staplehurst
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Posts posted by staplehurst
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This developing line of elevated convection over western Britain is not unsurprising as it had been signalled in most model output, the difficulty is gauging how much lightning it may produce given somewhat weak instability. From the convective forecast:
"Late in the day, showery bursts of rain will spread into W / SW Ireland associated with a cold front. This will continue to track east across Ireland and W Scotland during the evening and night hours, for a time being engaged by a sharpening shortwave approaching from the Atlantic. This may, for a time, increase the potential for some embedded elevated convection, however as the shortwave overruns the cold front the focus will shift to a new corridor of elevated convection that is likely to develop in the vicinity of the Irish Sea / W Britain during the early hours of Thursday - ahead of the cold front. Given rather weak instability, the risk of lightning is considered relatively low but a few isolated lightning strikes may be possible, hence the inclusion of a LOW threat level."
Convective Weather
WWW.CONVECTIVEWEATHER.CO.UKForecasting thunderstorms and severe convective weather across the British Isles and Ireland for up to the next 5 days.-
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2 minutes ago, Norfolk Sheep said:
The only alarm I've been setting is the one to remind me to book a shopping delivery slot on Thurs night/Fri morning
Still trying to work out if I need to get up early tomorrow morning, forego my nana nap in the afternoon or stay up later after booking said delivery slot on Friday morning or set an alarm for later on Friday........
07:30-08:00 at the earliest for a delivery tomorrow all gone to the North Sea by 12:30-1pm
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That'll be the 09z UKV driving the Met Office website graphics. Differences between the 06z (left) and 09z (right) are largely related to the increased moisture in the mid-levels...
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The biggest issue with Thursday is the quality of mid-level moisture - as we've mentioned before, the key to getting elevated thunderstorms to develop on Thursday morning is substantial moistening of the 700-850mb layer, which in turn will significantly increase the amount of CAPE available. The shortwave trough will move through regardless of whether it fires any convection or not, and in general thunderstorms require three main ingredients - instability, moisture and lift. We have a lifting mechanism, we potentially have instability (although this is dependant on the moisture content of the atmosphere too). The biggest concern is mid-level moisture (or lack of).
THURSDAY MORNING
Similar to my post yesterday morning, the models vary in how much mid-level moisture will be drawn northwards tomorrow morning coinciding with the approach of the shortwave from the west. It's a tricky overlap - the best moisture will be towards the SE, while the best forcing may be further to the NW, and so the better overlap between the two is probably over East Anglia, but I still think elevated thunderstorms are likely in SE England too. AROME is consistently moistening things nicely first thing tomorrow morning (5-7am onwards) and has quite a bit of elevated CAPE available (below left). The magnitude of CAPE, and reasonable shear, suggests lightning could be quite frequent with the most active cells. But other models are much drier in this layer, and less keen on generating convection.
Th shortwave trough will be moving through during the day tomorrow, followed by significant subsidence as an upper ridge quickly follows from the west. This creates a fairly marked boundary between available moisture in the mid-levels, and a rapid drying of the profile. Map below is a rough sketch, based on 00z EC, of the boundary between this moist/dry mid-level profile and its position during the course of Thursday. Essentially, ahead (or to the east) of this line there is a chance of elevated convection occurring, once the line has moved through then your chance decreases significantly.
THURSDAY AFTERNOON
As you can see, by the afternoon profiles become much drier with substantial warming in the ~500mb layer which acts as a cap to any surface-based convection that may try to develop. If elevated convection develops through the morning hours, the lack of surface heating combined with rain-cooled air will have an impact on potential maximum temperatures. If elevated convection doesn't occur in the morning, or is more limited in areal extent, then there will be better scope for surface heating - but I still think there may be quite a bit of medium-level cloud moving through. Either way, should temperatures climb substantially into the 20s Celsius by the afternoon, sea breeze development is likely (perhaps even reinforced by outflow boundary from morning convection). While this may provide the lift required to generate surface-based convection, I'm still concerned about (a) how dry the profiles are likely to be by mid-afternoon and (b) capping inversions both in the mid-levels and near-surface (depending on how high surface temperatures can reach). I've modified the forecast sounding above for a surface temperature of 24C and dew point of 11C. There may be quite a marked dew point boundary along this sea breeze convergence (single figure values inland, low teens near the coast).
