems to be some key risk areas like south Wales, but particularly the Scottish borders down to Liverpool (in addition to Ireland). What does concern me is that the jet stream pattern showing up on satellite images is deviating slightly from forecast (jet to the east of the storm is weaker than expected and the trough to the west a little sharper). Maybe a few twists and turns still to play out.

Just coming into range of some shorter range models like the lightning wizard ones which can give some further insight into expected conditions. Wind gusts over 90 knots (100mph+) for southern Ireland for example. I have also being trying to assess the potential for embedded super-cells which could pose a lightning, hail or tornado risk as well. Looking at mid level and low level lapse rates and overlaps between the steepest lapse rates for both, then I don't think there is much of a tornado risk (perhaps a waterspout risk for southern Ireland coasts). The parameters for an embedded non surface based super-cell are not that good either despite what some charts suggest apart from very close to the center of the storm. There is some marked vorticity advection from the cold front and some weak instability which would have to be watched. Theta e and Thompson index values suggest some very heavy rainfall for Ireland and perhaps to a lesser extent from the cold front associated with it. Assessing whether there is a possibility of a sting jet scenario is also tricky, as there is rapid cyclogenesis caused by the Jetstream, but also an amount of frontal wrapping around the center of the low. Despite quite a narrow gap between the cold front and central core for a period and the jet streak over the cold front, I am not at the minute thinking a sting jet is on the cards. Some of these factors which I don't think will be applicable for the UK (convective potential) may not hold true for parts of Europe as it weakens and moves across the north of Europe, so European forecasters should probably keep an eye on events as well. Longer range modelling differences between some European models and GFS have shown differences in the handling of a trough down towards the Baltics with European models I think picking up the trend earlier. I think makes the eastward shift in the forecast from European models more likely. Ensemble members were a little confused about a low pressure system developing along the frontal boundary Saturday into Sunday which moves across to northern Europe late Sunday. This looks likely and is also part of the reason for the slight eastward movement.Its possible with the Baltic trough moving a little more south east than south than there could be further eastwards movement in the forecast, but its unlikely to be far at this forecast range. I not too happy though with the modelling of the transition to extra tropical and think the warm core will only breakdown just south of Ireland, increasing the risks of stronger gusting winds for southern Ireland.Looking at satellite pictures (eumetsat) then there has been a slight upper jet to the north of Ophelia slowly wrapping around to the west , which has caused the hurricane to spread out eastwards. This is causing some sheer and perhaps a little dry air to be drawn within, so we should expect a little weakening and perhaps a slight eastward shift again. While focus is drawn to Ireland we should note that with many trees still with considerable foliage across England then even moderate winds could cause some disruption. Equally some parts of the UK are already fairly water soaked so a strong deluge of rain could cause some localized flooding issues. Lastly a couple of forecast images from GFS which show how the storm will be wrapped by a small jet streak to its east, with divergence aloft ahead of that streak. Whether its a hurricane, hybrid or plain autumn storm, this probably means rapid cyclogenesis as it approaches Ireland (Deepening of the low).

Looking at the storm as a hybrid between hurricane and extra tropical storm might give some different insights particularly looking at it as extra tropical. In this case we have a jet streak looping around the storm as it moves up from off the coast of Spain to France with divergence aloft ahead of the storm. This would suggest rapid cyclogenesis (deepening of the low) as it approaches the UK.As the storm moves near to the coast of Ireland that jet streak swings across England. Strongest winds for a hurricane are near the eye whilst the strongest winds from a non tropical storm are under the jet stream. Looking at the ECMWF 850mb winds shows wind strengths in excess of 70 knots at 850MB (That is 90mph winds at 5000 feet) across much of the UK. Now ECMWF brings the low closer to the UK than some other models and winds at the surface will be a lot less than that, but it is worth bearing in mind. So we must be careful not to assign purely tropical attributes to this storm.Having said that Southern Ireland looks to have the brunt of this according to current modelling.

