staplehurst

Been keeping an eye on Saturday night for a few days (let alone any other potential beyond that) but forecast profiles just don't look that unstable. The models naturally have subtly different thermodynamic profiles which is partly why they differ in placement on areas of showery rain - not unexpected given the complexity of the evolution but also how far out we are. There is some decent shear (indicative of the warm air advection regime), but the profiles just look too warm aloft to create any decent instability. Primarily skinny and/or restricted in depth CAPE with fairly moist profiles which never really bodes well for a prolific lightning display. If we could get the mid-level low over the Celtic Sea to shift further east and help cool the profile aloft that would help, but right now my gut feeling is most lightning will be on the French side and very little makes it into southern England. Still some showery rain from high-based convection though, some of this evaporating before reaching the ground. This is just Saturday night, beyond that there is less obvious upper forcing so we're relying more on subtle shortwaves around the upper ridge over Europe and so I suspect the detail for Sunday onwards will chop and change over the next couple of days as the models attempt to resolve these smaller scale features and their influence on the thermodynamic profiles.

East Anglia / Lincs was an interesting one today (mentioned it in the text discussion but too low confidence to add to map). If something could break the cap it could've been noteworthy, but the cap (so far) has been too strong. High-res guidance suggests showers and odd storms may continue to develop NE across the Midlands and perhaps NW / Cen N England through this evening, so we may have several hours yet with odd things popping off

I mentioned this to Lance the other day, but Sunday 3rd April 2016 is one of the earliest-in-the-year elevated thunderstorm events in recent times, with surface temperatures of 10-12C that evening under an easterly undercut - the warm air arriving from France riding over the "cold" surface air. The maximum temperatures of 17C in SE England and 20C in France earlier that afternoon are not too dissimilar to today's. Notice also the orientation of the lightning tracks (SSE-NNW) - not something you see very often in SE England!

Same frontal boundary stalled over eastern England on Sunday providing the focus for convergence given light surface winds. Less ridging aloft will allow deeper convection on Sunday, which with high precipitable water (~33mm), weak steering flow and potential for backbuilding along the same line could result in local flash flooding. There may also be some upscale growth as the front reinvigorates on Sunday night, which will just add further water on already locally flooded ground. Some high-res models have >60mm just on Sunday daytime, without taking into account further persistent rain during Sunday night (that's assuming the front remains in situ and doesn't wriggle about to the east etc). Very fine margins as to which areas receive the heavy rain.

New paper recently published in Weather, with free access: https://rmets.onlinelibrary.wiley.com/doi/abs/10.1002/wea.4057

The main risk for East Anglia is this evening/overnight as the upper low over Ireland slides across southern Britain. There is a small chance for the likes of Yorkshire this afternoon with some weak mid-level instability, but the main interest is the post-frontal environment as cold air aloft overspreads the existing humid airmass in place. I don't recall the Met Office issuing a thunderstorm warning for Thursday, unless I missed it?

Not expecting much in south Wales tonight to be honest. The 12z KNMI Harmonie has a reasonable handle on events at the moment which suggests this first cluster will rapidly weaken as it edges closer to the south coast. The 2nd line near the Channel Islands remains active for several more hours, but may also weaken (perhaps temporarily) as it nears the coast - especially the western end. There has been a broad consensus that this may reinvigorate as it moves inland during the early hours, and especially towards Kent and the Sussexes late in the night. Whether it pans out like that or not is another matter, as thunderstorms create their own environment regardless of what model guidance thinks will happen!

Yeah I was wondering the same thing, perhaps a couple of gravity waves due to the earlier/ongoing convection over the Brest peninsula. The ICON has consistently flagged this potential, but it seems 2-3 hours ahead of schedule which would suggest a landfall along the south coast around 6pm this evening. Very small cells with occasional lightning, probably quite shallow in depth and very high based - perhaps some nice castellanus with virga visible from afar?

