staplehurst
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Posts posted by staplehurst
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Worth noting, today isn't a particularly simple setup - there will be a succession of shortwaves rotating northwest then west from the Continent which will aid both elevated activity (as we are seeing in East Anglia right now) but also surface-based developments. And so it seems likely that 'surprise' thunderstorms will keep cropping up quite late into the evening across some southern parts of England, well after daytime heating, and then a signal towards the end of the night (much like last night) for north Norfolk towards E Lincs.
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A very impressive sounding from Herstmonceux at 11z. Unmodified it would convect freely with air parcels rising from the surface, but it requires a trigger - ideally low-level convergence (or some forcing aloft but that isn't likely until the evening). But essentially the cap is ready to go over southern England where low-level convergence develops/strengthens in the coming hours.
The radiosonde yields 2,800 J/kg CAPE - this is probably near the high end of UK climatology. I studied CAPE in the UK for my uni dissertation using a WRF simulation, and the highest in the UK during the period of study (2002-2012) was just over 3,000 J/kg CAPE during the August 2003 heatwave - so just highlighting how remarkable this spell is currently.
More info in this paper: https://rmets.onlinelibrary.wiley.com/doi/full/10.1002/joc.3976
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Just now, Paul Sherman said:
Yh Arome 6z offers a small hope at least
Watch the next couple of frames...
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2 minutes ago, Tom Jarvis said:
Yep cap is too strong today.
The stronger the cap, the longer you have to wait for initiation. As was the case yesterday, this happened between 6-7pm in general and EURO4 implies 7pm kick off
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1 minute ago, Chris.R said:
I have a feeling this is going to be a South and east event from today onwards. Just feels like that way.
Is that such a bad thing for a change?
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Given some reduction in dewpoints around the London area, modifying the 12z Herstmonceux ascent suggests a temperature of 33C would be required (in combination with lift from low-level convergence) to ultimately push through the EML and generate deep convection. Sounding yields just shy of 1,800 J/kg.CAPE. Given the large temperature-dewpoint spread, cloud bases are likely to be quite high (6,000ft or so).
Shear is very weak (10 kts) and in the absence of any larger scale forcing aloft any storm that does develop, while doing so explosively, will ultimately be of pulse-variety and likely rain in on itself and so have a short life duration (1-2 hours). Likely to be some very gusty downburst winds (hazards for Heathrow etc), and this outflow could help spawn daughter cells. Steering flow aloft appears to be southwesterly, so in theory storms will drift very slowly (15mph) to the NE, if daughter cells form southwest of the parent cell then this will give an element of training and heighten the risk of flooding. Slow storm motion and high PWAT brings the risk of point flash flooding. Magnitude of CAPE and steep mid-level lapse rates could produce some large hail.
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Temperatures have reached 32-33C in and around London, a cumulus field is starting to bubble just south of London over Surrey and northern parts of E / W Sussex. Convergence evident in surface observations too. Let the attempts to try and punch through the cap commence...
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6 minutes ago, viking_smb said:
@staplehurst What are your thoughts on the next 2 days mate? please
Stay in bed
Apart from Scotland, tomorrow is very much a 'where will the cap break' type day. Similar requirements to today - need to find a convergence zone (should be a few about), or a decent hill/mountain and hope it breaks the cap during the late afternoon / evening hours. Large hail and flash flooding the primary threat, shear is weaker tomorrow so cells may struggle to organise as much as they did today. Temperatures near 29-30C for Wales / W Mids to 32-33C for SE England required ideally. If any cells fire near NW Eng then these may benefit from a shortwave running in from the Irish Sea during the evening to grow upscale across northern England and then Scotland overnight, but some uncertainty about that. Models having a tough old time, with all sorts of strange convective feedback issues / random areas of southwards-moving precip which doesn't really help.
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Just now, SalopWatcher said:
Can see forks of red lightening right up at the top of this distant storm. Crazy looking.
sounds almost like sprites!
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The problem we have is, these storms are surface-based and generated by low-level convergence - they eventually managed to punch through the cap, leading to explosive development. However, the cap is still largely in place, and while the storms create their own environment and tap into the moisture-rich surface air, they are primarily forced just from the surface - so as temperatures at the surface begin to cool and low-level convergence reduces through the evening, that only leaves the storms with one choice: to become rooted at some elevated level instead and feed off the moisture available there. Problem is the lack of any substantial forcing aloft to support elevated thunderstorms at this time, and so ultimately after an initial flourish they begin to weaken...