It is way too early for any sort of realistic forecast , but realistically the worse case scenario for Scotland is probably some Gale force winds and heavy rain. By the time the system reaches Scotland the storm will have transitioned from a tropical system with warm core (convection around a central low ) to your typical north Atlantic low pressure system. Once it is fully transitioned to extra tropical it should not be any worse than you typical autumn low pressure system. The only difference is that it might contain tropical moist air giving lots of rain, and as it interacts with the Jetstream it might deepen to give strong Gale force winds. The only question is really how and when the warm core dies and how the remnants of the storm interact with the Jetstream. Both of these should have happened by the time the remnants reach Scotland and most likely before it reaches the UK. There are some slight scenarios where southern parts of the UK and Ireland could be affected by unusually high winds and rain for this time of the year, possibly even near weak hurricane strength winds. Living in Scotland you are unlikely to experience much beyond your typical autumn low pressure system, with rainfall and flooding a more likely risk than abnormally high winds.

Its quite an unusual path for tropical storm and forecast modelling will most likely change over the next week, but storms like this can produce a few unpredictable results. Hurricanes Gordon (2006) , Vince 2005, tropical storms Ana (2003), Grace (2009) come to mind. The prime concern does have to be for the Azores, but we should also consider what exactly happens when this type of storm goes extra tropical. Based on past observations the storm can become embedded in a frontal system and tropical aspects decay. Unlike storms which go extra tropical mid Atlantic as they lift out, what can happen is that the warm core shrinks in the front and tightens up before breaking up totally. The core moves quite rapidly up the frontal system and in its death throws can just about reach the southern UK bringing localized hurricane force winds. In effect the core tightens, shrinks and its rotation speeds up as it dies with a possibility of producing winds in excess of 100mph. In this particular case I don't think this will happen as the tropical storm is not strong enough and the core not warm enough with a result that the core dissipates long before it gets anywhere near the UK. So the prime risk to the UK would be tropical type rainfall and a particularly deep Atlantic low developing as the remnants move towards Iceland. Current modelling would suggest that western Ireland could experience some very strong winds next Monday afternoon and Northern England and Scotland a deluge of rain Monday evening. We should however note that ECMWF and UKMET moved things eastward in the last run.

06z forecast modelling looks a lot better than the overnight modelling. Suggests French storms will pass over the south east of the UK with further initiation of elevated storms south west and south later this evening trundling on through to East Anglia. Key area to watch would be around Exeter this evening. Not sure how far north storms will get as things move a little faster eastwards than previous thought.

It looks to me like the forecast initiation of storms forecast for this evening is linked to how far the trough extends south. So I would question where and when storms initiate. You might end up with thundery rain in the south west or you could end up with a much more severe storm that predicted due to being rooted in the boundary layer. Best guess is that perhaps things have become very interesting and it could alter the severity risk from storms.

I was thinking much the same thing. Satellite imagery to me looks like it is lifting the trough out slightly quicker than modelling suggests. It could bring storms to the south west earlier than forecast. The lobe of Vorticity out over the Bay of Biscay does not have quite the tail to the south that I was expecting. Think it is definitely a case for now casting while still bearing in mind that 9 times out of 10 the models get it right. Will probably wait to have a look at the 06 modelling.

Upper trough moves into the bay of Biscay area mid afternoon tomorrow helping to destabilize an elevated mixed layer. My current thinking is that there is an area perhaps over the isle of wight early evening where low level dry continental air begins to mix with more maritime air lifting dew-points both at the boundary layer and at the CAP. I don't think this will root storms in the boundary layer but may make instability above the CAP more pronounced. Large instability above the CAP could produce a significant lightening show with perhaps a risk of large hail. Key areas for me according to current modelling would be Southampton to Suffolk 5pm through to 3am. Interestingly GFS modelling suggests a turning cell moving from the south coast heading towards Bristol between 9pm and midnight. The models will change quite a bit between now and Tuesday evening so we should not read too much into this and storms are not likely to be rooted in the boundary layer. Wednesday daytime suggests some homegrown storms could develop as the upper trough approaches from the west. It does look as if the CAP will remain in place across much of the south and perhaps begins to break somewhere in the north midlands. Cloud cover might limit convention and winds are more unidirectional, so modelling would suggests severity of storms is likely to be limited (It is however a scenario where I have seen modelling severely under estimate storms, so I wait to see what models closer to the time show).

