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staplehurst

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  1. 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.
  2. New paper recently published in Weather, with free access: https://rmets.onlinelibrary.wiley.com/doi/abs/10.1002/wea.4057
  3. 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?
  4. 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!
  5. 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?
  6. 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)
  7. 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
  8. 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
  9. 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.
  10. 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.
  11. 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.
  12. 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.
  13. 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...
  14. 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.
  15. 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!
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