[Storms and Convective discussion - 1st August 2022 onwards]

Upper low near south Wales will slowly trundle across southern parts of England enhancing mid level lapse rates and instability as cooler air overrides moist surface air. Satellite imagery suggests a vorticity lobe coming in to the south of the upper low which may begin to wrap slightly around the upper low. Approaching vorticity is a good trigger for storms and may indicate a little more wind shear than currently forecast.

 Forecast SkewTs suggest a moist profile all the way up to the tropopause which tends to lead to messy convection and weak storms. Wind shear is forecast (need to monitor this) to be low and this would tend to lead to pulse like storms which tend to have limited intensity. Locally veering low level winds could lead to spout type very weak tornadoes but weak updraft strengths may limit this.

 Storms possible today and generally speaking the biggest risk is localised flooding due to slow moving storms. Risk of severe storms looks low but the vorticity pattern needs watching. 

[Storms and Convective discussion - 1st August 2022 onwards]

Nothing really to add to what others have said apart from the risk of over shooting tops in the south east in the early hours (increased lightning risk).

Precipitation patterns similar to yesterdays suggestions but a little delayed. Localized flooding risk for the south east possible looking at the modelling. Satellite imagery currently consistent with vorticity waves being ejected northwards from the cut off low near Spain. First indicators likely to be on the Spain France border early evening.

Current suggested precipitation patterns.

 

[Storms and Convective discussion - 1st August 2022 onwards]

Thursday early hours is a different prospect in terms of storms compared to Wednesday afternoon. Not sure that I would look at the early Thursday morning risk as your typical Kent clipper.  The atmosphere looks moist all the way up and it is not clear that instability will be surface based. It has more of a nocturnal squall line look of it (wind convergence at the 700hpa level).

Risk of lightning , heavy downpours and even gusty winds, but possibly limited in severity due to being elevated.  So we have an non event Wednesday afternoon that might surprise us and a Thursday morning Kent clipper which isn't which also may surprise for different reasons (localized flooding). I guess I need a few more model runs to get a better handle on things.

[Storms and Convective discussion - 1st August 2022 onwards]

If only Wednesday conditions were a month ago then we might be looking forward to some stormy weather just south of the front in the afternoon. What we have is cloud clearing south of the front giving showery weather and rising temperatures.

We have low level wind convergence and moisture pooling raising dewpoints.

A little instability is forecast and the models suggest some convection is possible.

Wind speed shear is present and hints of low level wind veering, which would give some ingredients towards low topped super cells.

The problem is that it is not June or early July and temperatures look like they need to be up close to 30C for cloud tops to get above mid levels and a slight shower.

If the models are a little bit out with there satellite imagery mapping then a surprise storm might be possible. Most likely this is another near miss but worth keeping an eye on the models in case they change.

[Storms and Convective discussion - 1st August 2022 onwards]

I was looking at a few parameters and thought storms might be possible today. Forecast models though are having non of it.

The first thing I noticed was a thread of colder air moving across southern England. Colder air high up usually increases lapse rates and instability.

It does show up as a band of vorticity on satellite images but is not as marked as the above charts suggest. The next thing I noticed was low level wind convergence zones which should pool surface moisture.

Yet forecast models do not think there is much chance of storms. Even low resolution models are not keen.

So it is back to forecast SkewTs for some clues.

So forecast SkewTs suggest a strong CAP due to a dry warm layer which prevents air rising beyond that layer.

 

There some unanswered questions for me with that forecast SkewT. The first is temperatures of 18C and dewpoints of 10C when they are already 20C and 14C. Where is that sliver of very cold air aloft. Is the CAP consistent with overnight weather soundings.

On balance I think the forecast is probably correct but it looks a lot closer than you might expect. Temperatures hitting 25C might just. spring a surprise storm.

 

 

 

[Storms and Convective discussion - 1st August 2022 onwards]

Usual warnings on this information that I simplify things and don't exactly follow the rules so this will be a flavour of things rather than some of the very good technical explanations on the internet from professional meteorologists (My interpretation should be taken with a pinch of salt). Maybe somebody will do a proper pinned post of this but in the meantime check Nick's very good pinned guide to thunderstorms in the UK which covers many of the ideas which about SkewTs.