Looking at the forecast models for today then I can see some marked low level wind convergence for parts of the south east and East. The suggestion is that some thunderstorms may develop in this region from mid morning through to early afternoon. Key areas are likely to be along east cost areas with perhaps somewhere like Suffolk having the greatest risk of a storm. For the rest of the UK no storm are forecast for today as a weak cold front begins to sink across the rest of the UK. This weak cold front brings drier warmer air in aloft which will reduce mid level lapse rates which limits the ability of low level warm air rising. What I have noticed is that some models yesterday kind of liked the look of storms developing at the leading edge of that cold front and they don't like it today. Typically the models are updated with a lot of up to date information over night which usually makes the forecast more accurate. The question we should ask ourselves is whether this is always true. This is where I want to go a little of track and explore something which nobody should constitute as a forecast. It occurs to me that overnight cloud cover may affect models accuracy negatively in special circumstances. As the atmosphere cools a little overnight you are likely to get a little more cloud forming. If this was fed into models then there is a possibility that cloud cover could be increased in the modelling and temperatures could be forecast a little on the low side for the following day. The first test of a theory has to be measuring actual against forecasts and the only thing we can rely on is surface temperatures. If temperatures are close to approaching 20C now in many areas, is it realistic to expect them to top out at around 23C today. So what happens if temperatures are a little higher than forecast and cloud cover is a little weaker.Looking at the castor bay (northern Ireland) sounding from last night then it looks to me like thunderstorms are possible along the leading edge of the occluded cold front when temperatures reach about 26C (maybe in the late afternoon towards the south east). The people who work out how forecast models work are pretty smart people and I am sure they have made allowances for such things, but there is always that one small scenario where they could improve. Just maybe that is today, or more probably those smart people have actually got it spot on.

Latest Satellite pictures show some Kent clippers developing, which should put on a show. Some development north of London as well. I am just not that convinced by the potential today now. It looks to me like the weak cold front is moving across a little quicker than forecast and looks to be a weaker feature than I was expecting. It currently looks to have cleared the UK by mid afternoon. Looking at the Soundings from midnight last night then I can see evidence of two CAP's at different levels and dewpoints not as high as I would have expected. Comparing Belfast and Nottingham soundings would suggest temperatures in the region of 27C are required for deep convection which would be a stretch. I am also comparing satellite with forecast vorticity and there may be a few differences which would change forcing, so that we are relying on low level convergence rather than a vorticity advection trigger. I am also concerned that mid level lapse rates drop off rapidly across most of the UK apart from the south. I am now looking at south of a line from Bristol to the Wash as being better for convective development , where low level wind convergence takes place. Am watching a slight disturbance in the Bristol channel to see if that develops. Probably best just watching the radar rather than trying to over think things as usual.

Tricky to forecast due lots of factors involved. Once thing we should perhaps point out is that early storms across the south may not be rooted in the boundary layer. See forecast skewt for London. Further north the picture is a little different. See forecast skewt for west midlands. This does not mean that you will not get an intense storm with lots of lightning and rain fall , just that inflow is not in the boundary layer and so the likes of tornadoes would not be on the cards where storms are not based from the boundary layer. High level cloud due to high level moisture may also limit boundary based convection. Having said that there are signs that we should expect significant rain fall from storms developing across the south. The chart that has caught my attention is lightning wizards 0-2km deep convergence chart. This suggests a significant storm passes across the south of the UK with moisture converging at the outflow boundaries. Lightning wizard explanation of the chart. It shows regions of mesoscale ascent, which are sometimes more and sometimes less established than convergence of the 10 meter wind. It occasionally shows expanding rings of deep convergence when GFS blows itself up into a big convective system. Any storms forming at outflow boundaries have the ability to develop dangerous characteristics. At the moment there is little suggest this will happen, but it is worth keeping an eye on. It will also be interesting to see where any drier air aloft will be positioned ahead of the approaching cold front. Typically models move storm development eastwards and downgrade during the day before storms, only to move them slightly back west and upgrade on the day. So do not be surprised if the next couple of runs move every thing eastwards and downgrades the potential. (Similarly at 3 days out the modelling shows things too far west). Think at the very least somebody should get a good lightning show tomorrow.

