Convective Outlook - Wednesday 30th May 2012
Posted 30 May 2012 - 09:57
At the surface, the UK is covered by a surface pressure col, with high pressure to the north and south and low to the west and east. Surface confluence/convergence zones will be present in this col, accentuated where sea breeze fronts converge with the gradient wind flow/outflow boundaries etc etc. Daytime heating over inland parts of England today, combined with the approach of the shortwave trough(s) should allow a shallow surface low/trough to form (most likely across the Midlands and eastern England this afternoon/evening).
Certainly a greater risk of heavy showers and thunderstorms compared with yesterday, better surface moisture (focussed along convergence lines) and cooler mid/upper temperatures creating better instability and a weaker capping inversion.
HRV satellite imagery shows largely sunny skies currently across much of central and eastern England (following the clearance of extensive low cloud and mist across south-eastern coastal counties). Strong insolation here is helping to lift temperatures quickly, with 20C/21C already being recorded from some inland areas of S England and the Midlands. A band of cloud and intermittent rain lies across N Ireland, W Wales and SW England, marking the approaching trough. Main thunderstorm risk today will be ahead of this feature as it continues to move slowly eastwards and encounters the greater instability/moisture over central and eastern England.
All models suggest increasing CAPE and negative LIs from late morning across central and eastern England (especially the Midlands, Lincs, East Anglia and inland parts of SE England and Cent S England). GFS once again is overdoing the dew points (and therefore CAPE/LIs) for this afternoon. Max surface temps of 23-24C and dewpoints of 14-15C seem more reasonable.
While the surface trough moving eastwards across SW England and Wales into the Midlands and Cent S England will provide some surface wind convergence/confluence, stronger convergence/confluence is likely in a zone from Kent and London northwards across western parts of East Anglia, the N and E Midlands and Lincs during this afternoon and evening. If a shallow surface low forms here then this convergence will intensify, assisted by a sea breeze from the east coast. Therefore, eastern England from Kent to the S Pennines/S Yorkshire looks like being the best location for thunderstorms this afternoon/evening. Areas south and west of the M40 could well experience some heavy showers and thunderstorms for a time, but a freshening westerly breeze late this afternoon/evening and reduced convergence will tend to kill off the potential. Over the NE Midlands, Lincs, E Anglia and southern parts of N England the approach of the second small shortwave and sustained wind convergence should keep heavy showers and a few thunderstorms going quite late into the evening.
Deep layer shear is generally weak, but increases slightly across the Midlands and northern England this evening. Storm organisation will therefore be quite low, but potentially increasing over the N Midlands, Yorks and Lincs this evening.
MAP (see below) shows where I think the greatest risk of a thunderstorm is this afternoon/evening (red box) and where there is a low risk (<10%) of a thunderstorm (blue box).
Modified 06z Larkhill sounding (probably most representative of the atmospheric column in the risk zone), shows fairly dry low levels early this morning (supported by limited cloud growth at the moment). The low levels should warm and moisten during the day (surface obs already verifying this) and mid/upper levels should cool slightly, increasing the CAPE and instability. I have modified the sounding with surface temp of 23C and a dewpoint of 14C, yielding 600-800 J/Kg (estimate) as a result.
This post has been edited by Matt D: 30 May 2012 - 10:38
Posted 30 May 2012 - 11:45
Please use this thread for today's convective forecasts/analysis only. Cheers
Matt, Thanks for the excellent heads up for today's TS potential.
My wife is off on a girls pitch and putt jolly this afternoon in Milton Keynes so I shall be watching the sky with interest and hoping your forcast comes to fruition a few miles to the East of here as you indicate