In theory better forcing aloft arrives from the west in an hour or two, so should hopefully see an uptick in elevated thunderstorm activity over mid/north Wales 9-10pm, and then later across NW England...
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20 minutes ago, Ben Sainsbury said:
New ConvectiveWeather forecast update includes removal of MDT area:
@staplehurst Any reasoning behind removal of the MDT area? Definite feels like there is less agreement among the models this morning from my own opinion.
Convective Weather
WWW.CONVECTIVEWEATHER.CO.UKForecasting thunderstorms and severe convective weather across the British Isles and Ireland for up to the next...Yeah, I have concerns about the cap - latest models are perhaps 1C warmer at 850mb in the 'area of interest', and given just how finely-balanced the whole thing is it's all or nothing in that type of situation. Probably higher confidence of elevated storms breaking out over NW England and the Irish Sea later this evening than surface-based storms in the Midlands, but we'll see...
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1 minute ago, viking_smb said:
Cool, so by the looks of things, it wont arrive till about 6-7am then, where is to now?
Well, the Theta-E ridge will be over Wales for most of the night, so in theory a few showers/thunderstorms could develop at any time, but they are more likely towards the very end of the night and perhaps more widely in west Wales during Monday morning
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57 minutes ago, viking_smb said:with your forecast, is there any update regarding tonight for south wales area?
No real change from what I've seen so far - as is often the case, tonight is a rather complex situation. The elevated mixed layer (EML) will cover most of England and Wales tonight, pushing north towards southern Scotland - so any minor upper forcing may just be enough to develop the odd rogue shower/thunderstorm almost anywhere. However, the main focus is across western Britain where upper forcing will be stronger as a couple of shortwave troughs move north. These help to cool and moisten the upper/mid-levels, generating more instability and encouraging lift.
Forecast profile for Wales (below) yields some quite deep elevated convection, IF profiles can moisten sufficiently (they are rather dry right now).
This is most likely to occur as one of these shortwaves approaches. From what I can see there may be a minor shortwave (dashed magenta below) running north from north Wales / Midlands into northern England during the night, followed by a more substantial shortwave (blue line) that approaches Cornwall around midnight and heads into Wales late in the night.
Elevated thunderstorms often develop along Theta-E ridges, and I've tried to identify the location of this through the night (right image) - notice how as an upper low develops near the Channel Islands, the ridge axis remains more-or-less locked here while it bulges more significantly westwards across Wales through the night.
Essentially what you'd be looking for is where either of these troughs (left) overlap with the Theta-E ridge (right) for your best chance of elevated thunderstorms developing - but it is likely the more substantial trough will arrive quite late in the night, so activity may be fairly isolated until this stage.
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13 minutes ago, Flash bang flash bang etc said:
Anyone know at what temp the cap would erode?
Over SE England, around 35-36C, but profiles are very dry aloft
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18 minutes ago, Thunder and Lightning said:
There sure is very strong capping through Friday afternoon, so even though I know its too early to make assumptions and loom at the details, I am not feeling super excited just given how much of a cap there is.
This is the information bit on the forecast sounding for London at 8 o'clock on Friday Afternoon:
1 CAPE and 360 CIN. It's going to be hard for a storm to form in that
Don't forget that's for surface-based convection - storms on Friday will be largely elevated, and very high-based at that! (10-12,000ft)
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50 minutes ago, Lance M said:
ECM 12z is even worse than it looked yesterday. Does the GEM fair better or worse in these sorts of setups?
I think in general they all vary in their performance - some handle certain situations better than others, and vice versa. Surface-based convection is easier to verify which models handle it best because ultimately the air parcels are always being lifted from the same level, i.e. the surface (for example GFS has a moisture bias and is too keen on develop convection in capped environments). But when it comes to elevated convection, it all depends on which level you're lifting the air parcel from, how much forcing / moisture is available etc, so it ends up being not a fair comparison between events.
GEM from memory was rather (rightfully) unenthusiastic for the 21st May event (which ended up largely busting), while also had a pretty good handle on the Aberdeenshire MCS several days in advance on Sat 27th June. ICON-EU probably had the best handle on the elevated thunderstorms that affected Northern Ireland and SW Scotland early on Thurs 25th June (most other models were too far east over the mainland initially). In some cases, such as French imports, it almost ends up being a case of following climatology rather than what the models are suggesting...