SkewT Diagrams are usually seen with respect to soundings. Here a weather balloon is released to rise through the atmosphere and the temperature, dewpoint, wind speed and wind direction are recorded at different levels as the balloon rises and then plotted on the SkewT diagram. From this information you get a snapshot of the air conditions and this is useful for predicting weather conditions downwind. For example the Nottingham soundings are sometimes useful for weather prediction later in the Netherlands or Germany. The only upstream sounding for the UK is Castor Bay in Ireland which tends to be useful for Scotland and Norway. UK weather models rely heavily on Satellite monitoring which give a good idea of the air conditions upwind but has some limitations. What I tend to have a quick look at is the forecast (predicted) SkewTs and because they are not based on real measured conditions I often take certain liberties just to get a rough feel of conditions. 
 The types of things I can see from the diagrams is how humid air (dewpoint too close to temperature at any height may mean messy convection), is convection possible, is there cooler air aloft making things more unstable, is there wind shear which could enhance storms or make them long lived, is there CIN or a CAP stopping convection and how low cloud bases are and how high cloud tops are. Looking at a forecast SkewT I have numbered some features and lines and will try to explain what they are and how I interpret things (shortcuts included) 

1. Winds going up through the atmosphere giving direction and strength (number of barbs). The more winds turn with height and more speed difference there is between the surface and upper levels (Wind shear) the more severe a storm might be.
2. Isotherm lines (blue) and saturation mixing ratio lines (yellow dotted) are used to calculate the lcl and ccl(cloud base). Everywhere along an isotherm has the same temperature.
3. Saturated adiabatics (green dashed lines) (dry adiabatics are the red lines but I don't really use them but should). As moist air rises it condenses out into cloud and becomes cooler. If the air is still warmer than the surrounding air after the cooling it will continue to rise. So this line is the rate at which air cools as it goes up and produces cloud.
4. The actual or forecast temperature of air at a particular height above the ground. If the temperature drops rapidly (lapse rate) between the surface and high up at the tropopause due to cooler air aloft and drops quicker than the saturated adiabatic rate then air will rise and produce cloud (convection) up to a point where warmer air is encountered. There other mechanisms for example fronts that produce cloud so it is not the complete story.
5. Dewpoint temperature which is a measure of humidity of the air. If the dewpoint and temperature are very close together then humidity is likely close to 100% and cloud will form at that level which might affect temperatures at the surface and lapse rates. Humidity at the surface will tend to dictate cloud base levels and whether any moisture is available to form clouds.
6. This is a line I add myself to guess at the cloud base level. It should be parallel to the saturation mixing ratio but I tend to use the isotherm line. To do this properly you should draw a line from the surface temperature parallel to the dry adiabatic (red lines) and draw a line from the surface dew point parallel to the saturated adiabatic (yellow dashed lines). This is the place where a surface parcel of air will begin condense out into cloud provided the meeting point of the two lines is above the temperature line.(This is where I take a bit of a rough shortcut)
7. This a line also added by myself which estimates the level of free convection (cloud top height). This should be a curved line from the intersection point determined by 6 (cloud base level) following the saturated adiabatic until it intersects with the temperature line. The larger the area between this line and the temperature line the more unstable the atmosphere is.
8. Things like elevated convection (The lines I have drawn don't work) and other scenarios may show up here. The higher the cape number the more unstable and stronger storms will happen. Li represents the lapse rate and the higher it is the stronger storms will be. CIN is a rating of prevention of convection and the higher the number the less likely storms will occur. Pressure 1000mb tells you convection should be from the surface and not elevated.

Strictly speaking the forecast SkewT shown may not lead to a storm due to the CIN level, but considering it is estimated from satellite images certain liberties when taken in conjunction with precipitation charts don't seem unreasonable.

