BrickFielder

When you are looking at thunderstorm potential then there are a number of things you can do to judge whether it is likely to occur. The first is that all models are consistent in showing thunderstorm potential. Models are in reasonable agreement that there is potential for a thundery out break in the UK Tomorrow.The next thing to look at is modelling of the jet stream and whether it is consistent with the EUMetSat real time images. Here it is hard to judge but it looks like the southward jet for the trough behind the low pressure out in the Atlantic is a little weak and there is a follow on weak low pressure system coming in behind it. This suggests to me that the timing of any thunderstorms could be inaccurate in the modelling. Once I am happy that there is some potential I start to look at where the winds at different levels are coming from. Tomorrow it looks like low level winds will be sourced from deep in Europe which gives very hot but not that moist air. Any modelling which suggests dew points above 15 Degrees Centigrade is likely to be suspect. At the 700Hpa level then winds are coming up from the Spain and Portugal area. For the south east this looks very much like a Spanish plume which is lifted over the mountains of Spain and Portugal. Towards the south west and the winds just skip past the mountains of Portugal. This makes a difference whether during the day storms are likely to be surface based or elevated. Upper winds are coming from a more south west direction. These winds from different directions add up to give wind shear or a spinning effect on storms. Having said that directional shear is limited in the mid to upper levels so only surface based storms should really be affected. Looking at upper vorticity charts then there are suggestions of two troughs ahead of the frontal system (South West to North East and across the south east) and localized storms creating their out localized vorticity environment.One of the things we should point out that the parameters for hail for Tuesday are significant. The limiting factor with many scenarios like this is cloudiness dampening instability and messy convection so that discreet cells do not develop (MCS). Upper cloudiness might be an issue but the general messy clouds from french storms is unlikely to be a factor. Putting it all together then I would say it is too early to say (You half expected that didn't you) as I have some concerns about timing. I am reasonably happy that we should see elevated storms across the UK with significant lightning with almost any part of the UK at risk with storms trundling northwards during the night (Different models show different areas most affected). There might be a short window for surfaced based convection affecting south western parts in to the midlands, but at the moment this is marginal (i.e. very much linked to timing concerns) and my hunch is that we get mostly elevated storms. It does need close monitoring as bringing things 6 hours forward would bring us into supercell and tornado risk scenarios. Lightning, localized flooding and hail are the prime risks and we should remember even at this stage it could all move 500 miles east or west.

Quite a serious risk of a severe storm for parts of Ireland today with low level convergence and significant wind shear. Elsewhere cloud cover and lack of instability will most likely limit things. I am however watching a lobe of vorticity running along the front and approaching from the south west. This could affect Wales later on. The complicated frontal pattern it is tricky to call. We should probably keep an eye on Cumbria and the Scotish borders as storms over Ireland drift that way later. on. For tomorrow then there is a suggested risk on the modelling for convergence zones to set up in bands across the country. Winds will be uni directional and instability not that strong. Something to watch out for tomorrow.

Fairly good instability over parts of Scotland and Ireland today with some low level convergence in places. Winds up through the atmosphere are not that strong so we should expect slow moving storms with a risk of localized flooding. Key area is likely to be north east Scotland. Slight risk of weak spout type tornadoes where low level convergence occurs. Tomorrow is tricky to forecast at the moment. Looking at forecast SkewT's there is a short window 3pm to 5pm for Ireland and parts of Wales and the midlands. I am not entirely sure whether the instability is post frontal or on the trailing edge of the front. What we can see is potential for very high cloud tops and significant wind shear, however instability looks very weak. Perhaps more significantly there is suggestion for high cloud which could limit surface temperatures. It will probably need a closer look and assessment tomorrow morning. Saturday gives potential instability for central and eastern parts of the UK. Here cloud tops are a bit more limited but instability is stronger. Wind shear is rather limited and uni directional meaning slower moving pulse type storms leading to localized flooding. The key area to watch at the moment looks like being the south east. Timings can change so I wont go into any more detail at the moment.