The forecast SkewT shown shows unidirectional but increasing wind strength with height above 700hpa. A turning of the wind with height as well as increasing wind speed with height is traditional associated with super cells. So this probably would not classify as a classic super cell environment. However increasing wind speed with height does suggest seperation between updraft and downdraft. This tends to mean storms which develop can be longer lasting. If instability is strong (high CAPE) then a rear downdraft can be quite strong and affect the low level wind flow. Effectively you get a winds blowing outwards at the back of a storm before circling back into the south of the storm into the indraft. The forecast SkewT will probably not show any localized effects of a large storm. Its also possible for storms to split or form at the edges of outflows in high instability regions which alter the dynamics. I do agree in principle that the low level wind vorticity weakens and limits supercell / tornadic development. I am still hesitant about whether there will be any storm let alone a supercell or tornado due to the CAP. Met Office unlikely warning seems about right, so I would agree with you.

Yet another tricky potential thunderstorm breakdown coming up, so I thought I would put a few thoughts down about the different aspects I am looking at. The first scenario I am looking at is the possibilities tomorrow afternoon with a warm sector trough moving across the UK. Scenario 1. During tomorrow light low level winds will begin to swing round to come from a more maritime direction bringing in a little extra moisture. Above this is still the elevated mixed layer with a marked CAP or warm nose at around 800hpa to 850hpa. This CAP appears to be slightly weaker the further north in the UK you go. Forecast models don't appear to particularly agree about whether this CAP will go and far to often we have seen models predict storms based on knee deep high dew points. We have also seen high instability produce not even a speckle of rain because triggering is not in place or the CAP is stronger than forecast. The extra ingredient I am watching closely is the potential for a little wind convergence (maybe around Oxford) which may trigger a pulse storm. Hodographs are curved marked in the south of England but less so further North so supercell development initially is very questionable. With inflow being curved into the south of any pulse storm then there is potential for a pulse storm to develop a rear downdraft and slowly develop tornado potential. It is not clear cut and parameters are likely to change so the forecast at the minute would have to be classed as a very slight risk. Warm sector troughs in the UK are however one of the scenarios which can produce moderately severe storms in the UK. Best guess is that the east and north east would be favored for storms at the moment. Scenario 2 I looking at the cold front and low pressure system moving across the UK in the early hours of Thursday. We have marked vortcity advection, potential for very high cloud tops, but the timing is such that surface temperatures are low. I am thinking there might be a small window of opportunity where the front becomes rooted in the boundary layer sometime between 6am and 10am. So again this would be a very slight risk mainly in the East and North East due to timing. Summary Looks like both a forecasters delight and nightmare. Delight because they get to examine and explore charts and detail they would not normally do. Nightmare because the potential for getting it wrong is very high. My best guess is that for scenario 1 the high level cloud showing up in the warm sector on satellite images may put a damper on things. For scenario 2 then the vorticity driving this scenario looks concentrated further south on satellite images than I would have expected. When in doubt I tend to look at jet stream forecasts and upper level divergence and this looks fairly strong for the early hours of Thursday. I expect a lot more detail to come out as models re-align with actuals and at this point I cannot make an accurate assessment. I don't suppose this was much help, but might point to things to watch.