OK lets test out the theory on a forecast SkewT from tomorrow. (I will try do it properly for once - if I am interpreting the lines correctly)

I can see the dewpoint line close to the temperature line at higher levels. This suggests convection might be embedded or very messy. There are very strong winds up near the tropopause which would suggest a jet stream (high fast winds) and cloud tops being spread quickly at higher levels. Wind speed and direction differences are not in the key areas to affect storm development much (lower and mid levels are more important). CIN of zero shows no prevention of convection and the 200 of Cape suggest weak instability and hence weak storms. 

Looking at precipitation charts I would take that SkewT with a pinch of salt. Forecast SkewTs can be useful but need to be looked at in conjunction with other charts and the latest satellite imagery. Looks like multiple low level convergence zones occurring  which will tend to cause moisture pooling. Moisture pooling will increase the surface dewpoint and alter any forecast SkewT which does not take that into account. Wales looks like a good spot for storms again tomorrow but any cloud clearance may spark storms so keep an eye on the radar.

 

Hopefully this might start to clear a little fog around SkewT's but it is not easy to explain and after a while it becomes instinctive for some storm enthusiasts and you begin to take liberties.

 

[Storms and Convective discussion - 1st August 2022 onwards]

I see a few discussions about supercells in the last day and thought a simple explanation might help. If winds change direction as you go up in the atmosphere then clouds will have a tendency to twist as air rises. If winds at a low level are light and winds increase with height then louds will tilt along with the higher winds aloft. Tilting will separate updraft and downdraft regions  and the twisting will rotate the cloud giving a supercell. If winds at the surface are very light and higher at the cloud base level  then winds will tend to have a rolling barrel effect below the cloud. This rolling or horizontal vorticity can be picked up by a twisting updraft region to form a tornado. This is very rare in the UK but not so rare in Europe hence why Estofex alerts tend to happen more in Europe.

As for today then the models would suggest storms for the west , east and north but not central areas.

Upper level cooling is not really there like yesterday  for Northern parts so maybe not as high a risk of severe storms although I notice some model anomalies.

 

Looking at Satellite images then I see a frontal system moving eastward across parts of the west with embedded storms. That front seems to be moving faster than forecast so I am not totally convinced by modelling of where and when storms will occur. 

 

 

Tomorrow is a different scenario with a frontal system turning  thundery across Wales and central areas through the afternoon. Upper level cooling is better but the atmosphere profile has a tendency to be moist all the way up to the troposphere so there is a risk of very messy convection.

Think radar watching might be a better option at this point.

 

[Storms and Convective discussion - 1st August 2022 onwards]

From Sunday onwards the jetstream is forecast to dip south across the UK giving a long wave trough and colder air aloft.

Typically when hot air from the continent gets replaced by maritime moist air and cooler air aloft spreads over the UK we get a thundery breakdown. There are worrying signs that the low wave trough becomes a cut off low over Spain and it becomes business as usual high pressure over the UK.

Even with the long wave trough modelling suggest high pressure at the surface lingers over the south with easterly winds still prevailing until we get a rapid switch to Northerly winds.

 

Forecast models do at the moment give a brief window where winds are from the south west (maritime moist air) but there is little time for surface dewpoints and humidity to rise in some southern areas.

There are sings that some areas will not get a thundery breakdown and little sign of rain for the immediate future. Dewpoints of below 10C and low humidity are not good for thunderstorms.

 

The problem with the modelling is that it maybe trying to handle a transition and holding on to the existing pattern too much. The confidence in what will happen Monday and Tuesday will be low at this point in my opinion so keep an open mind and don't be surprised by last minute changes in the forecasts.

 

[Storms and Convective discussion - 1st May 2022 onwards]

The shape of the upper trough is more elongated than I was expecting and moving north east across the UK slowly during the day.

 

Forecast SkewT's suggest very weak instability but if skies clear a little then warming temperatures could initiate convection. The wind field looks very slack with very little wind shear although trough movement suggests some upper level wind flow. Low level turning winds could support weak spout like funnels.

I guess focus might be low level wind convergence zones overlapping some clearing skies.

