staplehurst
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Posts posted by staplehurst
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12 minutes ago, Harry said:
Line of torrential looking downpours developing west of London…wasn’t expecting that
Shortwave swinging northeastwards this evening/tonight likely to continue to set off random showers/storms for a wee while yet.
QuoteWhile most activity will tend to slowly fade through the evening hours, there may be a relative uptick in activity into the early part of the night from E Wales/NW England/West Country into the Midlands, aided by a shortwave/cold pool aloft migrating northeastwards across the area.
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Slight concerns about the mass of thundery rain over northern France at the moment, possibly reaching the south coast around 12:30 (give or take). Cirrus ahead of this may dilute surface heating across southern England, and this could essentially eradicate these areas out of the home-grown storm risk. Also concerned about how quickly things will upscale, especially with this mass of rain coming in fairly early. Still think there's a window of 3-4 hours for fairly discrete thunderstorms over the Midlands/western East Anglia, but likely become fairly swamped with rain by early evening the way things are going.
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11 minutes ago, summer blizzard said:
Out of interest is there a meteorological reason that everybody is punting severe for the south while the models are showing that strongest precipitation in the north.
Is it simply that by the time it's here, it's likely just to be very heavy rain and no longer electrically active.
exactly that - at some point there will be a transition, and it's trying to find the location where that happens etc
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7 minutes ago, Sprites said:
Is the Euro 4 model hereabouts? I've heard folks big it up for convection.
sadly it was discontinued at the end of 2021 (from memory), but it was generally pretty good at convection
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Always good fun when subtle disturbances aloft appear at relatively short lead times - compare the difference over the eastern Channel between the 00z run (left) and the 09z run (right), highlighting some minor PVA in that area (still subject to placement error) aiding the development of the elevated thunderstorm over West Sussex.
Incidentally, there are growing hints at an arc of elevated showers/storms developing late evening onwards from Wales-Cen S Eng lifting NE through the night to reach NW Eng-Lincs by 06z Sunday - this is ahead of any developments in the Channel/SE. Not nailed on by any means, but essentially given the unstable airmass in place expect the unexpected should any smallscale disturbances occur aloft.
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1 hour ago, ResonantChannelThunder said:Dan hasn't mentioned the cap down here in his outlook, and says levels are likely too dry to produce any notable lightning.
The 05z Larkhill ascent was very unstable, with >2,000 J/kg SBCAPE given projected temperatures/dewpoints this afternoon. However, this is representative of the air across Cornwall by this afternoon (as it continues to migrate west) and profiles will become even drier aloft through the day as much drier air continues to advect from east to west (this is evident in the 09z sounding, with a much larger gap between the red and blue lines).
The 09z sounding suggests you'd need 27.5C to get past the surface-based CIN but 28.5C for convection to initiate in the absence of any notable forced ascent. It's quite breezy today, but still potential for a sea breeze to develop just inland from the south coast of both England and Wales, and this may just be sufficient to create some convective cloud and a few isolated heavy showers. Once you get past the surface-based CIN there's no substantial capping aloft so the only thing preventing big storms developing is just simply how dry the airmass is, so it will be interesting to see if anything does manage to punch high enough to produce a few lightning strikes or not.
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8 minutes ago, Metwatch said:
One for @staplehurst to answer I think.
Just a moment...
RMETS.ONLINELIBRARY.WILEY.COMThis paper, some written by Dan Holley (who is staplehurst) could be useful too, quoted: the highest CAPE across Great Britain during Spanish plumes can locally exceed 3000 J kg−1. In an average year, 15 days produce CAPE in excess of 500 J kg−1 somewhere in Great Britain, 4 days > 1000 J kg−1 and 1 day > 1500 J kg−1
Yeah, the one caveat about that paper (it's over 10 years old now, based on my final year dissertation in 2012-13) is it was based on idealised WRF simulations to analyse sea breezes and so probably wasn't the best configuration/physics schemes for convection. Would be good to redo that paper someday with more useful physics schemes and a longer time period perhaps.
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17 minutes ago, Flash bang flash bang etc said:It’s trying
haha, love that photo - look how high the base is!