Notice on the 'trough timeline' graphic above how the dashed lines start bending back west over the near Continent - this is indicative of the shortwave relaxing to the north, meaning the mid-level moisture isn't 'scoured away' here as much. If the shortwave can relax/lift to the north quicker, then this will probably mean (a) less morning elevated convection and (b) perhaps a slightly better chance of surface-based convection in the afternoon. One to watch, but right now not overly optimistic about the afternoon prospects, I think any convection that does try to fire will struggle to maintain itself.
THURSDAY NIGHT
Perhaps a more overlooked potential is on Thursday night, as the Theta-W plume that initially gets advected into SE England on Thursday morning, then shunted away to the east, resurges back north on Thursday night from France ahead of the next Atlantic low. There is potential for a second round of elevated thunderstorms developing during Thursday night as this becomes engaged by increased forcing aloft from the west. However, this overall setup is less clear-cut, and so confidence in the forecast evolution is a lot lower. The main area at risk is possibly slightly further north, towards the Midlands / Yorks-Lincs / East Anglia, but I would rather wait a few more model runs given the uncertainties at play as to whether anything will develop or not. Forecast profiles are quite warm/moist in the upper atmosphere, and overall the magnitude of instability is probably less than on Thursday morning.
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6 minutes ago, shaky said:
Whats with the high cape around west midlands but then no cape around the east midlands and then back to high cape across east anglia?would that be cloud cover causing it?this is going down to the wire!!
Rain-cooled air most likely
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2 minutes ago, Mitch perrott said:
sooo i wont get any action?
You have to bear in mind this model right now only goes out to 12z Thu (1pm BST). The whole instability plume and trough will be moving east through the afternoon, and so the storm risk will increase eastwards during the day (as may the coverage of storms). Plus if the trough speeds up, then it may end up being The Wash is the western extent instead of the Humber currently indicated on these maps. Still 2 days away, lots of detail to iron out. The risk is probably greatest in East Anglia, but SE England still has a reasonable chance of elevated storms too - they quite often develop more to the southeast than models sometimes suggest.
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3 minutes ago, Paul Sherman said:
Yep great Forecast there Dan and really goes into detail about timings etc
But this is the Uk and if it can go wrong it WILL go wrong
MCS it is then after early Elevated Storms congeal and move North East - Hahaha
You know the annoying thing is, if this were the U.S. you could easily shift the risk areas 50-100 miles east-west and no-one would notice. Being a relatively small island surrounded by water means (surface-based) storms either happen or don't, rather than just shifting the risk to the east - there's no happy medium. Frustrating to say the least!
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Timing is key for Thursday - we've mentioned before how both the phasing and shape/characteristics of the mid/upper level trough will ultimately determine how many thunderstorms (if any) develop. Models are handling this trough slightly differently, including between runs of the same model. The overall picture is we have an upper ridge (red dashed line) over the British Isles for the next couple of days, this shunts eastwards as the upper trough (blue-dashed, quite a narrow / sharp feature) slides in from the Atlantic, but is quickly followed by another upper ridge. This means there will be a relatively small window of a few hours where the upper trough will be in a favourable position, before profiles become heavily subsided as the second ridge builds from the west.
The 4 panel below, of 500mb relative humidity, highlights the differences between each model, and indeed between runs of the same model (ARPEGE etc) with the handling of this trough. Notice how the 00z ARPEGE is way quicker with the trough than it's previous 12z run, and while both the 00z ECMWF and GFS have similar timings for the trough axis, they do have subtle differences in the shape of it. There has been a trend though for this trough to approach slightly faster than previous runs, and this is something to bear in mind if this trend continues (this ultimately shifts the area where storms may develop gradually eastwards). Notice also the marked mid-level dry intrusion (very low RH) immediately to the rear of the trough.
There are two possible storm evolutions for Thursday. Firstly, elevated thunderstorms could develop as the approaching trough cools and moistens the mid-levels, increasing the available instability and providing the forced ascent required to generate elevated convection above the elevated mixed layer (EML). Forecast profiles (below left) suggest this is heavily reliant on the moisture content in the 700-800mb level. Models vary quite a bit at how much moisture may be available in this region, and generally those with more moisture (higher RH) are breaking out elevated thunderstorms, while those that are drier are not.
The ECMWF has been consistently breaking out elevated thunderstorms during the morning hours for several runs now - until today's 00z run which has the 700mb 'moisture tongue' (below right) slower arriving and therefore generally too late to phase in with the passage of the trough. Elevated storms tend to affect a much wider area, so more places would in theory experience a thunderstorm if they developed - but equally tend to scupper potential for surface-based storms later in the day due to excessive cloud and less surface heating.