Conditions could be marginally convective for parts of the Midlands northwards tomorrow if modelling is to be believed. Forecast SkewT's suggest some high cloud so it could go either way. Low level convergence and moisture pooling combined with a potential trough in the mid to upper layers likely to play a part. Slow moving convection fed with moist air from the south can be a good combination. Looking at the EUmetsat images I am not convinced the modelling is correct though. Off the coast of Spain is a disturbance where there should really be high pressure. The low pressure towards Iceland has marked vorticity maxima to the south of it. This means it can actually go either way and needs to monitored closely. Here I would be worried about pulse storms developing localized characteristics due to weak warm air feeds from the south.

Albermarle and Castor bay soundings from overnight show weak instability will be available due to daytime heating. I can also see marked upper level divergence across north east Scotland during the Afternoon. Low level convergence occurs along parts of the east coast as well. This would suggest storms are possible today across eastern Scotland during the afternoon. These are likely to be slow moving and due to weak low level winds could give rise to a few weak spouts. Along the east coast of England and particularly for areas from Hull, through Lincoln down to Lowestoft could give rise to localized storms. Confidence is a bit low today in any forecast due to weak instability (the slightest deviation in instability could affect things greatly). The jet stream to me looks a little further south than forecast although it is supposed to move a bit northwards (not convinced by this). Modelling also suggests forked vorticity advection which looking at EUmetsat images seems questionable. Forecast SkewT's show a mid level cap which relaxes southwards as the day goes on which you would expect mid level cloud to slowly clear southwards (Not convinced by this). This means thunder storms should not totally be ruled out elsewhere than the highlighted areas and forecast storms may equally fail to materialize. Scotland is where the signals for thunderstorms are best and least subject to changes.

Tricky to forecast tomorrow at the moment. Chance of surface based convection from north midlands northwards with storm potential greatest in Scotland. Modelling currently shows limited cloud tops, but that does depend on upper level temperatures being estimated correctly from satellite imagery. If upper levels are just a bit cooler then you will get thunderstorms in the north. Triggering is likely to be occluded fronts by the looks of it.

In theory looking looking at the modelling then there is a small window between 6pm and 9pm over north east scotland where storms can marginally become surface based looking at the forecast skewt's. Equally cumbria , northumberland and many parts of Ireland and scotland could see elevated storms. The trouble is that the jetstream over Poland is further east than forecast and satellite imagery shows the current cold front weakening with a further front developing behind it. I am also not sure how quickly the low pressure lifts out northwards or whether it heads more north east. As modelling starts to catch up with actual conditions I would not be surprised to see some thunderstorm potential developing tomorrow. As for today then I am just nt convinced, but we should be looking at north east scotland as a potential focus point of storms early evening.

I am watching Saturday closely for the potential for elevated storms. Looking at the Forecast SkewT's then we can see a huge CAP preventing boundary convection, but above that there is some potential for convection to initiate (The instability looks to be mostly elevated rather than surface based). What I am looking for is signs of a 700hPa Jet ahead of the front which could initiate storms. Upper level instability more often that not is not released without a significant trigger and I am just not really seeing it at the moment. If some troughs occur ahead of the cold front (fax chart -- Northern areas) then there might be some possibilities. It should be noted that the CPC blocking charts are somewhat different to what modelling solutions are suggesting so confidence that models have a proper handle on the evolution synoptic conditions is somewhat limited. Best wait till closer to the time.

If sky's remain a little clearer over the east and depending on which forecast model you look at then there is a chance of storms for East Anglia around midday. I have a suspicion that the clearer air will not remain however. Most forecast SkewT's show moist upper layers which will mean high cloud cover so I am not convinced of any day time storms today apart from Ireland. What I do notice from the Satellite charts is marked divergence aloft over northern Spain at the moment. This is forecast to move northwards which could affect the prospects for overnight elevated storms. Positioning of overnight storms tonight are all over the place and the timing is not much better. Depending on timing storms could root into the boundary just after dawn possibly over East Anglia. Mid Level Lapse rates over the UK are of particular concern because they are not really conducive to storm formation (Tricky to assess because elevated mixed layers suggest lower lapse rates than they really are) It might be better to watch the satellite imagery and review the midday soundings rather than look at forecast models in my view for today.

