staplehurst
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Posts posted by staplehurst
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9 minutes ago, fine wine said:
Heavy snow with huge flakes in north Essex. Totally unexpected
Hi-res models have been flagging the potential for some sleet/wet snow late in the night/first thing this morning since Tuesday - light winds and heavy showery precip allowing some evaporative cooling. Very marginal when you look at the temp/dewp spreads.
Yesterday's 06z KNMI HARMONIE, as an example...
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Here's the longer-term trends in UK summers and winters, the dashed-black line is the running 30-year average
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5 hours ago, Froze were the Days said:
2 days in 1987 with ice days recorded!?!...that can't be right.
Late to the party, catching up on posts - the year relates to when the winter started (so for Jan 1987 you'd be looking at 1986). This winter (up to 11th Feb) is listed as 2020 etc...
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17 minutes ago, minus10 said:
Can anyone tell me why after a lovely cold day with a lot of snow about the temp has struggled to fall this evening in wgc. With all the lying snow i would have thought it would have dropped like a stone. Just now it was barely -1degrees?
It's been quite breezy today, and I suspect that is helping to keep the air mixed for the time being. The wind should gradually ease through the night, so temperatures may drop quicker later.
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Right now the occlusion appears to be about 2 hours faster than most high-res models, and a tad further north - 12z EC seems to have a reasonable handle on its current position, which if that were to verify would pivot somewhere just south of Birmingham with ~7cm there, also means a much shorter-duration event for areas near/south of the M4 and hence smaller amounts. Would in theory bring more of East Anglia into play, but equally an easterly onshore wind would probably keep a lot of the precipitation along/east of the A140 and A12 as rain/sleet.
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2 minutes ago, WheresTheSnow said:
How reliable is the model forecasting this? Rain for me on that one
it's just the 06z KNMI HARMONIE, chosen mainly because it had a reasonable handle on the likely northern extent in East Anglia when the tweet was created this morning. We'd always suggest never to take any one model literally, but it offers one potential outcome. The turquoise blue colour is 'sleet'.
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You can see the difference between today and tomorrow looking at forecast soundings and the changes in the structure of the air above the ground as the front occludes and the main thrust of milder air aloft slides away to the south. Note the surface temperatures are very similar on both days east of the warm front (i.e. low single figures).
Reasonably deep layer of air >0C today, meaning snow falling aloft melts to rain on the way down. By tomorrow, the air will gradually cool aloft, allowing snow to survive to close to ground level - but still melting right near ground level. Hence if you're on a hill (i.e. 200m in this example) you have a much better chance of seeing snow, and if precipitation gets heavy enough through evaporative cooling then that'll bring the chance of some wet snow down to lower levels.
Probably going to struggle to settle given the already wet ground, but could see a slight covering on hills in particular, more especially north of London where air cools aloft earlier while the frontal band is still active before it decays through the evening and clears southwards.
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Just a quick summary of the global models from their 00z runs on Monday, for the 24hr accumulations 06z Wed to 06z Thu. Clearly most handled the event pretty well, with the ECMWF having a bit of a wobble. Yes some chopping and changing occasionally brought the precipitation slightly further north/south with each run, but the multi-model consensus was pretty clear - with only a very low chance of anything happening north of the M4...
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The 00z KNMI-HARMONIE has a reasonable handle on the northern extent of the precipitation at the moment, which produces up to 10cm of snow over Dartmoor, and even 2cm over parts of Dorset. Locally 1cm over the Downs and High Weald, but for most barely a dusting.
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1 minute ago, Spikey M said:
I'm a complete amateur so forgive a potentially stupid question;
Low pressure obviously = wind, so presumably even if the rain breaks up it's going to get pretty blowy?
Depends on the pressure gradient - if the low is fairly slack, then light and variable winds cause areas of convergence that helps aid showers/thunderstorms to develop (assuming other ingredients are in place). If there is a tight pressure gradient, then you'll get strong winds. Potential for gusts 40-50mph along the south coast this evening on the south side of this low, but in general strong winds inland are not really expected tonight (aside from the convective gust component associated with thunderstorms)
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8 minutes ago, Spikey M said:
Probably not the best source, but Weather.com has it weakening and breaking up. Where have you seen this?
Various model output suggests the surface low will slowly deepen over the coming 12 hours, for example the 12z ARPEGE has central pressure 1001mb at 18z, falling to 1000mb at 21z, 998mb at 00z, 997mb at 03z and 996mb at 06z
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Just now, Zak M said:
I don't think so, looks more like a mesocyclone/rotating updraft, but this is 2020. It could be anything!
This is the most recent chart:
There is a surface low in the vicinity of Southampton at the moment (~1001mb), tracking northeast behind the main convection and is expected to slowly deepen as it moves towards East Anglia this evening and tonight - primarily driven by divergence aloft.
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Reports of three fallen trees around Holton Heath causing extensive delays on the A351 between Wareham and the A35 roundabout. Also flooding and another fallen tree on the A31 near East End causing long delays.
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16 hours ago, Weather-history said:
So what triggered off the storms over the North Sea?
It was night, so no diurnal convection. It is not like winter with a deep cold airmass over a "warm" North Sea. It wasn't topography.