 

 

Very marginal scenario but most models seem keen on breaking out a little convective rain. The more marked upper trough is interesting though.

[Storms and Convective discussion - 1st May 2022 onwards]

The front currently over the South west is bringing in moisture at upper levels. This will lead to some upper level clouds and storms as it slowly sweeps west to east during the day. It should be noted that the boundary layer (surface) is very dry and there will be significant resistance to convective initiation (CIN). What is not perhaps picked up by the models well is the possible second frontal boundary which seems likely to play its part tomorrow as upper level cooler air crosses the southern half of the UK.

By early evening upper level frontal cloud band should reach the midlands and London areas and considering cloud top heights may give a lightning show.

 

The UKV Model also seems to like initiating convection along a low level convergence zone.

 

This is just one model but it does hint at a possibly of a cap going quite explosively. along that zone.

 

Since this is just a hint on one model we should not get too excited and even here the convection is likely to be elevated. Not convinced (overnight soundings were not that positive) but worth keeping an eye on.

 

[Storms and Convective discussion - 1st May 2022 onwards]

Watching the radar with animation on shows some cloud moving east and some north. Looks almost as if there is a surface low crossing the southern part of England. Further north looks like a convergence zone or developing front which looks slow moving apart from spreading northwards.

 

Maybe it is just my imagination. It could give a hint as to where storms will be later though.

Happy watching.

[Storms and Convective discussion - 1st May 2022 onwards]

We have a long wave trough dipping down across the UK which is bringing colder air loft. This will mean steepening temperature lapse rates increasing instability (Cape) but also bring down the tropopause (cloud top height).

Low wind speed and low level wind convergence along with orographic lifting (BWE possible) are thought likely triggers for convection.

Low cloud bases and slack turning surface winds could lead to very slow moving winds and very weak spout like tornadoes.

Models seem to give a signal for continued precipitation and instability into the night for some central areas.

Having looked at the Cambourne sounding from last night I am not totally convinced upper levels will give the forecast instability. Satellite imagery also suggests the long wave trough is more vertically stacked that modelling suggests (stretches further East). There is also a clear vorticity lobe out over the Bay of Biscay which I think will tend to force instability ahead of it.

 

Best guess is that many Eastern and Northern areas might see storms where modelling is not so keen. Very moist profiles and approaching vorticity from the south could lead to an MCS (Depends on whether vorticity begins to lift out or trundles further into France) .  Highest risk is localised flooding due to slow moving storms and due to steepness of temperature lapse rates through the zero degree isotherm some larger hail.  Storm training (In lines) could mean some places get repeated soaking and other places get nothing. Yet again not convinced by what models suggest in terms of coverage and placing of storms.

 

[Storms and Convective discussion - 1st May 2022 onwards]

Chance of storms across Northern Ireland with perhaps a low top supercell risk today. Biggest risk might be storm training and repeated heavy rainfall.

Overnight an occluded front moves across the UK from the west. A second front follows on behind which may turn convective in the early hours tomorrow. The suggestion at the moment is that storms do not develop.

That second front seems to fizzle out and we get some convection in the form of swift moving pop corn. Forecast SkewTs show no storms tomorrow but it looks marginally possible (models don't need to be far wrong to get storms).

Next up is Wednesday night when there is the possibility of severe storms (overhead jet) developing in France moving up across the South East.

Long wave trough is properly over the UK with cool air aloft on Thursday. Low winds speed and localised wind convergence means some potent slow moving storms might develop. Central South and Manchester Northwards look most at risk.

Still early days and there is a good chance parts of the UK will not see any storms.

 

 

[Storms and Convective discussion - 1st May 2022 onwards]

Forecast models are not that keen on storms developing today apart from across Ireland towards the center of the low pressure. Models show very weak cape (instability) and a marked cap on convection.

I am not convinced about Dewpoints being 4C but the forecast is consistent with the Herstmonceau sounding from last night apart from the cap being a little lower.

The Cambourne sounding from last night though is different being more unstable and without the dry air in the mid levels.

It is difficult to tell which is most representative of conditions 12 hours later but my hunch would be the Cambourne sounding likely shows conditions just ahead of the front. 