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Forecast sounding from 00z ARPEGE for Friday afternoon over Cen S England, associated with instability plume arriving from France. Elevated convection rooted from 650mb (cloud bases 12,000ft!). Very dry both aloft and below this level, so precip will likely largely evaporate before reaching ground, and also convection may struggle to maintain itself given how dry it is aloft. Probably why the EC (and most models) look fairly dry with no major rain totals at the surface. This increasing mid-level convective cloud may also impact how high the temperatures reach across southern England on Friday afternoon, for example.
Substantial elevated mixed layer and deeply mixed boundary layer would require surface temps around 40C or higher to develop surface-based storms, and even this assumes a dewpoint of 15C which may be too high - which is a shame, because the wind profile is very nice!
The whole instability plume will continue to advance NE across Midlands, northern, eastern and southeastern England during Friday afternoon and evening, clearing to the North Sea during the early hours of Saturday. Uncertainty at this stage at how upper forcing will phase with elevated instability, and therefore how widespread any elevated showers/thunderstorms may be. Still a few days away for this to chop and change...
Given these profiles, strongest convection could produce some rather gusty winds / heat bursts.
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12 minutes ago, Zak M said:
What's with all the activity down south?!
Pockets of elevated convection in the warm advection regime - judging by satellite and radiosonde data, the convection probably wasn't that deep but went up in a hurry...
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2 minutes ago, Chris Lea-Alex said:
So despite not really being electrified, would this glub be classed as an MCS? And if so could it spawn its own MCV?
Just a wave on the front - yesterday's 12z EC did a reasonably good job with it, while many of the high-res models struggled with the extent of the precipitation. This is very common with frontal waves - they are often more extensive and further north than models would imply...
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5 minutes ago, matt111 said:
I’m surprised it doesn’t show the same here considering I certainly haven’t seen anything this year other than the distant stuff in the channel the other night.
If you zoom in quite a bit you can see the extreme tip of Hampshire is in the pale grey (1 thunder day) shading. But it's quite a quick gradient to 3 days just to your north - where there were some lightning strikes within the 10km radius on Thursday 11th June, for example...
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Just now, AMeltedFlake said:
What are the reasons behind a poor end to a heatwave? timing?
It hasn't ended yet!
But potentially not enough upper forcing - we'll just have to see how the next 36 hours plays out, there is plenty of potential, will there be enough trigger...
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Looks like an elevated thunderstorm has developed just SE of Falmouth with recent lightning in the area.
12z Camborne ascent (typical environment that will be creeping into Wales this afternoon) requires a surface temperature of at least 29C (preferably higher) combined with dewpoints around 17C to break through the cap, and develop surface-based convection. Modified profile yields 2,000 J/kg CAPE with some reasonable shear. Any storms that can fire, particularly W / NW Wales but perhaps also NW Devon, could become severe with large hail and strong outflow gusts.
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14 minutes ago, Josh Rubio said:
How’s it looking for the East Midlands in relation to this? Thanks.
For the East Midlands (profiles below from west Lincs) there is potential for elevated thunderstorms during the middle of the day, rooted from around the ~750mb layer, so bases would be around 8-9,000ft.
The low-level environment is capped to surface-based convection until perhaps late afternoon or evening, and so it's a bit touch-and-go as to whether something can fire, but if it does it's more likely during the evening hours...
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27 minutes ago, bradleywx said:
I’ve got a hunch that tomorrow afternoon’s SB activity is currently being underplayed, models are notoriously with them in particular anyway..maybe I’m being optimistic but based on what I’ve seen there’s no reason to believe it’ll be restricted to just far eastern/north eastern counties, proper ingredients are in place well away from there.
edit: met office have increased likelihood on tomorrow’s warning, shape looks similar? I suppose that was always expected however.
Environment is too capped (CIN shaded green, CAPE shaded orange) across SE England for surface-based storms on Friday afternoon, the hot air is just too deep to overcome the cap - you would need surface temperatures of around 34C or higher, and even then the profile is very dry aloft. This does not rule out the odd elevated shower/storm developing, but most of these will have cleared to the east by the afternoon - and the morning cloud cover associated with these may also hinder surface temperatures somewhat.
Conversely NW England has much less capping, and with surface temperatures of 25C or higher, and dewpoints in the mid-high teens, the forecast profiles yield substantial CAPE and likely explosive and potentially severe thunderstorm development.
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Storms and Convective discussion- 7th August onwards
in Storms & Severe Weather
Posted
sorry for late reply, just catching up - some are rather tricky to identify as they occur at different levels. In general look for areas of PVA (positive vorticity advection) at 500mb is a good starting guide, and also pockets of slightly colder temperatures or troughing to the isobars at various mid-levels