It was essentially the edge of a plume being drawn northwards from western Europe across the North Sea towards Scandinavia, becoming engaged by a pronounced shortwave swinging northwards (that also aided the increase in lightning activity near Wales/SW England too). The models had been playing with the idea of elevated thunderstorms erupting close to the East Anglia coast as early as Monday from memory, so not bad for a Day 4 forecast.
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22 hours ago, staplehurst said:There appears to be a small low centre over N / NW Kent (Medway vicinity) judging by surface observations. Will be interesting to see what happens when this thunderstorm drifts westwards over / near this feature - the added low-level vorticity may become ingested into the cloud base to bring the risk of a funnel...
We have a candidate!
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Annoyingly the Environment Agency rain gauge in South Hornchurch hasn't reported for a couple of hours, but radar-estimates would suggest >80mm over Rainham/south Hornchurch and a PWS rain gauge (so, unofficial) on Netatmo in Rainham has recorded 101mm so far today...
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34 minutes ago, Mitch the motorbike storm said:
Kent clipper
That'll just be one run of the EURO4 (00z by the looks of it)
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There appears to be a small low centre over N / NW Kent (Medway vicinity) judging by surface observations. Will be interesting to see what happens when this thunderstorm drifts westwards over / near this feature - the added low-level vorticity may become ingested into the cloud base to bring the risk of a funnel...
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Where can one buy a H.A.A.R.P. to attract all these storms to Norfolk? Asking for a friend...
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26 minutes ago, Harry said:
Totally agreed - I’m struggling to see where the energy will come from. WRF-NMM is modelling up to 1,500 J/Kg MUCAPE around London and Home Counties at 19:00 but I’m not sure how reliant this is on solar heating!?!?!? @staplehurst is the CAPE guru so maybe could advise.
Sunday evening/night has my attention though as models suggesting a fresh plume of instability accompanied by low pressure.
Biggest concern today was the extensive low cloud, which is rather stubborn to break. The most prolonged breaks so far have been in E Norfolk and NE Suffolk, allowing temperatures to lift to 24C at Tibenham Airfield in south Norfolk. Some gaps are also noted in SE Kent, and here temperatures have crept up to 23C in places.
12z Herstmonceux sounding suggests there is a subtle warm nose at ~850mb that requires a surface temperature of at least 22C to get past - many stations across southern England have not yet reached this threshold. The subdued temperatures have also meant not a significant temperature contrast between land and sea, and so the sea breeze convergence is only really evident across Kent/Sussex, and also Devon, otherwise the winds are largely offshore even on the coast. Without the convergence and >22C temperatures, surface-based convection is going to struggle to develop.
Based on this, the most likely areas right now to see convection initiate are E and SE Kent into East Sussex, and E Norfolk into E Suffolk - I particularly like this location due to the length of sunny breaks here so far, and a small surface low that is located in east Suffolk. Anything that does fire here will then drift WSW through Essex and perhaps towards London during the evening. Convergence is also noted in west Wales which may combine with upslope flow, and the fact there has been more sunshine here, to trigger some convection too.
The sounding reveals tall convection is possible (cloud tops potentially up to 36,000ft, which is quite deep for UK standards) but CAPE is rather skinny with fairly saturated profiles - and so lightning probably won't be as prolific as we've seen in previous days. As has been the issue all this week, shear is weak (10kts), and so storms will pulse and then collapse, bringing the risk of significant rain totals given their slow movement to the WSW (PWAT is 42.3mm!!)
The Bury St Edmunds storm right now I suspect is still elevated given the cooler surface temperatures and lack of any noticeable surface convergence there. Herstmonceux suggests ~800 J/kg if temperatures get to 24C.
Edit: obs just updated and the sea breeze is beginning to show signs of developing along other south coasts in southern England
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Satellite imagery reveals growing cumulus in the vicinity of the northern Chilterns around Aylesbury and Luton, and some radar echoes have appeared in that area too. Elsewhere, quite a few sferics have also appeared in the Harlow area
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10 minutes ago, UKSupercell said:
Is it me or is that Cambridgeshire cell moving fast as a tortoise?! Every time I've refreshed the radar it's just lingering there!!!!!!
That storm is intriguing - it's been there for hours! 12z Nottingham highlighting the instability aloft (nearly 600 J/kg CAPE), with convection firing from ~825mb level above the cool boundary layer (so it is elevated and not surface-based). Can only assume there's some locally enhanced convergence at that level in the area, perhaps outflow from the earlier storms helping to maintain it.
The arrival of elevated showers creeping towards east Essex is somewhat encouraging, implying there is additional forcing aloft approaching from the east - which may help additional showers/storms develop later this evening. Time will tell!
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5 minutes ago, *Stormforce~beka* said:
Why are they all dying instead of lasting hours like the Welsh ones? Wales is the place to be!
Looks like some fairly extensive cold pools have developed - there's not a great deal of shear today, so these are pulse-type storms that collapse fairly quickly, gusting out and while that can develop daughter cells nearby (as we saw multiple times yesterday) there's a lot of rain-cooled air now over southern England (the outflow also disrupting and distorting the shape of the nice convergence zone that was in place earlier)
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Storms and convective discussion - 1st April 2021 onwards
in Storms & Severe Weather
Posted
My gut feeling is this morning's activity will possibly be more noteworthy than the evening's offerings - hopefully not, but we'll see how the day evolves