Risks of elevated storms tomorrow night as well. Looking at forecast skewT's surface based storms during tomorrow daytime are not out of the question either. The moist upper layers mean that cloud cover is likely to affect convection tomorrow as well, but it pretty touch and go whether storms develop.

The dawn chorus this morning was fairly clear which is often a sign that the air is calm and fairly moist. I did wonder whether surface based storms would develop today but the lapse rates did not look promising and the cloud cover limited temperatures. During tonight an area of increased lapse rates moves in across the western half of the UK. This I think could trigger one or two elevated storms. Here I am looking at an area from Bristol across eastern wales and across parts of the west midlands. There is a remote chance that tail end storms could reach down to the boundary layer. I am also looking at Lincolnshire and northwards up the east coast.

Still a bit early to be looking at too much detail for Sunday into Monday, but there are a few points which we can think about ahead of a more detailed look. The first thing to look at is where the low level moisture and heat is coming from for the UK. It looks to me like the low level air is coming from deep within the continent over Germany. This direction of wind is not known for particularly moist air, although with the amount of thunderstorms recently in this region the moisture should be there. The next thing to look at is the mid level lapse rates which really don't really start to increase until Sunday evening and into the night. This means convection will struggle to reach the heights needed for a thunderstorm until the best daytime heating is over. Looking at the forecast SkewT's for that period then I can see an elevated mixed layer towards the east of the Country which only just reaches the UK after convection can be rooted in the boundary (Nice lightning show but limited thunderstorm severity). Further west shows a bit more of a maritime air flow but is more saturated all the way up (Messy convection without discreet storms, limiting storm severity). Wind sheer looks to be significant Sunday evening , so super cell development is possible ,but without convection rooted in the boundary layer you wont really have an updraft region again limiting thunderstorm severity. Conditions for thunderstorms on Monday look better with steeper lapse rates, and convection rooted in the boundary layer, but wind sheer has begun to drop away significantly. What this shows is that timing is very critical to the severity of storms and small changes in the speed of the approaching low pressure and jet streaks make a big difference. This is why looking at high thunderstorm indications three days before hand is often considered Fantasy Land forecasting.If past modelling performance is any guide then we should expect convective potential to move a little eastwards and speed up, which would probably increase the severe thunderstorm risk but move it eastwards. Current modelling suggest we will get thunderstorms but it might be a close miss for a more severe thunderstorm (OK maybe there is a insy winsy tiny window)(Dew points above 16C are usually questionable for the UK )

Storms did eventually fire over north France yesterday late afternoon which gave some impressive lightning displays for some south east coast areas , but storms struggled to reach down into the boundary. Pretty close miss for severe storms in the south east last night, but still gave plenty of rainfall and a good lightning show for some. Today some showers across parts of Wales and the midlands as skies begin to clear and low level convergence zones set up. A warm nose at 550Hpa is likely to limit convection unless temperatures rise significantly. Best risk for thunderstorms is in the south east where another convergence zone sets up late afternoon. Although it does not show up well on the GFS derived forecast SkewT's there is some marked turning in the winds from the surface upwards. The best 700hPa winds do not overlay the convergence zone very well but residual low level moisture could spark some storms. Confidence on this is fairly low at the moment in this due to cloud cover conditions.

I looked out the window this morning and saw high level cloud cover and birds circling on thermals and thought if the sun breaks through then convection could initiate today. Then I looked at the modelling,Satellite images an Forecast SkewT's and thought maybe not. Looking at the Satellite Imagery gives a bit of a different picture to what I was Expecting. It looks like the surface low over the north of France is beginning to decay with not much convection along the trailing front across France. What is left is not much more than a trough at the triple point which has given some heavy rain across the channel islands. This I think can re invigorate a little as it crosses the south east tip of the UK during the afternoon. The wind dynamics are good enough for a moderate storm, but I rather think temperatures will mean storms will struggle to root into the boundary. You can also see the warm air convection beginning to stall across Spain on the Eumetsat real time images. The second low which is still of the coast of North Spain and has much better upper air dynamics is beginning to fill. This has developed an area of MCS at the junction of the fronts out to Sea. This is forecast to lift out and cross the south east of the UK in the early hours of tomorrow morning (I think it might be a bit later looking at the Satellite imagery). We do have an elevated mixed layer across the UK and France which is making it rather humid but I think the low will have a mix of maritime air in it as it crosses the UK (EML. Still across France). So the south east might see some elevated storms over night but it is looking less certain in my view, however there is a chance of some moderately severe storms for the south east corner during the morning tomorrow depending on timing. Other southern parts of the UK may see some outflow dominant stormy rainfall (Just wet not really electrical) as well. Still not convinced I understand the dynamics here and will wait and see.