We also have a trough over the Bay of Biscay which is trundling around the low. Forecasts suggest this stops its forward motion and slowly moves off into France. At the very least a I would expect the approaching vorticity to spark heavy showers over Northern France which will be steered northward to southern counties of the UK.

I guess storms are possible in an area just ahead of the front and along southern counties based on some of these observations. Second guessing models is always fraught with danger though.

 

Modelling is really not that keen on storms apart from some embedded in the front over the borders and north wales. Northern Ireland also at risk early on before the low begins to lift out. Maybe someone will get a surprise storm so keep tuned to the radar.

[Storms and Convective discussion - 1st May 2022 onwards]

I would not expect wide spread thunderstorms today. Generally speaking we have the jet stream with dry warmer air roaring over head. Even if clouds managed to get up that high the jet stream would shred the cloud top. We do however have steep mid level lapse rates and plenty of wind speed shear. That means strong updrafts and updraft downdraft separation.

 

This looks like very squally heavy showers. If there is low level wind convergence then there is a slight risk a more potent heavy shower.

 

Even though we do not have much risk of storms there is a very slight risk of weak tornadoes due to strong updrafts and wind speed  shear. High cloud bases does mean it is very slight. Over all not really much expectation for storms.

 

 

[Storms and Convective discussion - 1st May 2022 onwards]

Not exactly sure where low level wind convergence will take place ,but it looks like it will play a part.

Other factors coming into play are cloud cover damping things down.

Temperature and dew point will be key and I guess late afternoon early evening upper level cooling.

Models favour Wales, Manchester and Liverpool areas but I would not rule out the south coast, London and East Anglia.

Possible elevated storms over night.

Looking at current conditions I can see Dew points above 15C in East Anglia while they are still down at 11C in parts of the West. Just a hunch but I would not rule out storms in the East.

Best forecasting now will be looking out the window  and watching the radar.

 

[Storms and Convective discussion - 1st May 2022 onwards]

15 minutes ago, viking_smb said:

Hi brickfielder 

bit puzzled with some of the wording 

'but with the upper low having been cut off from the jetstream so long maybe a lot of the energy (cool upper air has warmed'   What does that mean please? 

cheers

rob 

For convection to take place you need warm moist air at the surface and colder air aloft. The temperature gradient between lower and higher levels needs to be high enough to make a parcel of air to rise ( temperature gradient steeper than adiabatic ). That parcel of air needs to have enough moisture so that it condenses water out as cloud as it rises.  Normally for storms to develop you also need a trigger or focus as well (low wind convergence, band of upper cooler or drier air, approaching vorticity or low pressure, broken cap).

The jet stream is a band of high level winds and separates cooler air to the north and warmer air to the south. It undulates and loops and can create cut off pools of cooler air.

 

These cut off pools of cooler air at high levels air generally associated with low pressure and thunder storm systems.  Recently we have had a pool of cooler upper level air off the coast of Portugal and the Bay of Biscay which has been creating storms over Spain , France and Germany. That pool of cut off colder air was cut off in a jet stream undulating loop over a fortnight ago. This means that upper cooler air has been warming over that time. 

 

These charts show the air at upper levels. You can see the very cold air in the recently cut off pool of cold upper air over Eastern Europe along with the less cool air moving up from the Bay of Biscay overnight as the cold pool aloft lifts out north over the UK tomorrow.  Going back to the temperature difference between the surface and upper level you perhaps can see that instability (Rising air due to temperature difference) is going to be less for an older cold pool of upper air.

 

I hope this helps a little or at least points you in directions where you can investigate further. It is quite complicated and I have somewhat simplified the explanation even if it does not seem so.  Maybe the UK thunderstorm guide can help as well. All questions are welcome even if I cannot answer them or miss the question at busy times.

[Storms and Convective discussion - 1st May 2022 onwards]

Not much changed from the scenario suggested yesterday for storms tomorrow. Predicting low level wind convergence and timing is a bit tricky. Models some what differ and do not seem that keen on strong surface based storms.

