[Tony Gilbert]

9.30pm Sat

Slight Risk of Thundery Showers with Hail Southern UK 11Z-17Z Sun 18th March 12

Weak surface troughing sags southward through Sunday initiating another round of showers with small hail primarily through the afternoon as steep upper troughing advects eastward. Slight to moderate risk of some showers turning thundery though I do not expect this to be wide spread in nature. Weak upper support and saturated mid levels expected to impede any lifted parcel. Though overall vertical lapse rate looks sufficient to overcome this restriction if pockets of increased insolation can occur. Some indication that a west to east line of convergence Mid Dorset through to Sussex could develop for a time increasing forcing at surface. With the gradual increase of lapse rates across this zone I do not rule out the odd early season funnel cloud observation though ATM lack of potential instability might suggest otherwise. Certainly good potential for some photogenic anvils with a clear horizon back drop, maybe!

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This post has been edited by Tony Gilbert: 17 March 2012 - 21:44

[Tony Gilbert]

UPDATE 9.50am Sun

As per earlier forecast with additional risk as follows;-

Slight Risk of Funnel Cloud Observations W.Sussex through to Kent 12Z-17Z

Based primarily on the GFS and NMM models there would be some better confidence for thunder today across southern UK. Additional boxes are added into Cornwall and S.Wales as per map below. Lifted Index expected to peak at around -2 and upper trough 2 degC cooler at 500mb compared with earlier expectations. GFS advects a layer of dry air at mid level across SE regions increasing potential instability for this zone. Hence a risk would now become more likely for funnel cloud development where surface convergence is strongest.

Vertical shear remains very weak so the best activity looks to occur early on as surface temps peak and the first signs of convection lift skyward. We can expect rapid cooling at surface from this point onwards where CAPE's will rapidly diminish. Surface moisture is somewhat limited today but should be enough to work with. Models suggest a drop in humidity values just below cloud base which will typically develop smooth charcoal dark cloud bases. This is a result of up/down convergence.

A great day for storm chasing for those within the convective box!

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This post has been edited by Tony Gilbert: 18 March 2012 - 10:02

[PJB]

Satellite Imagery shows a good deal of Vertical Cu Developing, though it appears quite flat, which indicates that much vertical development is going to be difficult to achieve. Radar Indicates a couple of sharpish echoes developing over Salisbury Plain, and a few other lighter echoes over the West COuntry. Scattered Heavy Showers south of the front seems to be the best call. The Hirlam favours Dorset across to Essex and SE wards. The NAE is not really interested in anything much outside the main frontal zone to the north.

Paul

[Dave W]

Certainly some beefy showers developing around Derby/Lincs area now..

[Tony Gilbert]

UPDATE 2.36pm Sun

Surface obs currently show very little convergence ATM. Lack of convergence will certainly reduce today's potential for southern regions regarding lifting any parcel into its LFC!
Whilst some Towering cumulus and small hail has occurred, some concerns regarding actual thunder today now. Next couple of ours or not at all!

This post has been edited by Tony Gilbert: 18 March 2012 - 14:41