Storms and Convective discussion - May 2024

[Eagle Eye]

Convective Outlook️ 

Overnight on Wednesday, severe thunderstorms look fairly likely to hit parts of the south coast bringing the risk of frequent lightning, low-end severe hail and some flash flooding risk. Before that, some daytime showers are possible across England and Ireland with 200+ J/KG of SBCAPE possible. The daytime risk looks fairly limited on the lightning front without much energy but a few lines of showers may grow strong in Ireland whilst lacking in much lightning threat along a PV lobe. Forced ascent into clusters along convergence zones in Ireland and less organised showers across the midlands ahead of and connecting to the approaching frontal system. Generally weakening into the evening as surface energy weakens. Then the main risk is the likely linear system evolving overnight. Initially forming in a large theta-E pool east (ahead in this scenario, or rather being upglided next to the warm front) of the warm front, perhaps connecting into the warm front itself. That may weaken the western portions of the system, but only over time and unlikely to make it's effect too much before the coast. Most of the energy is associated with the eastern extent of the system where the severe risk is as well, with development into the system acting as momentum inducers. Also, bringing the risk of elevated Supercellular development given hooked deep-layer shear and strong energy. If so , then the severe hail risk is very much there because of the increasing severity of updrafts associated if a Supercell does form, as they take up more energy into the storm than non-Supercelular storms do, and given the buoyancy available, the threat for 1-1.25 inch hail is there despite not particularly amazing background signals such as the mediocre lapse-rates and the strong low-level shear. With the MCS itself, winds are unlikely to be severe but 200+ J/KG of DCAPE may risk some strong wind gusts but it's not particularly looking severe on the models. However the below cloud winds do suggest that they will take away somewhat from the hail risk. Initial development of the MCS should take place in the evening over upper central portions of France, the exact positioning still contested about. With surface storms evolving into an elevated MCS as night falls and they enter 1000+ J/KG of MLCAPE associated with the intensifying trough and moisture convergence towards the MCS as an inversion undercuts the surface-850's portion of the system and the moist but cooling air beyond the inversion takes advantage of the ejecting Theta-W plane upgliding along the warm front. Dry air entrainment into the mid-levels means that the saturation weakens past the surface towards 50-70% which is more than enough for frequent lightning to take place as the saturation signal doesn't interfere with it.

Dry air forces itself behind the MCS leading to moisture diverging away from the surroundings and converging into a tight band wrapping up into the MCS itself and increasing the 850's lift and ascent occurs rapidly as this occurs in the evening or overnight depending on speed, near the northern France coast. This is where severe weather is likely to be at its strongest but some severe weather may linger into southern England as I said earlier.

Exact alignment is still very poorly modelled, however, the steering currents tend to align with any Belgian or northern France developments hitting the southern coasts, with a more Belgian development perhaps having more energy to feed from but will move more WNW rather than the NW of northern French development, but that's generally the difference between the SE and southern parts of east Anglia being hit full on or relying on new storms forming further east in the channel which has been a scenario on a lot of models, likely because of the sheer amount of energy eventually out-strengthening  the convective inhibition on some models, but we have decided not to extend the moderate into the SE, because of the large model split over Belgium MCS development or relying on further channel development.

Convective height looks unlikely to be an issue with 10-12km of cloud height development and given the previously mentioned kinematics such as forcing and deep-layer shear, it may become even taller. With most unstable ascent being -8 LI's on some soundings. Very rare for the UK and hence the severe potential. 

As it hits the south coast, there is a pool of 100+ J/KG of 3CAPE extending across most of the south coast and that could well force surface storms ahead or within the system itself and an increase in strength again. Very interesting surface conditions with sea level pressure falling ahead of the trough of itself forcing that surface energy, which whilst it may take up some of the MCS's energy, this is a different pool of energy and rather the MCS is moving with it's separate energy pool so I find that an unlikely bust scenario. However, very limiting lapse-rates means that the lift is suppressing convection in this area and I don't see it overcoming the surface inversion.

There are some models that are still limited in it's width development andor whether it can properly cross the channel itself. Hence why it's currently a moderate risk and a rather small one at that based off not particularly expected strength as most places hit by this, especially on the eastern extent in the south and southeast should get frequent lightning, but rather the most likely place to be hit.

Given the moisture that is available, there is risk of torrential rain. and hence the flash flooding risk. Some of that may evaporative with the distance between surface and cloud base and limit that to another low-end severe risk though. The main risk is frequent lightning currently.

The general movement of this appears to be along the south coast across all the way to Wales by 6 am on most models. However, weakening as they do. There may be a few re strengthening attempts in local areas based off kinematics in certain areas but that's very difficult to forecast so a slight generally covers that.   

That'll do for now. A very weird scenario as I keep saying. This is only my best attempt at a discussion. 

Edited April 30 by Eagle Eye

[Jamie M]

AROME with a casual 2200 J/kg over Ramsgate 

[LightningLover]

 Jamie M Should *just* be enough for a rumble or two!