The second potential is for surface-based thunderstorms, which will require substantial surface heating to overcome the low-level capping inversion. There is likely to be a fairly deep EML, and given surface mixing will likely occur I suspect some of the models are overdoing the surface dewpoints. I've adjusted the second forecast sounding below (somewhere in Essex) for a dew point of 12C, which seems reasonable, but which would require an air temperature of at least 26C to overcome the cap. Assuming the development of a sea breeze / surface low, then moisture pooling along convergent boundaries may lift the dew points up another couple of degrees potentially, and this may lower the required 'trigger' air temperature by a couple of degrees as well. Such low-level convergence would ideally be needed to provide the trigger for deep convection to occur.
This particular forecast profile is very dry (but it is based on the 00z ARPEGE which I feel is clearing the trough through too quickly - the alternative was to use a GFS forecast profile which is excessively moist, a known bias in the model), and so one issue is potentially the lack of moisture aloft to sustain convection. There is some reasonable speed shear, which would help separate updraft/downdrafts and combined with potentially significant CAPE would aid in storm longevity. As such, some fairly large hail would be possible. The high cloud base (due to the large surface temperature/dewpoint spread) would severely limit the tornado potential, unless bases were able to lower once convection gets going and the boundary layer moistens somewhat. Low-level winds may become increasingly backed, especially later in the afternoon, as sea breeze / heat low develops.
So, to summarise - concerns exist over the timing of the mid-level trough (if it continues to speed up then it may move through too quickly for peak diurnal heating). Forecast profiles also look quite dry, and so deep convection may struggle to develop - especially at first. But IF surface-based storms do develop, then they could produce some reasonably large hail. And there is a chance of elevated storms developing late morning onwards, depending on how much moistening occurs in the mid-levels - but this may scupper, or at least limit in areal extent, the potential for surface-based storms in the afternoon.
In essence, lots to play for, much of which is determined by the shape/timing of the upper trough...
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19 minutes ago, Zak M said:
Hmmm... has anyone seen the GFS 18z? (Or the GooFuS as @Paul Sherman likes to call it ) It has thunderstorms developing in the morning at 9am on Thursday... bit strange as that's the first model to show the thunderstorms developing that early but the GFS has its swings so it probably won't show it the next run
General consensus amongst model guidance is for two main areas of interest - potential for elevated showers/thunderstorms in the morning hours, not necessarily imported from France but perhaps more developing 'overhead' as the approaching upper trough engages with the warm plume over E / SE Britain. Assuming morning convection clears smartly then there would be potential for surface-based thunderstorms to develop in the afternoon / early evening across eastern England.
However, all of this is hugely dependent on the exact shape of the upper trough, and this varies from model to model, and even run to run of each model, hence the uncertainty. If the upper trough is less developed/sharp, then less engagement with the plume will occur and consequently the chances of anything developing will be less. We also need to consider timing issues - if the trough swings through too quickly, then it won't coincide with peak diurnal heating for example. Then of course, if elevated convection does develop in the morning hours, this notoriously produces a lot of cloud that tends to inhibit surface-based development in the afternoon.
So lots to play for, and in forecasting terms a long way away. I've been keeping my eye on Thursday for a while now, but probably won't get too excited until at least Tuesday. there is potentially some reasonable shear at play, so any storms that do develop on Thursday afternoon could become fairly well-organised for a time - but again this depends on how the upper pattern evolves, and we'll have to wait a few more days to be able to iron out that detail...
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1 hour ago, staplehurst said:
A shower, driven by low-level wind convergence where an easterly wind along the south Pembrokeshire coast meets a northeasterly from the north Pembrokeshire coast, and probably partly aided (initially) by nearby hills too. Many models had a good handle on this feature and the potential for isolated showers to develop here just as they move offshore.
Perhaps bigger potential down in Cornwall with more pronounced sea breeze convergences, and perhaps some orographic uplift from Dartmoor playing a part too.
Cell now developing just offshore from Plymouth/Looe and already produced a couple of lightning strikes - must've gone up in a fair hurry as the latest radar scan (7 min delay) is not particularly impressive! Expect other showers/storms to develop further west across into 'mainland' Cornwall over next few hours...
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4 minutes ago, Zak M said:
What's that building off the coast of SW Wales?
A shower, driven by low-level wind convergence where an easterly wind along the south Pembrokeshire coast meets a northeasterly from the north Pembrokeshire coast, and probably partly aided (initially) by nearby hills too. Many models had a good handle on this feature and the potential for isolated showers to develop here just as they move offshore.