Hmmm conditions at first glance suggest that there is a possibility of a sting jet occurring as already mentioned (been watching to see which way the models will go). Quickly developing storm with an occlusion beginning to wrap around. In theory around the tip of the occlusion if the gap between the occlusion and the cold front is narrow you could get a sting jet. There are one or two negative signs for sting jet occurrence over the UK in current forecasts. Firstly as Atlantic storms go it is not that big or deep, next the gap between the occlusion tip and the cold front does not really narrow until it starts to leave the UK. Key areas likely to be south of a line from Liverpool to the wash with some models suggesting winds gusting to over 80mph with a second area Wales through to South East with gusting winds up to 60mph. With quickly developing storms being notoriously hard for models to pin down I would take those areas with a pinch of salt. Slight risk of weak tornado on the back edge of cold front crossing the south east and maybe at the triple point crossing the borders (unlikely as the instability is not right) and by the looks of it blizzard conditions in the Glasgow to Edinburgh corridor. Unfortunately or fortunately depending on your view its all best guesses based on modelling and there have been plenty of occasions where potential strong winds , sting jets or blizzard possibilities have been downgraded very close to the event. Stay safe.

So the Forecast SkewT has two main lines on it. The left one is the dew point (The temperature at which moisture turns into water) with height and the right one the temperature with height. So you look at the dewpoint at the surface and then work out at what height a rising parcel of air turns to water (I have marked that in purple). You then look at the adiabatic rate the green lines and work out when a parcel of air stops rising (thats the thick green line I have added). The other chart is the humidity (moisture content of the air) at 300hPA (which would be thunderstorm top height). This tells me that there is drier air aloft where the blue is on the chart. Drier air usually means colder air which would shift the right hand line on the skewt leftwards meaning air parcels can rise higher where there is cold dry air aloft.

Slight chance of a few thunderstorms tomorrow as upper level troughs (occluded front) move across the UK. Looks like very slack winds until you reach the very top of the clouds, so slow moving with heavy downpours might be the order of the day. Could be good for photographers. Very slight risk of spout development (very weak spin ups) due to convergence zones developing. Dry air aloft should enhance storm severity slightly. Too far out to put much more detail on things at the moment, but key risk might be localized flooding.

Primary risk looks to move from Cumbria Lancashire towards the North East when looking at the Radar. Modelling suggests that north east (hull north wards) mid afternoon may be at moderate risk of a severe thunderstorm. Backed low level winds suggest spout like weak tornadoes are possible here. One concern is that the CAP is barely broken until later in the day. Main risk is torrential downpours and lightning due to high cloud tops. The zero degree isotherm also looks a bit high for significant hail, but that is no guarantee. Secondary risk is across midlands to east Anglia perhaps a bit later in the day, but this requires a number of factors to come together which models find hard to predict under these circumstances. Net weather modelling suggest that some low level convergence will take place perhaps coinciding with the CAP beginning to relax a little late afternoon. When I saw this on the modelling yesterday I was expecting it to disappear, move significantly or be a lot weaker on the modelling today, but that does not appear to be the case. Once factor which has changes a little is the wind speed sheer through the mid levels which is a little lower making updraft and downdraft separation less likely. The slightly drier air coming in aloft is still there which is thought by some to affect any RFD (Rear Flank Downdraft - which is associated with tornado development). Vorticity charts suggest that if the instability is strong enough then these storms can create their own wind sheer environment. For every 10 times I have seen something like this in the modelling then 9 times it has not worked out as forecast. Limiting factors are that the convergence zone may not appear, the CAP may not break and the wind sheer profile is not really that conducive to severe storm development. Something to watch out for, but very much secondary to the storm potential in Scotland and the north east.

A couple of straggler storms for late tomorrow afternoon showing up in the modelling (Not reliable at this stage) due to low level wind convergence. Slight relaxing of the CAP could make it possible I suppose. High instability maybe just enough wind sheer for updraft and downdraft separation and drier air from 500hpa upwards could create some stronger downdrafts. Something perhaps to keep a very close eye on if they do materialize tomorrow (moderate severe storm risk) (One model run should not really be a basis for any sort of forecast) This is of course separate to overnight elevated storms and storms across northern parts of the UK and Wales throughout the day tomorrow (North East maybe in the firing line)

Bit of a Spanish plume type scenario Wednesday into Thursday. Current charts would suggest elevated storms coming in over Wednesday night which might become boundary layer active as they move out over the north sea. This is then followed by further storms become active from mid morning onwards, possibly with an MCS scenario. Typically these types of events do tend to move eastwards as we move closer to the event, but there is no guarantee. Dewpoint temperatures at 20 C seem a bit high for the UK and 18 C is usually where they top out. So its way to early to predict at this point.