Yeah, similar situation to southern England - sea breeze convergence and assuming dew points near 17C then an air temperature of around 27.5C or higher according to the 00z Castor Bay ascent. There's also a subtle shortwave trough aiding ascent a little more there too. The sounding gives over 1,200 J/kg CAPE, hence they appear to be quite electrically active, but very dry profiles keeping them spaced apart (for now, until potential daughter cells form etc)

Only if Worksop and Mansfield are in the SE Here's the rain accumulations from yesterday, to give an idea on the coverage of showers/thunderstorms

The 00z Herstmonceux would suggest 29C, 00z Nottingham convects at 27C but has a warm nose at ~790mb that would realistically need surface temperatures closer to 30C to overcome (or if there was strong forcing from below). Lack of upper level support today (unlike yesterday) means convective cloud will probably struggle away from any marked convergence zone to provide the necessary 'kick' from below. Convergence at the moment seems most pronounced over W Kent into East Sussex, and perhaps between Southampton and Bournemouth. Hopefully the sea breeze can do its thing in S England this afternoon (smaller risk in East Anglia), but overall I have lower expectations today than on Monday

12z Herstmonceux sounding modified for 28C air temp with a dew point of 18C. Profile suggests 28C is required for deep convection to occur from the surface, which we've now achieved across SE England (locally 30C in the London area). Aside from the moist layer between 780-850mb (probably due to continued attempts at convective cloud so far today helping to moisten the layers), the profile is very dry with evidence of the dry, sinking air aloft characteristic of high pressure. This will mean it's a bit of a tug of war going on in the vertical - air trying to rapidly rise from the surface, while also experience sinking aloft attempting to suppress it. So we are heavily reliant on strong forcing from the surface (sea breeze convergence aided with lift over the Downs etc) to get convective cloud to grow tall enough for heavy rain/lightning etc. Impressive SBCAPE of 1,600 J/kg, however this doesn't necessarily mean much if convective cloud is unable to use the whole profile (other than fast updraft speeds, at least initially). Evidence of some directional and speed shear, with 30kts 0-6km, which will help separate updrafts from downdrafts and enable cells to potentially last sometime - subject to them not being overcome by the very dry air aloft. Fairly slow storm motion of 12kts could result in some local flash flooding, and if cells can be sustained and utilise this magnitude of CAPE then some marginally-severe/large hail will be possible. High cloud bases due to the deep mixing of the boundary layer, with bases around 4,000ft, but the inverted-V profile beneath the cloud bases could allow some rather gusty winds to develop.

12z Camborne (Cornwall) ascent highlights the mid-level instability responsible for the elevated thunderstorms in the vicinity of Devon and Cornwall - convection initiating from around 1.5km (5,000ft) above the ground, but very moist profile below this level so areas of low cloud beneath the main convection. Just over 200 J/kg CAPE, much of this in the mixed phase region - ideal for lightning production. Overall the profile matches the 00z ECMWF run reasonably well, however it appears the 450-550mb layer is cooler in reality than modelled - the net result is steeper mid-level lapse rates and therefore a more favourable environment for thunderstorms. This has clearly been instrumental at increasing the amount of instability and therefore producing lightning more than perhaps initially expected.

12z Nottingham ascent slightly modified for conditions in East Anglia. Produces around 500 J/kg CAPE, with steep low-level lapse rates (rapid decrease in temperature with height). Reasonably fat CAPE below 700mb but becomes much skinnier above due to dry/warm layers aloft. Therefore scope for heavy showers, but little thunder due to being too shallow with much of the cloud layer above freezing. Winds are light throughout the cloud-bearing layer, with minimal shear resulting in slow-moving, pulse-type mode. Very moist through the cloud-bearing layer, which coupled with vorticity along convergence zones being stretched by updraughts has aided numerous funnel cloud reports so far.