Perhaps bigger potential down in Cornwall with more pronounced sea breeze convergences, and perhaps some orographic uplift from Dartmoor playing a part too.
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Subtle forcing aloft sliding southwards across northern England tonight may engage with the remnant instability over Yorkshire and the north Midlands to maintain or even pep up some of the showers here, perhaps especially after midnight - and so an ongoing risk of a few heavy showers developing 'out of the blue' with the odd lightning strike possible, probably weakening towards the end of the night.
Plus increasing (weak) elevated instability creeping across the English Channel towards the coast of Kent/Sussex during the night, although models vary on whether showery precip will break out or not but a few isolated lightning strikes not totally ruled out across the eastern English Channel overnight.
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3 minutes ago, AMeltedFlake said:
Main risk areas tomorrow?
Wales, N + W Midlands, NE England, Cumbria and more of Ireland
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Just now, Zak M said:
Is the BBC really going for this?
well, the EURO4 is...
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4 minutes ago, Paul Sherman said:
BBC really getting on board with Saturday showing a rash of Showers on some of their forecasts now (Pretty sure they get their data from Meteogroup)
Whats your early thoughts on this Mr Holley ?
They'll be using the EC to drive their graphics. Most models are in reasonable agreement for a scattering of showers and a few thunderstorms on Saturday afternoon / evening, particularly north of the M4 - but still a few days to iron out that detail. Might need to keep an eye on what comes out of northern France Saturday night too, but that will depend heavily on the upper pattern and how quickly the cold front moves south, so more confident on daytime showers and less confident on any weakening convection over the Channel before the northerly comes crashing in.
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7 minutes ago, Zak M said:
That might be the smallest SLGT I have ever seen
Certainly rivals this one!
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14 minutes ago, Zak M said:
Surprised at that storm in the Channel Islands - formed very quickly with very frequent lightning and only 13-14c temps there.
They are quite likely elevated - cloud bases are 3,000ft which is around 890mb, riding atop the warm nose that is present just above the cool surface easterly undercut
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Any line convection today is likely to be a lot shallower than past events - these are vertical cross-sections from model output for the squall lines associated with Storm Ciara (left), Storm Dennis (middle) and today (right). This is, of course, based on model data so may not have captured the vertical structure perfectly, but gives an idea at just how shallow today's convection will be vs previous events, as @Mapantz has alluded to...
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On 13/10/2019 at 08:15, Downburst said:
It would be better if the cycle was always referred to a Solar Sunspot Maxima or Minima, or some way to emphasize the sun spot. I’ve always worried about the general public thinking, oh a weak sun etc. Not saying it doesn’t contribute to weather, but the certainty in some people’s mind even given the facts such as you’ve shown. Better off praying for global impacting volcanoes as they seem to have more real frequent correlation to cooling on a global scale. Still the Thames was frozen......
Here's how UK winters (DJF) compare to the sunspot cycle historically. Of course events such as the Beast from the East were only 1 week long, so have less impact on a 3-month average etc
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8 minutes ago, Mitch perrott said:
do we call this a bust?
depends on your initial expectations...
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That is the issue with a sparse (official) observation network. If you used just the Met Office stations then the wettest location was Tibenham with 47mm, if you include Environment Agency gauges then 68mm near Harleston comes in, and if you include unofficial PWS then Blofield with near 100mm would lead the way - but it is difficult to verify these reports as we don't know how reliable a given rain gauge is.
Naturally rainfall varies significantly over just a few miles, so it will be very difficult to give an accurate picture of how much has fallen in a given location - even radar coverage in this part of the world is rather poor, we are far from any radar sites, and so the beam is quite high and there is a lot of overlapping in the composite data with other radars, so even a radar-estimate wouldn't be ideal (and probably underestimate).
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11 minutes ago, zmstorm said:
I JUST SAW A FLASH OUT IN THE SEA WTH
you must have a good view from that far inland
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1 hour ago, Mapantz said:
I'm not sure about the channel lighting up?!
Mainly after midnight, and moreso towards the very end of the night into the morning 'proper' - though should add this is not a plume event, this is typical autumn surface-based convection over warm seas that then develop more widely inland as surface temperatures warm through the day on Monday. It will be hit-and-miss, not widespread.
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5 minutes ago, AMeltedFlake said:
How will this event tonight compare to Tuesday 23rd July? Less chance of storms this evening?
overall a lot less lightning and more hit-and-miss I suspect
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Storms and Convective discussion - 10th April 2020 onwards
in Storms & Severe Weather
Posted
Only really in the 10-13z period, after that there's too much mid-level capping