When looking at the convective prospects for tomorrow and into Wednesday we may need to think about two separate thunderstorm potential conditions, neither of which is fully nailed down by the models yet. Broadly speaking we have a low pressure system to the west of Portugal which has quite a sharp vorticity maximum associated with it. This is causing warm air to be moved northwards and up over the Iberian peninsular (North Spain). The key thing to watch is how much convection takes place over Northern Spain today (just starting to occur) as this will give some indications as to which modeled solutions are correct. It is always worth remembering that typically modelling moves things eastwards a day before any potential and then slightly westwards on the day. However the vorticity embedded in the low of Portugal is more than Forecast so all of the modelling might be a bit off (Looks like it could give some pretty windy conditions as well if it does not slacken off as forecast). The modelling suggests tomorrow the warm air destabilizes as cooler air from the trough start to over run it. This should occur out over the Bay of Biscay and then significantly over France. In effect a surface low develops ahead of the storm out towards Portugal as thunderstorms build over France. For the UK the suggestion is that destabilization occurs in the Bay of Biscay around Midday with convection build as this area comes on shore. Instability might be weak during this period but wind shear profiles look fairly potent. Later on during the night we have elevated storms (Not surface based) moving into the south east corner of the UK. Instability is likely to be high and tops should reach a decent height so we could be in for quite a lightning show. Elevated storms do not always produce lots of rain but in this case the moisture profile suggests some significant down pours. The trouble is that at the moment I am not convinced by any of the modelling. The storm to the west of Portugal looks more potent than thought, time for the warm air to rise over North Spain is relatively short and discrepancies in model output mean any forecast is relatively worthless in my view at this point.

Moderate chance of thunderstorms today depending on cloud cover with a low possibility of a more severe storm. I am particularly looking at parts of the midlands across to Humberside. A low level convergence zone from the south west through to east Anglia should also produce some convective activity. I did think that the trigger for convective activity in the north midlands was low level trough, but having had a closer look it might be a mid level dry slot, which shows up on the forecast skewT. Satellite imagery confirms there is a drier slot but it might move through a bit quicker than forecast. With a jetstreak overhead updraft and downdraft separation will likely lead to longer lived storms and with slight wind veer at low levels and speed wind shear through the upper levels then storms could become quite tilted. This means localized flooding is a real possibility. Worth keeping a close eye on especially for low topped supercells towards the north east.

Modelling today shows very little convective potential above mid levels, so not much chance of storms today. What is interesting is that the modelling might be a little late over the continent. Looking at the EUMETSAT real time images I can see a pronounced jetstreak with a exit region developing a surface low over Belgium and Holland. In theory this area moves over Denmark heading for Norway and Sweden, but considering the Satellite images it might move a little more out into the north sea. The raises the possibility of a little more cloud clearance over the UK. All forecast SkewT's show a marked warming above 700Hpa to 500Hpa which will CAP convection. If I take the Herstmonceux sounding from last night and raise the surface temperature to 16 Degrees you can get significant convection. That however will not be representative of the air mass later today as warming aloft and increasing cap are forecast. Here I would be looking at a short window of time on the southern boundary of the occluded front where low level convergence occurs. So realistically there are no thunderstorms today, but depending on how things develop I would not totally rule out a weak thunderstorm especially if cloud clears. Probability of a thunderstorm today is less than 5 percent, but keep watching.

