[Tony Gilbert]

8.20am Thurs

Moderate Risk of Scattered Thundery Showers with Small Hail 11Z-20Z Thurs 12th April 12 East Eire & much of UK

Small Risk of Funnel Cloud Observations as per Yellow Box

A similar day to yesterday Weds, with a scattering of thundery showers for the said regions. Once again weak upper support and saturated mid and upper levels of the atmosphere will reduce the potential for any severe weather or organisation of convective cells. I have once again added a marker box for a small risk of funnel development with today's atmospheric conditions marginally superior to yesterday. Models predict better surface moisture with lift generated from closer to surface. A convergence zone is expected to develop through the afternoon along the South Downs and up into E Anglia. The risk for Funnel Clouds (as with yesterday) is forecast as 'Slight' in nature; This does not mean that FC's will occur but that it is deemed that there are conditions which are marginally conducive to their development! Though once again a lack of dry air within the lower mid levels reduces the risk somewhat for today. Interesting to note; that the presence of dry air above a layer of moist at the top of the boundary layer causes lapse rates to fall rapidly over a very short vertical height. This in turn increases the speed of an upward rising parcel and hence stronger updrafts are created at much lower levels. The congestus cloud may not necessarily develop any great height or thunder but may posses a strong low level updraft for a short duration. The only additional factor is moisture and dynamic lift that can push the rising parcel into its LFC with some level of vorticity available from below.

Red box indicates an approximate zone of a sharper converging surface vector regarding FCs

PS. anyone living around Colchester/ Ipswich your feedback regarding todays results, if any would be much appreciated!

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This post has been edited by Tony Gilbert: 12 April 2012 - 08:04

[PaulKn]

Looks like some funnels might develop down the east coast of Yorks/Lincs from now until perhaps mid-afternoon, as sea breeze convergence zone is in that area.

[PaulKn]

Look at the current observations rather than model - plenty of sea breeze down the North Sea coast.

[PaulKn]

[John Mason]

I suspect sea-breezes were involved with the Welsh storms today, too. I had visitors so wasn't chasing or anything, but could see that line from the west. At Borth, a due westerly was blowing stiffly, enough to kick up white horses on the sea surface. Inland, clear skies stretched for some 7-8 miles then bits and pieces in front of a large Cb that had Cu towers shooting up quickly to its NW, cells then visibly tracking SE. Returning to Machynlleth, by 1800, this westerly was again evident sat out with friends in the beer-garden although it was pleasantly warm in the sunshine. By 1900 the westerly had suddenly gone and dark-grey murk was seen to be spilling westwards from the main Cb to our east. Within 15 minutes we felt the cool easterly breeze that accompanied what I guess was the outflow-boundary spreading west following 1) the end of primetime heating and thus the sea-breeze and 2) the collapse of updraught-columns; some spots of rain followed soon afterwards.

Had I thought that sea-breezes might be important here today (not normally until May) I'd have taken the visitors chasing. Dunno what they would have made of that!! Key time would have been early afternoon around rain-free bases from past experience. Reports coming in of the mountains around Dinas Mawddwy white with hail. It would have been a tricky one for positioning though: the area of initiation was over the Arans, not known for their easy road-network and westward vantage-points! Anyone walking up there today would have had a hair-raising encounter..... literally!

Cheers - John

[Tony Gilbert]

Chris Lloyd, on 13 April 2012 - 13:37, said:

Let me see where Chandlers Ford is on that model analysis that I posted above - oh yes, right where those convergence line winds are indicated. Nice funnel cloud that.

All credit to Tony because I would not have even looked at the predicted windstreams were it not for the convective thread

It seems Wales had its fair share too yesterday.

Cheers Chris! Though always hard to be sure when predicting funnel clouds because they can be so illusive in nature. Whilst it is of course possible for FCs to form under any congestus cloud where ambient vorticity is ingested, the past decade or so has shown that in fact the majority of reports do in fact tend to suggest that they are more prevalent over areas of convergence or confluence! And even more so if several other conducive atmospheric conditions in place.

All in all it may seem to many that I may throw in a FC potential box maybe too often with my forecast but in reality I am always considering several atmospheric a factors before doing this!