Cloud tops could be quite high but with the upper low having been cut off from the jetstream so long maybe a lot of the energy (cool upper air has warmed).

It is important to point out that there is a chance of further elevated storms overnight. Modelling suggests these might be stronger than the late afternoon storms.

I have kept quiet about Friday so far but it looks like we have an upper trough leading to thunder storms traversing the UK from South West to North East (including East Yorkshire).

Modelling suggest Wales might be a place of storm focus late afternoon tomorrow. Overnight then Southern England and Central areas look in the firing line. Tomorrow looks more like a Midlands northwards event. Some modelling really is not that keen on storms and with dewpoint around 12C max then you can perhaps understand the reluctance. My best guess is that there will be storms as this is a scenario modelling has got wrong before. 

[Storms and Convective discussion - 1st May 2022 onwards]

The bay of Biscay, Spanish low is just about running out of steam, but will be slightly enhanced by a small low feature which has dived south past the west of Ireland.

This combined low will become cut off from the main jet stream and high pressure will ridge across the UK given stable conditions on Wednesday.

On Thursday a long wave trough approaches the UK from the west as the cut of low over the bay of Biscay and Spain lifts out northwards. The result is cooler air aloft with plume humidity spreading northwards over the UK during the day. Aloft some jet stream exit wind divergence could act to enhance convection.

There are some features on Satellite imagery which seem questionably handled by the modelling.

 First the merging of the low diving south with the Spanish low is an unusual scenario. At the least I would expect the low diving south to have enough energy left to take the Spanish low a bit more southwards. The next feature which gets sort of ignored is the disturbance in the Azores high which has the look of a tropical storm. It can fade away as modelled, but at the least it is the sort of feature you expect to see when the Strong Azores high is about to move southwards. The strong long wave trough pushing east behind the feature would suggest it will no longer be stationary and might affect any ridging. I also note the disturbance over North Africa which over time will begin to affect the equatorial Atlantic. In other words there is scope for weather forecast modelling to be wrong.

What we have on Thursday is maybe low level wind convergence with cooling and more humid air aloft. Wind shear both directional and speed wise is fairly low giving conditions conducive to pulse short lived storms.

The trouble with making assumptions on rough parameters is that they can ignore micro scale conditions. Low level winds from the south west colliding with south west winds would tend to give back building (training) storms. Cooler moist air meeting hot dry at a convergence zone will tend to move the updraft region eastwards and make the west of the storm more outflow dominant. That outflow wind will tend to increase low level winds from the south west. Updrafts are likely to be strong at this time of year, but updraft downdraft separation is unlikely to happen due to wind shear limiting storm longevity. This does not take into account updraft regions dying and being replaced with new ones. Storm steerage direction, low level wind humidity and directions can work at a micro scale level to create surprisingly severe storms where you might not expect them.  

Storm potential could disappear and details will not be clear yet, but it seems like a reasonable chance of a few potent storms. Worth getting a little bit excited and a little ramp even if I have perhaps over egged it a little.

 

Hope you enjoyed the discussion even if you need to take it with a pinch of salt. 

[Storms and Convective discussion - 1st May 2022 onwards]

So we have a Spanish plume which is being undercut by colder air. This is acting to lift up the plume air and increase the humidity at upper levels. This is not convective but may lead to deep cloud. Look at these two forecast Skewt's and you will see what I mean.

So the developing cold front across the UK will mostly be elevated and not convective, but the potential depth of cloud may give it some storm like aspects. 

Separate to this Saturday evening may see the plume destabilise over the south east of the UK. Evening cooling aloft may lead to destabilisation with elevated electrical storms developing.

 

I still cannot get over the drop in day time temperatures without a proper storm.

 

Looking at Satellite imagery suggest the cut off low off the coast of Portugal is lifting out northwards a little earlier and the front is a little further north than modelling suggests at the moment. This in my opinion increases the risk of storms for the south east. Rainfall across central areas looks as if it might be considerable so worth keeping in mind.