An upper level trough extends down across the UK and parts of western Europe with a surface low over Northern France. Circling around this trough are a number of jetstreaks with associated left exit regions where winds diverge aloft. As the JetStreak over the continent moves north the main exit region moves north with a surface low developing over north west Europe and the north sea. A smaller jetstreak appears across the central part of the UK for a short time this afternoon with its associated left exit. Similar to yesterday low level convergence zones will develop across the south east moving north westwards slightly during the afternoon. Along these areas weak thunderstorms are likely to develop which are likely to be slow moving and lead to localized flooding. Winds are fairly lite so pulse storms are likely. Forecast SkewT's show a moderate cloud top heights and a moist profile making heavy rainfall likely. Not quite sure of the impact of the short lived left exit region for the jetstreak across the UK this afternoon. My thinking is that the divergence aloft could give some isolated over shooting tops. Even moister air profiles across central England could then give rise to some very heavy rainfall with some weak embedded thunderstorms.

I have been looking at the Satellite images this morning to get some clarity on the weather effects today. What I have been struck by is the amount of spin on the wrapped occlusion at the center of the low at the moment. The way it spins almost makes me think of a tropical storm, except I know there is no warm core. It could give some pretty strong winds along the south coast and in to the channel which may not be fully on peoples radar. The caveat on this is that the low should start to fill and possibly split at lower levels. In terms of convective activity then I am looking at a number of potential areas. Firstly the wrapped occlusion and here I am looking at the south east and East Anglia. Next I am looking at some post frontal surface trough denoted by some wind convergence in models and here I am looking at Midlands to East Anglia moving northwards. Thirdly I am looking at the potential (Not sure about this looking at the satellite imagery) new surface low over Wales which could develop its own wrapped occlusion. Fourthly a potential surface trough between the Wales surface low and the main center of low pressure over the south east. Which of these actually happen is a tricky call. I think we can say that Wales looks like getting some pretty heavy and consistent rain. The wrapped occlusion over the south east is likely to initiate new convection as a little sunshine will have been present and surface moisture will be available (London Maybe). Cloud Tops could be quite high but the front clears fairly late in the day for full daytime heating to take effect. Wind speed Shear is mixed, but I think the chances of significant severe thunderstorms is low. The spin in the Satellite imagery still bothers me as the jet stream left exit does not really move on until late afternoon.

Tomorrow (Friday) Still trying to get a grip on what exactly is the scenario tomorrow even the Met Office guys trying to do the fax charts seem a little unsure. At lower level it appears that a second low pressure develops within the original low pressure system whilst at upper levels it remains a single low. According to the models at the moment there is a risk of thunderstorms after the main rain band moves through with key areas being the south coast through the midlands and up into the north of England in the evening. According to forecast SkewT's quite high cloud tops are possible although they may be limited by surface temperatures (i.e. breaks in the cloud are pretty essential).Untill I can get a real grip on the scenario I will not venture any further into analysis as it seems possible that convective developments might be further east than forecast at the moment.

Today The upper level trough still stretches across Wales, the central UK and Northern England.Where low level wind convergence takes place then some convection could initiate. Models don't exactly agree where that convergence is, but seem to point to parts of Wales and the Liverpool Manchester area (Perhaps parts of the Midlands). Cloud Tops and wind sheer are limited so we should not expect any severe storms today. Perhaps an isolated rumble of thunder although this is based on temperatures around 15C which I think is on the low side.

My concerns about yesterdays convective potential when looking at the satellite images were warranted. I thought it might be due to the warm nose at 650hPa being stronger than forecast but analysis from actual soundings suggest that upper level lapse rates were not as steep as forecast.Weather models are initialized with a mixture of sounding (actual) data and derived from satellite data (especially out to sea). It seems to me the satellite data analysis out over the Bay of Biscay estimated upper level temperatures to be lower than they actually were and underestimated moisture at certain levels. Today's potential for convective activity is very limited. We have some potential in central Scotland, Southern Ireland and maybe the south west of England with an outflow dominant frontal band clipping the east coast later. Thursday we have low pressure near northern Ireland with a potential trough swinging across from north wales across to Humberside. Here we need to bear in mind potential satellite imagery analysis bias , cloud cover again and low surface temperatures.It will need more assessment tomorrow especially since there is disagreement in the modelling. Friday we have a developing low pressure system pushing up from the south in the Bay of Biscay bringing with it a lot of rain and potentially some embedded storms. It is also worth keeping an eye on any post cold front troughing, the wrapped occlusion and the triple point. The low pressure looks slightly tilted and modelling suggests a slight split at lower levels as it crosses the UK. We should bear in mind low level lapse rate steepness is poor and it is just the type of scenario models will struggle with so not worth looking into any detail yet.

