Storm Forecast
Valid: Wed 29 Aug 2012 06:00 to Thu 30 Aug 2012 06:00 UTC
Issued: Wed 29 Aug 2012 07:21
Forecaster: PUCIK

A level 1 was issued for southern UK mainly for tornadoes and severe wind gusts.

A level 1 was issued mainly for Northern Spain, Southern France and Massif Central mainly for large hail and severe wind gusts.

SYNOPSIS

A quite wavy pattern has established over Europe, with deep trough over the Atlantic, a ridge over Central parts of Europe and again, a trough (transforming into a cut-off low) centered to the north of the Black Sea. Especially towards the end of the forecast period, the Atlantic trough should begin its eastward movement with attendant frontal system passing over parts of Western Europe. Closer to the surface, rather deep low pressure systems are forecast over the British Isles and over eastern Ukraine, slowly filling in during the day. A string of high pressure systems stretches from Scandinavia through Central Europe towards the Azorian high. The main frontal zone during the day will occupy Northern Iberia and France, another already occludded one is affecting area of British Isles.

DISCUSSION

... Southern UK ...

Some window for isolated severe threat might exist over this area as the trough with attendant frontal system spread eastwards. The main time-period should be probably the beggining of the forecast period, possibly between 9 and 12 Z as the cyclonically curved jet-streak favorably moves over the region, with some attendant forcing. Consequently, some marginal instability with more than 20 m/s of DLS will be available in the region. Sounding profiles from 00 UTC also suggest quite strong LLS, around 10 m/s and enhanced SREH, as south-southeasterly surface flow underlies the mid and upper level southwesterlies. The biggest limiting factor might be the fact that instability will be marginal and also the factor of embedded storms within larger scale precip. Nevertheless, isolated severe wind gusts and tornadoes are forecast in case that supercells manage to develop in this setup.

... Northern Spain towards Massif Central ...

An EML has spread towards northeast in the southwesterly flow and is well sampled by Nimmes and Barcelona soundings. Barcelona 00 UTC measurement has over 2000 J/kg of MLCAPE, but strongly capped and favorable low level veering. This veering is also nicely visible in Nimmes measurement, albeit CAPE was lower in this case. Generally speaking, moderate to strong instability should be present over the region, with the highest values towards the southeast (Ebro valley), but as usual, CIN will also be the highest here. Moderate to strong DLS with low level wind veering with height contributing to the enhanced SREH should be available for enhancement of storm organisation. Well organised multicells or even supercells are possible, posing threats of large hail and severe wind gusts. High LCLs will limit the excessive precipitation threat although over Massif Central, backbuilding in case of multicells might lead to higher precip accumulations, especially later into the forecast period.