Convective Outlook - Monday 28th May 2012
Posted 27 May 2012 - 11:34
Posted 27 May 2012 - 11:49
Yes, might as well put up gfs for SB, LI & Wind (10m), again all for 15z Monday.
Posted 27 May 2012 - 18:17
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.
Posted 27 May 2012 - 19:23
Posted 27 May 2012 - 21:03
Posted 27 May 2012 - 22:04
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.
Posted 28 May 2012 - 08:26
Great chasing weather but the risk of flooding from these precip cores is a substantiated risk for those prone to such flash-flood events.
Posted 28 May 2012 - 09:46
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
This post has been edited by Matt D: 28 May 2012 - 09:55
Posted 28 May 2012 - 12:23
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...
Posted 28 May 2012 - 13:22
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.
This post has been edited by Matt D: 28 May 2012 - 13:23