It's only a bust if something was predicted in the first place The 09z Larkhill (Wiltshire) ascent highlights the profile over southern England fairly well, convection is generally capped around 500mb giving cloud top temperatures near -20C (locally colder where stronger convection can punch a little higher). Probably why the showers look very "blobby" - convection goes up, hits the "cap" and spreads out. Around 300-400 J/kg CAPE sampled, and given ~half of the cloud layer is above the freezing level still scope for the odd rumble of thunder, but nothing too significant other than numerous heavy showers.

Model sounding for 11pm BST in roughly that area from the 12z ECMWF - highlights 300 J/kg CAPE with parcels likely lifted from the 900mb level (notice no surface-based CAPE due to cooler temperatures down at sea level relative to aloft). CAPE is quite skinny but would give you cloud tops colder than -40C, and some decent cloud-layer shear, albeit unidirectional. Water vapour imagery also suggests this is forming on the leading edge of some very dry air associated with a PV anomaly, providing extra 'lift'. Several models have been toying with the idea with a period of uptick in activity over the Channel tonight, probably eventually merging with the mass of rain currently over the IoW and environs, but as to how long lightning lasts for is uncertain...

Just lacking in thunder - although we've said all along (see post from last night) that because the profiles are so moist this tends to reduce the lightning potential and you just end up with heavy downpours and very little lightning.

With each model run the progress northwards of rain has become slower, and as a result the area at risk is further south. Currently the winds are broadly NE'erly along the south coast, however over the next few hours convergence is expected to develop as coastal sites develop more of SE'ly onshore wind. We've seen some sunshine lifting temperatures to 20-21C, dewpoints are around 15-16C. Therefore, eyes on Dorset and Hampshire into the afternoon where we could see some surface-based developments, the focus then gradually shifting northwestwards through the afternoon. If storms can develop, as mentioned in previous post, scope for 70+mm very locally, these totals probably focussed on Wiltshire (but also perhaps extending into some adjacent counties) where training of multiple cells over the same areas could occur. Very much conditional on enough surface heating and sufficient convergence developing though!

Tomorrow exhibits some similarities to 16 August 2020, which produced some extreme rainfall totals in south Norfolk (240mm in 6-8 hours). There is a paper currently being published on the meteorology of this event. It seems likely a convergence zone may develop in the afternoon somewhere near or just north of the M4 across to Hertfordshire. North of this boundary, a brisk northeasterly wind will, in theory, allow a continuous supply of 'heated' air from East Anglia (where the best potential for cloud breaks is likely by midday and into the afternoon), and this will result in pooling of warm, moist air along this boundary during the afternoon. In @Paul Sherman speak, "spreads" could be 72F/61F over East Anglia. Winds aloft are fairly unidirectional (mostly from the ESE), and would in theory steer any storm cells to the WNW or NW, while inflow winds near ground level will be perpendicular (from the NE) and of similar speed to the storm motion. This will help restrain cold outflow / gust front from advancing too far away from the parent cell, and in theory aid the development of daughter cells in a similar location, such that *somewhere* could get multiple cells training over the same area. Profiles are very moist, with a substantial amount of the cloud below the freezing level, so these could deliver a LOT of rain over a period of a few hours. BUT this is highly dependent on many factors, namely the convergence zone developing in the first place and sufficient surface heating, both of which could be scuppered if there's too much cloud/rain or it advances too quickly northwards. We often see in the U.S. how the shape/progress of convergence boundaries can get distorted by rain-cooled air from earlier convection, etc. The main difference between this event and last August is the magnitude of instability is lower, and the window of potential shorter - so both will mean rain totals will be lower. Either way, currently keeping an eye on the south Midlands / M4 corridor for the risk of some fairly large rainfall totals (various NWP guidance suggests 50-70mm possible, KNMI Harmonie fancies near 100mm on some runs). Perhaps Bucks / Oxfordshire / Berkshire / N Wiltshire / E Glocs most at risk, but need to keep an eye on trends during Sunday to see if this potential still exists. This of course is not including the more widespread heavy rain that will be affecting other parts of Cen S / SW England. Note: I'm not necessarily promising loads of lightning because profiles will be very moist, but scope for some torrential downpours and flash flooding etc.