It is a bit tricky to forecast convection today and even the Estofex forecaster is hedging his bets. What has me most concerned is the satellite imagery and cloud cover as it does not appear to be exactly conforming to modelling. What we can see is that there is an upper level trough approaching from the west reducing temperatures at upper levels. Upper levels also do not appear to be as moist as modelling suggested yesterday which should tend towards more discreet developments rather than an MCS (Big stormy mess). From low level wind modelling we can see areas where low level winds converge and veer. This is likely to be a focus for convective initiation. Forecast SkewT's show a marked warm nose at around 650hPa which begins to relax as the trough moves in. This should mean mid level cloudiness should begin to clear and allow temperatures to rise. Also shown is some marked wind and speed sheer in the lowest level of the atmosphere. This can mean any strong updraft could potentially be rolled and twisted upwards for a short lived tornado. Directional sheer through the mid and upper levels is not that great so super cell development is low risk (There might just be enough for a low topped one). Speed sheer in the upper levels is moderate which means storms can tilt a little making them more likely to be long lived. Being realistic then updrafts may not be that strong, cloud cover is likely to limit convection, cloud tops are likely to be limited and the timing of the correct conditions for convective initiation may not match up. Areas to watch according to the models are Wales with perhaps some embedded storms along the front, perhaps the south east for a continental clipper and the midlands moving north. I would probably identify Oxford from mid day moving northwards through the east and west midlands mid afternoon and trundling on into Humberside and parts of the north later. Due to Satellite imagery concerns, my confidence in any storms is a bit low, but the conditions from modelling would suggest a possibility of a moderately severe storm through the midlands with perhaps a weak short lived tornado. I guess storm chasers might head towards somewhere like Leicester although there is every chance today would be a bust. Its still interesting to make guesses from the modelling and from here on in it is just a case of watching to see.

Timing of how quickly the upper level trough moves in tomorrow differs in the different models, but there is a low level chance of storms tomorrow. For me there are two potential areas of interest. Firstly parts of East Anglia where low level convergence sets up and moisture gets trapped on the warm side of a developing cold front. Secondly along or just ahead of the cold front there is a potential for an MCS with moist profiles. Here I would be looking closely at models for low level jets and considering where the leading edge of any MCS would be. Both scenarios have some low potential for an isolated moderately severe storm, although we should bear in mind potential cloud cover and temperatures.Primary risk is likely to be heavy rainfall moving in from the west and anything else needs to be reviewed early tomorrow when models begin to agree.

It is a bit tricky forecasting convective activity today and I am struggling a little bit to point to specific areas to watch for storms. The forecast charts show marked low level convergence zones from North Yorkshire to London and along the south coast and parts of Wales. Looking at the forecast SkewT's for London then it becomes clear that weak mid level lapse rates are likely to limit cloud tops. Looking at the forecast SkewT's for Cardiff then mid level lapse rates are even worse, but you can definitely see some drier air moving in to the upper mid levels. Looking at the west midlands SkewT's then you can see similar problems with a cap at about 550hPa as that drier air starts to move in aloft. Lightning Wizard cloud top charts suggest perhaps an area of extended cloud tops as that drier air aloft starts to move in. What I am having particular difficulty with is steerage winds and although they are generally fairly light there appears to be a marked southward steerage across the midlands this morning below 650hPa. This is comparison to a generally steerage from the south west above that with very little movement below 700hPa apart from the midlands. This suggests to me that a strong convergence zone will setup along the south coast later. Key area in my opinion is likely to be along the south coast between Southampton and Brighton from mid after noon. My confidence is not very high and weak mid level lapse rates are likely to prevent storms.