So, we've surpassed the temperatures required for air parcels to start rising, however they have a lot of dry air to battle, both in the low-levels (hence the high bases) and in the mid-levels. It will take some time for convection to have repeated goes at gaining height, collapsing/detaching, and another tower will give it a go, reach slightly higher, then collapse etc. Eventually one will manage to punch upwards, and given the instability on tap you should be able to see it shooting upwards without needing to timelapse it. The best chances will be where forcing from the low-levels exists, such as convergence zones and/or upslope flow over high ground.
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3 hours ago, Willsy said:
i know this is old news but i witnessed a very large waterspout/tornado last year. After torro did their research they found that the parent cell traveled 62 inland producing at least 3 tornadoes maybe more . i caught the first one as it came in at barton on sea. was quite a sight to say the least. wasn’t your standard water spout as had a broad rotation with lots of spin ups within. seeing the damage first hand was like something out of a movie lol. i know i’ll probably never see anything like this again but with a warming world who knows! These aren’t my photos neither is the written info but thought i’d share it for. the torro forum i’m on
The video you captured as it came ashore was fascinating to watch!
There's a whole 60-minute presentation on both the meteorology behind it and the impact at this year's TORRO spring conference being held in Oxford in just over a week's time if anyone fancies coming along - completely free, full programme and to register:
The Tornado and Storm Research Organisation (TORRO)
WWW.TORRO.ORG.UKTORRO is a privately-supported research body specialising in severe convective weather in Britain and Ireland-
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14 minutes ago, staplehurst said:
Hi all,
I've resurrected our Snow Depth reporting tool (was originally made for the easterly in February 2021). It was very popular and hugely valuable back then to see the spread of snow depths across the region, so if possible could you submit a snow depth reading once the snow has ended at your location and then you can see all the other reports nearby too. Thank you!
WQ Snow Depth Reports
WEATHERQUEST.CO.UKI should add, even a report of 0cm is useful in knowing where the rain/snow boundary is!
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Hi all,
I've resurrected our Snow Depth reporting tool (was originally made for the easterly in February 2021). It was very popular and hugely valuable back then to see the spread of snow depths across the region, so if possible could you submit a snow depth reading once the snow has ended at your location and then you can see all the other reports nearby too. Thank you!
WQ Snow Depth Reports
WEATHERQUEST.CO.UK-
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Just looking at the travel maps and clearly snow having a big impact on some of the routes in East/West Sussex. Who needs radar when you can see where the snow starts by finding the edge of the slow traffic!
Seemingly the boundary right now is just south of Staplehurst-Headcorn, for example. Several reports of sleet/freezing rain on the leading edge (suspect some is evaporating before the ground so the radar appears to look heavier than reality on the leading edge).
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The 00z Nottingham ascent modified for 23.5C with a dew point of 17C yields 2,000 J/kg CAPE. The 05z Larkhill ascent is essentially saturated through much of the mid/upper troposphere (indicative of the extensive rain across southern England at the moment from higher level cloud) but still yields 1,500 J/kg CAPE with 23C / 17C. It also has a decent southwest flow at jet stream level, which - assuming convection punches this tall - would help vent thunderstorms at anvil level. PWAT of 31mm at Larkhill, weak steering flow, large CAPE and saturated profiles (meaning less rain evaporates before reaching the ground), low cloud bases etc all point to slow-moving torrential downpours and a real flash flooding concern - especially if they occur over an urban environment and/or train over similar areas.
This does remind me of 16 August 2020 and 25 July 2021 in some respects, in which there was a lot of lightning detected on radars but visually you didn't see a great deal.
We've already reached 25C in East Anglia where some hazy sunshine occasionally breaks through, and there is already a marked convergence zone running from Norwich down to Cambridge and Luton. Once the first cells develop they will then heavily dictate how the rest of the afternoon pans out by altering the environment with their cold pools and outflow etc.
Remember, it is only 10z and most storms at this time of year often develop mid-late afternoon and evening, sometimes well into the evening (thinking August 2020). Yes there is extensive cloud, but the models expected extensive cloud and yet still break out a tonne of heavy showers and thunderstorms.
And for those in Kent/Sussex and perhaps Essex, there may be even some more activity overnight drifting inland from the Channel.
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50 minutes ago, Peachy said:There's a definite change in the air now. For the first time in days the clear blue skies here in the Midlands are being replaced with huge clouds growing by the hour.
I predict the breakdown has started and we could see some huge storms earlier than predicted.