[Kirkcaldy Weather]

Continues to be an interesting evolution with this event, I discussed in detail in the post below 

 

As mentioned there were many discrepancies between models on said evolution and indeed its not 100% resolved however as things look currently a few details to update and an overall summary from my POV

The main cyclonic Centre is now modelled to develop from Northern Africa (Algeria)  into the Balearic Sea wheras prior outputs developed it into the Tyrrhenian as discussed in my post above.

So looking at the period of May 1st through May 3rd from a significant rainfall and associated significant flash flood threat POV, Italy and France seeing the majority however the rainfall activity which extends from Southern into more Central and Northern France will progress into the UK with current signal being the most significant accumulations and flash flood threat being focused into Southwestern England extending toward Southeast + Southern Ireland.

GFS and some of the finer scale modelling is indicating this threat area might include Central South England into Southern Wales.

Cheers, KW

[Alderc 2.0]

So let me get this right, massive instability, a trigger, all over southern England, at night in early May all while I’m in a hospital bed overlooking the English Channel? What could go wrong? 

[viking_smb]

 Alderc 2.0

 

That trough over eastern England has disappeared on the latest fax 

[Southern Storm]

 Alderc 2.0 sorry to hear that Alderc, they'll be more opportunities I'm sure! I'll be out with the camera so I'll try to record something, but hopefully you get to see something from your hospital bed. Wishing you a speedy recovery 

[Eagle Eye]

 viking_smb trough or not the PV lobe forms all the same. Mostly Theta-E driven risk essentially. The trough is a byproduct of that. 

P. S

Sorry to hear that @Alderc 2.0

Edited April 30 by Eagle Eye

[Eagle Eye]

52 minutes ago, Eagle Eye said:

Convective Outlook️ 

Fairly aligned with my groups convective Outlook earlier as well, our severe threshold is based off UK not European standards. 

[Mapantz]

That level 2 will suck the life out of anything advancing in this direction.  

[Eagle Eye]

 Mapantz  It's a different system. Ours comes from the combination of the remaining energy of that system 'combining' with the developing energy over Benelux. So sort of a different system. Although for you the AROME might slightly be too north with the energy this still demonstrates my point. 

[viking_smb]

 Alderc 2.0  get better soon m8

[East_England_Stormchaser91]

If I get time tomorrow evening/night, I may head south and chase. Somewhere like Stansted might be a good starting point. Storm season is back! 

[Eagle Eye]

Another scenario from the southern forming MCS is a split as the UKV demonstrates. Arguably better potential for me but I feel bad for whoever house it splits over if it does that. This definitely feels possible.

 

[Josh Rubio]

Am I right that it’s the showers which will form NE of the main band of rain which is of interest? 

[CoventryWeather]

 Eagle Eye still no shower development before the main feature. 

I would have thought there would be storm development from sunset, easily across the Belgium and French coasts. Feel there could be a fair bit missing from modelling. 

Unless there's dry air or something interfering with that in the NE? 

[Eagle Eye]

 CoventryWeather

Tonight's risk has been downgraded but if you're talking about tomorrow. Lapse-rates ahead of the MCS are very positive (so very poor for storm formation) and lift isn't there because of that. Limited depth of CAPE beyond the surface inversion means it's really unlikely to be anything before the MCS much at the moment.

@Josh Rubio In a way yeah, they form parallel next to the warm front. But a long time after it first hits the south as the theta-E trails slightly behind. So the front should hit in the afternoon, then the storms hopefully late at night or early morning.

Edited April 30 by Eagle Eye

[Thunderspotter]

 Alderc 2.0 sorry to hear this, hope everything is well and you get better soon! 

[Eagle Eye]

AROME is a bit messy but I'm personally hit full on so this potential outcome is interesting to me. 

The UKV appears to be beginning to go the same way but with more convection ahead.

 

 

 

[viking_smb]

ECM timings on windy look like I will be hit from about 8am tomorrow

[LetItSnow]

feels like it’s been ages since a proper thundery attack from the south / southeast. i don’t know what the odds will be here in central london of getting anything direct. it’ll be a night of model watching no doubt. 

[GreyCrag]

It sure is a tricky one to chase tonight as this morning's model runs have shifted things a bit south again, with the UKV now having a few home-grown storms breaking out over Reading at 2am to add to the complexity. The local METAR forecasts in the south aren't looking too promising with low-cloud bases (base level at 300ft with 1.8mi vis at Brize Norton for example) as the MSC complex becomes embedded within the warm-front.

It's looking less likely i'll be chasing now based on the visibility forecast, but if things change then it's 'only' a 4hr drive 

[Eagle Eye]

UKV 03Z now has a firm trend just slightly west of the AROME. Models perhaps beginning to try and converge on a scenario. Home grown storms are now being modelled with a PV band ahead of the MCS. Also a bit earlier on the latest UKV compared to others. 

[Ben Sainsbury]

It appears to me, the models have been trending in a good direction. I wasn’t originally overly optimistic about tonight, but that has all but changed.

[ChannelThunder]

On an office day in Winchester so I'll be quiet in here 'til later on! If I took my car over for these days I'd have been tempted to stay over on the mainland to open up chase opportunities for the night, alas I'll be back home hoping the storms come to me!

It does increasingly look like the most lightning activity will be over Kent, Sussex and then northwestwards of there, I think I'll be on the edge of that and could just get stuck with heavy rain .