12z Herstmonceux sounding highlights just how saturated the lowest ~1.5km is, with some modest surface heating this is aiding lots of 'tropical downpours' that are capped around 850mb. Ideally need surface temperatures nearer 25C to be able to punch through this cap and therefore utilise substantial CAPE. However, fairly extensive cloud is limiting the degree of heating so far to nearer 20C. Profile also highlights some medium-level instability, so it is an interesting combination of both shallow surface-based showers (Kent/Surrey) and bits of showery rain emanating from medium-level cloud (e.g. Hampshire). The 06z AROME suggests the chance of some heavier/deeper showers in the next few hours from Hampshire through Surrey to London and Essex, seemingly tied to a shortwave currently moving into central southern England. I think overall though the risk of anything substantial developing is rather low, but non-zero. Otherwise, towards the end of the night parts of east Kent could see some continental exports drifting up from the south/southeast, but questionable as to how much lightning activity may reach this far.

the joys of a bifurcating flow!

Reminds me a little of Monday 24th June 2019... (when the storms kept veering more to the east than originally forecast)

Unmodified, the 12z Hertsmonceux (East Sussex) sounding reveals the shallow depth of elevated convection (left diagram), hence weak updrafts and very little lightning activity with a whopping 50 J/kg CAPE! Some pretty cloud bases, but not much else! Some very dry, deep surface air too so rain initially evaporating before reaching the ground (and hence radar over-reading in some places). The profile suggests, in the absence of any significant forcing or changes in the thermodynamic structure, you would need an air temperature of 29+C to be able to generate surface-based "airmass" convection. Fortunately we have some developing convergence (red-dashed line) between the hot, dry and rather breezy easterly wind across central/eastern Britain, and the cooler/moister lighter south/southeasterly winds over central southern England. Currently dew point temperatures are around 12-13C in the hot air ahead of this mess of elevated convection. Underneath and behind this showery rain, dew points are higher (but air temperatures lower) courtesy of rain moistening the lower levels. With any luck, as this boundary continues to shift NE through the afternoon and early evening, this convergence may become more pronounced and will aid in pooling of moisture along the boundary. Assuming low-level convergence is sufficient to create forced ascent and we maintain 26°C temperatures (as per Hertstmoncuex), we would need to increase the dew point to at least 16°C to make air parcels buoyant (given the deep mixed layer). 2m temperatures will naturally reduce through the evening (and also as cloud continues to advance from the southwest), so lowering the air temp to 24°C you would need a dew point closer to 20°C. Some model output suggests 19°C dew points may be achievable by the evening, and we have to consider the Herstmonceux profile won't be identical across the whole of England/Wales so the profile may be slightly more favourable further northwest (i.e. Midlands) than across SE England where these measurements are made. Either way, it is very finely-balanced as to whether sufficient deep surface-based convection can initiate this evening along the convergence zone later, roughly near the M40 by this stage. Initiation aside, speed shear is weak today (Herstmonceux reveals 10kts storm layer shear) but there is some considerable veering of winds with height which will allow any surface-based updrafts to rotate but essentially any surface-based storms may struggle to remain sustained for any prolonged period of time.

If you ever wanted an example of how difficult elevated thunderstorm forecasting is, here's the past 4 runs of the EURO4. Hence it is near impossible to say if X location will see a thunderstorm in this setup, realistically you have to treat it fairly broad brush and not take an individual model/run literally because just a subtle difference aloft can make a big difference as to whether a thunderstorm develops or not...

Not a great deal to add to be honest, all going roughly to expectations - small cells firing over the western English Channel and it'll just be a case of watching radar developments through the remainder of the night. Some earlier high-res model runs were suggesting elevated thunderstorms not far from The Lizard/Lands End by midnight-1am, more recent runs have knocked this back to 3am (ish) but realistically this is a post-midnight event, and more especially the latter part of the night for Cornwall/Scilly, and perhaps eventually Devon at the very end of the night.