Models had a good handle on the increase in mid-level cloud cover today, especially over the Midlands and continuing to increase into the evening - e.g. see 15z run of the Harmonie from yesterday
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Yeovilton reporting a cloud base of 15,000ft which fits expectations from model soundings. We discussed this potential in our internal morning briefing on Friday - these are being generated from a fairly shallow layer in the mid-levels above the hot, dry airmass that's advecting slowly northeastwards over the next 48 hours. Much of the rain will likely evaporate before reaching the ground given such high bases and hot/dry air beneath. Would not be surprised to see further 'random' showers pop up periodically between now and Tuesday morning above the EML, before the main focus shifts to the cold front advancing from the SW etc.
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19 minutes ago, minus10 said:
Thanks for the explanation, really interesting...do you think if there had been more of a breeze to mix the atmosphere up that may have helped with surface heating and convection ?
If there was less cloud/more surface heating then that would drive the sea breeze inland and create quite a decent CZ to generate heavy showers/thunderstorms, but sadly (as is so often the case in the UK) the cloud has remained in most areas.
Very much a conditional risk on days like today, and it is hard to convey that in forecasts. It reminds me of a few events earlier in the spring in Oklahoma when all the parameters were there to create big supercells and possible tornadoes, but it relied on the cap breaking - and trying to convey the message to the audience/public that some really nasty weather was possible while they look up at clear blue sky can perhaps be difficult to understand. Perhaps there's a better way to flag up high impact low probability events?
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It's an interesting setup this afternoon - there is evidence of a CZ along the south coast (black-dashed line), more 'inland' over SW England where there has been better cloud breaks/surface heating allowing the sea breeze to advance further inland, whereas it's hugging right along the Sussex and Kent coast. This is likely responsible for the heavy showers in Cornwall at the moment. However, for most areas it isn't warm enough to break the cap and generate surface-based convection.
Meanwhile an arc of showery rain is developing/intensifying along the sharp Theta-W boundary (image is Theta-E but can be used vaguely interchangeably), this being engaged by positive vorticity [advection] around the upper low that is sliding eastwards through the western English Channel to create the necessary lift to generate precipitation. Because this Theta-W boundary and PVA will remain close together through the rest of today, this will likely continue to drive a band of showery rain progressively eastwards - we may find the orientation pivots from its current NW-SE to become SW-NE by the overnight period. Given the lack of surface heating to create buoyancy, and the rather saturated profiles, it doesn't seem likely much lightning will occur unless we can improve conditions at the surface.
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It does appear the French surface-based storms were (at least partly) to blame for the limited amount of thunderstorms in SE England last night. Obviously some did develop in the residual instability ahead of the French outflow over Kent/East Sussex, but this likely used up the remaining CAPE before being replaced by the area of "no CAPE". Whereas Dorset was less-impacted by the earlier French convection and had a much more plentiful supply of unstable air to tap into and so was able to maintain much more active thunderstorms for longer.
Always fun to see the massive differences in CAPE being simulated by models once they've realised a lot of it has been depleted by earlier thunderstorms - compare the ARPEGE 00z run last night with the previous 12z run from yesterday afternoon for the same timestamp. And if nothing else this serves to highlight how poorly models handled yesterday's convection (which was not a surprise as the risk was always there with a weak cap in France).
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Just now, sunnijim said:
Not sure any model had storms kicking off by 9pm on the South Coast?
Exactly, hence stressing to incorporate an error bar of a couple of hours because the instability plume is already there and it's a case of how quickly (or slowly) would it destabilise. Earlier initiation favours more of the south coast, later initiation and the whole thing would be further inland. Fortunately it's occurred earlier, although perhaps not early enough for West Sussex coast?
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14 minutes ago, viking_smb said:
What about south wales mate?
Would never rule out out (certainly scope for showers, whether they're thundery etc...), will be a case of radar watching as is often the case in these setups. Gut feeling is you're probably a bit too far west, but we'll see!
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8 minutes ago, Lance M said:
The same posts happened before the May events, too! Seems they're always going to happen unless it's > high twenties!
Thank you for the detailed update! Good to know that the instability is still there for the central South.Ha, yes. Think I prefer it when France has a strong cap, makes forecasting slightly easier...
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I've seen a few posts about it not "feeling stormy" or "warm/humid enough" for storms. That's the beauty of elevated thunderstorms - it can be as cold as you like at the surface. In some ways, the colder the air at the surface, the more 'forced lift' potential should a warm airmass be pushed northwards towards the UK (because warm air will be forced over a layer of colder, denser air, creating ascent). Ultimately thunderstorms have little interest in whether we think it's warm or cold at ground level, because it's all about relative temperatures rather than absolute. That's how you get thunder snow in the winter!
