[Uskys]

To get the ball rolling.........

Possibility of Thunderstorms over a large part of England & Scotland , around aline say from London to Edinburgh.

Chris Fawkes suggests there is a small chance of severe thunderstorms.

MLCAPE for 15z Monday:

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[CgStrikeMK]

A lot of SB Cape in this region as well Monday afternoon, along with a strong area of wind convergence / helicity in an area covering Herts northwards upto Leicestershire could see some very big sustained slow moving storms if they trigger.

[Uskys]

CgStrikeMK, on 27 May 2012 - 11:34, said:

A lot of SB Cape in this region as well Monday afternoon, along with a strong area of wind convergence / helicity in an area covering Herts northwards upto Leicestershire could see some very big sustained slow moving storms if they trigger.

Yes, might as well put up gfs for SB, LI & Wind (10m), again all for 15z Monday.

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[Matt D]

Still reasonably good agreement of a 'corridor' of maximised surface wind convergence/confluence between Kent/London/Essex/N and E Midlands and NW England during Monday daytime. Strong surface heating, increasing amounts of low level moisture along the narrow 'corridor' of wind convergence and slight cooling of mid/upper temperatures through the day as the upper ridging weakens *should* allow a scattering of heavy showers and thunderstorms to develop.

Limited upper level forcing and weak wind shear though, with gentle flow at mid/upper levels (due to the proximity of the upper ridge overhead), will limit storm organisation and longevity so a series of pulse storms seem the most likely mode. Gusty winds and torrential downpours with hail and lightning are certainly possible in the stronger cells, with storm motion likely to be very slow so high spot rainfall totals should result.

My pick of an area to choose for highest storm risk would be London/Essex/Herts/Bedfordshire, with persistent strong convergence here though the day (E flow, assisted by sea breeze front, against a WSWly flow developing eastwards from Cent S England during the day).

As ever, GFS dew point predictions of 18-19C in the convergence zone seem TOO high. I would suggest dewpoints of 15-16C along the convergence zone, but with temps of 25-27C, this will still yield a few 100 J/Kg of CAPE, perhaps capped until mid-afternoon.

[PJB]

The KNMI Hirlam develops a convergence zoone from the NW Midlands, down the M1 towards London. The NAE agrees largely with this zone, with both models targetting the London area and surrounding N Home COunties as the best areas for a zone of slow moving storms to develop. Moisture is not great in the lower Mid Layers and thus profiles will be quite capped until sufficient heat is obtained and convergence focuses moisture. The T+30 NAE Forecast Ascent for Brize Norton indicates CAPE already in existance at 1pm and as Temps rise through the afternoon to 22-25C larger CAPE develops.

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[AndrewJM]

If a storm were to fire in say Herts which direction would it drift?

[Tony Gilbert]

Certainly some good potential for scattered thundery showers as mentioned above. Initial activity looks to be slightly elevated though later in the period as dew points increase we should see some surface based development. Probably a bit messy but some good scope for intense sferic returns! Particularly so just NW of London.

[Matt D]

AndrewJM, on 27 May 2012 - 20:56, said:

If a storm were to fire in say Herts which direction would it drift?

NNW. Though flow is very weak and individual cells likely to be quite disorganised and slow moving. *IF* any do develop they are likely to be big rain producers with some quite frequent lightning as cells reach maturity.

[Uskys]

gfs 00z sb cape / ml cape 15z today

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[JOKE_SNOW]

Remnants of the ever dissipating CF evident on vis Satellite today. Were looking ahead of this however for where storm potential initiates as the CF provides enough lift through forcing alone that developments can finally sustain updraughts and inflows long enough to break whatever little capping is left. Moisture injection along with +1300kj of CAPE is enough to see strong cells anywhere north-south from the Cheshire Plain right down to Essex. The emphasis being these storms won't be moving anywhere fast and barely cross the Pennines by midnight tonight since initiation being progged for early this afternoon.
Great chasing weather but the risk of flooding from these precip cores is a substantiated risk for those prone to such flash-flood events.

