[Tony Gilbert]

9pm Mon

Slight Risk of Isolated Thundery Showers North Eire and South Central UK Tues 29th May 2012

At the moment some uncertainties regarding the influence of an upper ridge over France. UKMO fax chart have likewise drawn (a potential cold front) for southern UK. If the GFS prediction holds together then we could well see isolated thunderstorms break out by mid afternoon, regions as per map. Whilst CAPE would seem less than of late, a combination of better moisture and stronger convergence all look to be factors which may well increase the risk potential locally.

Once again we have a weakly sheared environment where any convection looks to be rather disorganised though certainly some potential for a sea breeze much closer to the coast whereby a small risk will exist for funnel reports.

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[Tony Gilbert]

8.40am Tues UPDATE

As per earlier forecast with UKMO fax chart now translating a small surface trough likely to develop SW UK. Storm steering winds are estimated to be around 8 kts ENE based on the mean average flow between 700 mb and 500mb. In short the storms will be slow moving but capable of relocating themselves toward the ENE. This should allow some recovery of surface temps between each convective cell and hense low level instability could be sustained longer than if there were no upper flow.

GFS and NMM break out prec by late morning though based on the earlier weaker surface moisture I believe the real time might be around 4pm maybe Horsham / W.Sussex. Better confidence in today's outlook remains with a predicted 100% humidity at 900 mb just below cloud base. Therefor the current calculation for ML CAPE ATM would seem more realistic than recent days. Convergence will be the key to the best storm activity. The stronger the sea breeze the harder the surface confluent vector will become!

This post has been edited by Tony Gilbert: 29 May 2012 - 07:49

[Tony Gilbert]

Tony Gilbert, on 29 May 2012 - 07:47, said:

8.40am Tues UPDATE

As per earlier forecast with UKMO fax chart now translating a small surface trough likely to develop SW UK. Storm steering winds are estimated to be around 8 kts ENE based on the mean average flow between 700 mb and 500mb. In short the storms will be slow moving but capable of relocating themselves toward the ENE. This should allow some recovery of surface temps between each convective cell and hense low level instability could be sustained longer than if there were no upper flow.

GFS and NMM break out prec by late morning though based on the earlier weaker surface moisture I believe the real time might be around 4pm maybe Horsham / W.Sussex. Better confidence in today's outlook remains with a predicted 100% humidity at 900 mb just below cloud base. Therefor the current calculation for ML CAPE ATM would seem more realistic than recent days. Convergence will be the key to the best storm activity. The stronger the sea breeze the harder the surface confluent vector will become!

PS. Above should have read trough developing 'SE' UK

[Tony Gilbert]

Please remember that this outlook is posted as a Slight Risk, only. ie there are certainly no guarantees here!

ATM plenty of cumulus around but not seeing any real vertical height. The surface trough is rather weak and we need a forcing trigger to get things going. Portsdown hill looking NE is as good a location as anywhere. The next couple of hrs will either explode into action or not, as the case maybe!

[Tony Gilbert]

12Z Larkhill skew-t sounding below shows a moderate cap at 850 mb though with good CAPE ready to be released right up to the tropapause. Conversely further west the Cambourne ascents give a double cap scenario with an addition capping layer at 800mb. Looking at the cloud locally here I would suggest that most of the cumulus building ATM is initiated from above the boundary layer. Probably due to the fact that an impenetrable layer of warm air as per the cambourne ascent has likely advected eastward this afternoon. The strongest lapse rates are by far within the boundary layer but forcing looks to have been too weak to breach this layer today!

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This post has been edited by Tony Gilbert: 29 May 2012 - 17:33

[PJB]

Analysis of the 11Z Herstmonceux ascent indicated a temp of at least 23.5C was required to break the Cap in SE England. Then only Skinny CAPE was available around 100-150j/.kg but with tops to around 300mb. Temps have reached this level and hence accounts for the small heavy shower seen nr Gatwick.

Wednesday sees much greater moisture and cooling aloft so a more preferential environment for development of showers.

paul