[Tony Gilbert]

Posted 9.10pm Thurs

Slight Risk of Severe Thunderstorms Central Ireland, Wales, and northern England 11Z-21Z Fri 17th Aug. Prime Risk Severe Gusts

Slight Risk of Tornado Development as per red boxed zones below

Quasi stationary low 980mb west of Ireland draws moist unstable air northwards from Azores. Some late surface troughing expected to develop over Ireland as surface temps increase.For UK the cold front expected to develop initially across Wales where UKMO Fax charts exhibit some surface wave motion which could allow for stronger low level veer of winds. Helicity values predicted up to 250 m/s2 though primarily based on speed shear as opposed to directional shear. With such totals it is nevertheless possible for storms to develop supercell characteristics and the chance of a tornado based on the strong 0-3km shear available. Forecast sounding give uncapped cloud top level. So the stronger upper winds are likely to be fully utilised by any isolated stronger storm development. Convective gust are calculated to peak at around 55kt (severe) based on the mean average winds between 800mb and 600mb. Hail will be possible on the taller isolated storms though with the elevated freezing level I doubt that hail will be the primary risk. With the upper trough only having marginal influence on the the regions effected tomorrow the risk would appear to be associated with the stronger low level lapse rates and dynamic forcing along the cold front itself. It is expected that the NW side of the cold front axis will be over shadowed by heavy cloud cover. It is therefore likely that the strongest solar heating will develop to the SE of the initial convection.Whilst some relatively strong CAPE values are forecast, the upper atmosphere looks saturated reducing potential instability somewhat. Hense I do not expect electrical acivity to be widespread.

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This post has been edited by Tony Gilbert: 16 August 2012 - 20:40

[Tony Gilbert]

Any moderator please change header to Friday the 17th. My usual mistake. Always get tired about now Thanks

This post has been edited by Tony Gilbert: 16 August 2012 - 20:39

[Tony Gilbert]

UPDATE 8.45am

As per earlier forecast with Risk Ireland removed. Risk period for tornado potential is shorted and shifted slightly east. Any tornado risk will be directly associated with a any surface wave development. In short 'No wave then no tornado'. A continued risk of severe gust possible though VV are slightly less than earlier predictions so not sure if many cloud will reach a sufficient height to release a full convective induced downdraft.
Any wave development should become apparent via radar and could well develop a single isolated stronger storm cell. (red box again)

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This post has been edited by Tony Gilbert: 17 August 2012 - 07:52

[Tony Gilbert]

Results for today might be attributed to the following;

Most striking feature today was the lack of surface heating toward the southern most periphery of the cold front. In fact Vis Sat imagery today was quite clear regarding the reason for this restriction with a very heavy mid cloud layer. Below shows Notts sounding 12Z with some basic indication of restricted potential instability but most of all lack of low level heating due to cloud cover which was the likely to be the prime restrictive factor here!.

This aside the shear potential was more than workable had the conditions angled that way!

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This post has been edited by Tony Gilbert: 17 August 2012 - 22:04