[Matt D]

Beware the GFS overdoing the dew points (a chronic and common problem in this type of set up). At 09z, it is already 3-4C too high with its dew point predictions through inland parts of England. Dew points of 13C to 14C are more typical within the expected development zone (note Charlwood has a 14C dewpoint at 09z).

Overestimated dew points obviously bring an overestimated forecast of CAPE and LI values too; the 900-1200 J/Kg suggested by GFS is likely to be a bit too high. Nevertheless, analysing the 00z Herstmonceux sounding (which is likely to be the most representative of environment through the storm risk zone this afternoon), with a surface temperature of 25C and dew point of 13C (quite realistic values for this afternoon in the convergence zone, IMO), then some reasonable CAPE (estimate 600-900 J/Kg) and negative LIs are likely (see plot below).

This analysis increases my confidence, somewhat, that we will see some convective development - even if it will be quite isolated in nature. Caveats are that the boundary layer moisture is pretty shallow in depth, with fairly dry air in the 950mb-800mb layer (look at the sounding for Herstmonceux below). This is where Mixed Layer CAPE prediction comes into its own, taking account of the dry air immediately above the surface and giving a more realistic impression on how much potential energy is available to growing updrafts/Cbs. Dry air may 'mix out' some of the near surface moisture during the day as surface heating increases the overturning in the boundary layer. This is why I think the GFS forecast dew points are very unreaslistic for today!

Things to watch during the next few hours are:

1. Surface temps and dew point obs, in order to find the best overlap of heat and moisture: http://www.weatheron...kuk&SORT=3&UD=0

2. Surface wind obs to fine tune the location of best surface wind convergence and verify the model predictions on this: http://meteocentre.c...&lang=en&map=UK

3. Watch the HRV imagery closely for signs of Cu development in the convergence zone - should be a little way ahead of the decaying mid level cloud from the cold front and probably (at least initially) close to London where surface temps are rising most rapidly at the moment. Once surface temps reach 24C then we should start to see some Cu building: http://www.sat24.com/en/gb

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This post has been edited by Matt D: 28 May 2012 - 09:55

[Dave W]

Yes its about time GFS fixed those parameters - its been rather off with a lot of its surface parameter temp /humidity forecasts for some time now..

[Uskys]

Nina Ridge's forecast for this afternoon also mentioned the risk of isolated Ts around the coast of Northern Northern Ireland.

[Matt D]

Surface temps of 24-26C now across the Greater London area and towards inland Essex/Kent and the Thames Estuary, with dew points of 14-15C. Coupled with converging surface winds in these areas, assisted by a sea breeze from the east, Cu are starting to develop quite quickly (as shown by the latest few frames of the HRV imagery). As reported in the chat thread, Cu are also developing further north, especially across the Peak District/Pennines.

Over the next few hours, peak heating combined with stronger moisture pooling along the convergence zone (sea breeze from the east should augment this) should see a few heavy showers and storms develop. N Kent/London/Essex/Herts looks a good spot...

[PJB]

The 12Z nottingham Ascent indicates reasonable MLCAPE once temps reach 25-26C. However will require focussed ascent from convergence to trigger substained convection as the mid layers are dry to some depth.

[Matt D]

Surface wind obs are showing a freshening westerly breeze across many inland parts of S England (to the north of a weak ridge that extends north from NW France). This has recently reached S and W London now, while N and E London remains in a slack/south-easterly flow.

http://www.xcweather.co.uk/

HRV imagery shows a line of towering Cu extending NW from mid-Kent across E London and further north into W Essex and Herts. Further Cu growth is occurring over N Wales and over the higher ground in the Midlands and especially N England. Similar cloud growth is occurring across parts of the Scottish Highland (north of the Central Belt).

For London, I think storm chances are pretty low in the west, now the westerly breeze has arrived. This has been handled fairly well by the models. As commented in the Chat thread, points just east and north of London look more at risk now, especially as the LHI will be advected slowly eastwards against the oncoming easterly flow.

The map below shows where I think there is a risk of a thunderstorm for the rest of the afternoon/evening. Bear in mind that any storms will be isolated and slow moving, with many places in these 2 boxes also missing out too! Where storms do occur, some very high rainfall rates are likely with quite frequent lightning.

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This post has been edited by Matt D: 28 May 2012 - 13:23