Similar to yesterday with low level convergence zones. Particularly north midlands to east Anglia but also Exeter to Suffolk. Lightning wizard charts show a risk of spouts (very weak dust devil type funnels), but I am not sure updraft strengths will be strong enough. Very slack wind flow so storms could be very slow moving with a risk of localized flooding. Cloud tops may not be quite as high as recent days, but we might see more of a break in the clouds in places, so its swings and roundabouts. Key area for me is east Anglia and Lincolnshire.

Very similar modelling outlook to yesterday except NMM model is not suggesting anything unusual. Much more in the way of low level convergence zones suggested over the UK but not always in the regions of best instability. Southern areas are much closer to the jet aloft so deep layer wind shear is a little better here. Trawling through a number of forecast skewT's suggest a little low level veering of winds in places although there is not the marked difference between surface an cloud base wind speeds. More detailed modelling suggests perhaps a little strong winds at 925hpa so will be taking the forecast skewts with a pinch of salt. Mid level humidity charts clearly suggest storm development behind the clearing frontal system, however looking closely at satellite pictures (EUMetsat seems to have a problem this morning) there is a suggestion that breaks in the cloud will be more widespread than yesterday. Modelling also suggests storm development may continue into the evening a little longer than yesterday. Main risks again are localized flooding.

I usually look at surface winds for convergence. 925HPA which is often around cloud base height. 500hpa (mid level) for cloud steering direction. 300hpa upper winds for strong winds or divergence.

Models are not quite in alignment but looking at the Netweather charts then I think if I lived in the M4 corridor I would be a little bit concerned. From the low level wind charts I see wind convergence areas developing during the afternoon along the M4 corridor and from the Southampton area to London. From the mid level humidity charts I see a drier slot where storms can develop. Vertical velocity charts show strong updraft potential along the M4 corridor. Deep Layer sheer charts show the potential for a storm to develop its own circulating winds. There is also a touch of cloud base (925hpa winds) wind coming in behind. The trouble is that when a model pin points a location like it is and other models don't quite agree then there is a risk that either nothing will happen or events will shift a little further north or south. I guess it does depend on whether we get some cloud breaks. The main risk will be localised flooding , but strong updrafts (particularly through the zero degree isotherm) and high cloud tops mean there is a risk of electrical activity. Low level vorticity and strong updrafts mean there is a slight tornado risk as well. There is no strong support for rotating storms but there are hints that it may be possible. Certainly worth watching out for anyway even if it does not quite pan out the way some models might suggest.

Looking at the broad view of the forecast today then we have very moist air right the way up to the tropopause which should give us some storms embedded in grey murk moving from the south west across the UK during the afternoon and into the evening. The prime risk will be for local flash flooding. Weak winds through the atmosphere and relatively weak instability should limit the ability for severe storms to develop. However some of the detail charts have one or two things about them that make me feel a little nervous about suggesting severe storms will not develop today. I am looking at drier air coming aloft behind the band of storms with a touch of wind around the cloud tops. Cloud tops could be a little higher than we have seen over the week. Low level winds could pick up a little ahead of and behind the storms suggesting storms could develop their own low level wind field. Mid Level (500hpa) vorticity and wind field charts show some anomalies which might suggest storms will deviate a little from the expected. EuMetsat satellite show a band of vorticity coming in ahead of the drier air aloft. Some pretty low cloud bases forecast and looking out of the window low level cloud is scooting along quicker than I would have expected for the environment. So the prime risk is still flash flooding, but there is some potential for significant lightning and I would not rule out a localised marginally severe storm especially if cloud begins to break ahead of storms. It should be an interesting day to see how it all unfolds.