The idea behind elevated thunderstorms is that they can be partly driven by a process called isentropic lifting/upglide, which in a nutshell is where warm, moist air is forced up over a cold, dense airmass near the surface. This is common in the warm season over the English Channel and southern England, and is often responsible for our overnight "plume" thunderstorms. I've modified this vertical cross-section of Theta-E to hopefully give a better idea of the lifting of the warm air as it moves further north...
The 12z sounding from Trappes unmodified yields 1,700 J/kg CAPE but it is capped by a warm nose at 850-900mb. However, the cap is small (20 J/kg CIN) and easily eroded by surface heating which is what we've seen over France this afternoon. If you modify the sounding to representative surface temperatures and dew points then you're left with explosive deep convection with 2,500 J/kg CAPE. These storms are surface-based, and so as soon as they dip their toe in the SSTs of just 13-15C in the English Channel then they're not going to respond too kindly and will likely decay (with lightning largely staying over the Continent).
The big issue with France getting all excited is it completely washes out the atmosphere in that area and significantly modifies the thermodynamic profile. The 12z AROME picks up on this signal, with a large pool where CAPE has been used and the atmosphere is much more stable.
However, the air over the English Channel (especially near the south coast) and across southern England is still relatively unstable, as confirmed by the earlier 12z sounding from Herstmonceux which yields 350 J/kg CAPE from parcels lifted from the ~830mb layer (if you could lift it from the warm nose at the ~920mb layer there would be much more substantial CAPE). You can also find convergence generated by outflow from earlier French storms can (perhaps temporarily) provided an uptick in elevated convection on the leading and/or western edge of the broader precipitation shield.
An elongated PV strip stretches currently from northern France across Biscay down to northern Spain, and the leading edge of this is likely driving the storms near the Channel Islands. So in the short term, it is this area that is likely to produce active thunderstorms, the focus shifting slowly towards Cen S England. However, as the PV strip eventually reaches the south coast, it will likely engage more with the residual instability 'tongue' late evening/towards midnight and this is when model guidance suggests a fairly rapid increase in elevated shower/thunderstorm activity over central southern England. That said, they are naturally struggling with the earlier French convection (no surprise!) and how that may impact things tonight. It seems, because the French exports are so slow moving (due to weak steering winds) that this may not hamper things too much across S/SE England later tonight - but those locations on the coast it is very much dependent on how quickly things can initiate before the instability plume moves inland. An interesting evening/night to come either way!
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1 minute ago, viking_smb said:whats your thoughts @staplehurst on tonight then mate
it'll be interesting to see how it plays out. CAM (convection allowing models) have been, generally, killing any surface-based storms once they start to cross the Channel - for some reason in the model world they struggle to become rooted above the boundary layer. There's also a fairly strong suggestion of cold pool development, with a broad/expanding precip shield and all the active storms on the bowing out eastern side (plus very strong gusty winds). At the same time, on the western periphery of the precip shield you can get convergence that aids a few storms far to the west too. So I guess it's possible you end up with one zone that's ever-shifting to the east on the eastern flank (aiming for SE Kent or even just staying in France), a broad area of (boring) rain, and then other cells on the western edge of the rain.
That said, there's also a reasonable signal in model guidance for elevated storms to erupt around 6-8pm onwards over the mid Channel/Cherbourg area and these may have a better chance of surviving given their origins/inflow level and would probably head into West+East Sussex.
These events rarely follow model guidance so will be interesting to see how the evening pans out...
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2 minutes ago, Lance M said:
If I had a quid for every time this has been (correctly) pointed out in this thread since yesterday.....
You could afford a tank of fuel to go chasing
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Storms and Convective discussion - June 2023
in Storms & Severe Weather
Posted · Edited by staplehurst
that was the main reason from refraining going any higher than a SLGT - Lincolnshire saw a number of active thunderstorms develop, and given the environment they had the potential to become severe. However, it became apparent this morning once the earlier rain/convection was further east than generally modelled yesterday that it was less likely severe thunderstorms would develop. It's trying to find the right balance between the chance of something occurring (low) vs the potential for it to be severe/impactful (high).