[Storms and Convective discussion - 1st May 2022 onwards]

Looks like a typical Spanish plume which gets undercut and lifted by cooler air coming south across the UK. This would typically give elevated storms.

 

There are however concerns and anomalies that make forecasting this far out unreliable. The first thing to note is that temperatures are likely to drop from daytime of 34C Friday  to 24C  Saturday and  this would suggest a further cold front boundary over France. Modelling does not seem to like destabilisation of the atmosphere over France which baring a trigger looks like it should. Looking at low level winds I can see convergence but even south of that zone surface winds are southwards. Surface conditions do suggest moisture pooling on the humidity and 0-1km mixing ratio charts.  So we have precipitation forecast Saturday possibly linked to surface conditions , yet we have forecast profiles that suggest an upper destabilizing plume.  In other words detail models are picking up some localised conditions that are difficult to understand. Modelling tomorrow may give a better idea and at this point I would be reluctant to associate the precipitation with storms at all. 

[Storms and Convective discussion - 1st May 2022 onwards]

Upper level vorticity lobe slipping down across Ireland into Wales and part of England. This increases mid level lapse rates and moves slowly east across the UK.

Low wind convergence is likely to initiate convection with generally unidirectional winds going upwards you should expect pulse storms which may back build if the convergence does not move. Instability does not look that impressive although under estimated surface temperatures may skew that a little. Ireland is probably the place most at risk from a more severe storm but Scotland and the north east look to be at risk of localised flooding.

Updrafts seem unlikely to be impressive tomorrow but there are hints of a low level jet and drier air aloft. Drier air loft tends to mean storms are isolated and the low level jet gives low level vorticity which is a possible ingredient for weak tornadoes( but not if updrafts are weak ). Probably needs a closer look tomorrow.

[Storms and Convective discussion - 1st May 2022 onwards]

I am keeping an eye on Tuesday for convectional potential even though at the moment the risk is low. During the day the UK falls under a left Jetstreak exit and upper trough. Slightly higher dew points and temperatures with slightly cooler air aloft could give storms.

 

Wednesday we have potential through a possible post frontal trough. Again the parameters for storms are not lined up but with a few changes to the forecast storms could be possible.

Temperatures are still on the low side (25C +) for good surface based storm development, but worth keeping an eye on.

[Storms and Convective discussion - 1st May 2022 onwards]

Tricky to forecast exactly what will happen and complicated conflicting indicators. There are a number of typical thunderstorm scenarios for the UK but todays has elements of typical scenarios yet is slightly different. Looking for upper air and mid layer air divergence can be a good sign of thunderstorms and frontal genesis. Whilst this is evident it is unlikely to be the main driver of storms today.

Most likely we are going to see surface based storms developing over parts of France late afternoon and these will trundle North eastwards and perhaps the south east of the UK. Temperatures in the UK will not be supportive of surface based storms generally and imported storms are likely to have anvils streaming ahead over the UK. 

There is another scenario at play where the plume destabilises as evening cooling aloft starts with initiation over Normandy and the channel islands and perhaps even the south of the UK. At some point storms are likely to be outflow dominant and less electrified (Just. heavy rain for some).

Most likely parts of the south will see storms and lightning tonight with further risks of convective activity tomorrow. A lot depends on what happens over France so the best forecasting is probably by radar and satellite watching.

[Storms and Convective discussion - 1st May 2022 onwards]

Thunderstorm risk again today with tops being a little higher than yesterday but still limited. Upper vorticity and cooler air increases mid level lapse rates making the atmosphere weakly unstable. Low surface temperatures somewhat limit instability but breaks in the cloud late afternoon could easily spark up a storm.

Limited depth low should give slack winds over much of the UK leading to low level wind convergence areas which could spark convection.  Backed low level winds and cooling upper layers late afternoon could lead to spout type weak tornadoes in East Anglia with Lincolnshire and Norfolk being areas of interest.  Area of interest possibly stretching to East Yorkshire and London as well. 

 

Satellite imagery suggests some cloudiness might be late to clear and surface low might be a little late traversing the UK so exact timing and positioning will be subject to change.