Extended Forecast
Valid: Mon 26 May 2008 06:00 to Tue 27 May 2008 06:00 UTC
Issued: Sat 24 May 2008 15:19
Forecaster: TUSCHY
SYNOPSIS
Large - scale trough over Spain and Portugal dictates the weather over most parts of Europe. Scores of speed maxima at 300 hPa rotate around this feature which keeps its neutral tilt during the forecast period. As a consequence, a highly meridional flow establishes downstream and a very warm / hot airmass overspreads the central Mediterranean and as far north as Austria and the Czech Republic during the end of the forecast period with reading between 13 to 16 °C at 850 hPa. This strong WAA backs strong ridging over the central Mediterranean where hot and dry conditions prevail. No major changes for the rest of Europe as cold / stable conditions can be found over far northern Europe, spreading southeastwards.
DISCUSSION
.... Parts of the western Mediterranean including the Alboran Channel, the Balearic Islands, the Balearic Sea and the Gulf of Lion...
A quite potent setup for this part of the forecast area featuring a developing African cyclone south of the Atlas mountains, moving northwards and a coupled mid - / upper - level jet structure, developing during the evening / night hours as a strong streak rounds the base of the trough while another streak exists the area towards the north. This results in a broad area with strong lift which gets only stronger after 21Z as a compact vorticity maximum enters the area from the south, reaching the Balearic Islands after 03Z. Finally we have to deal with a stalling north - south aligned frontal boundary crossing the Balearic Islands during the daytime hours. As surface depression / upper - level vorticity maximum approaches later - on, this front retreats westwards. So plenty of forcing mechanisms are present in an uncapped environment. Shear is very strong as DLS exceeds 25 - 30 m/s and strong LLS ( dependant on final strength of the surface depression ) is present as well. Expect a cluster of thunderstorms to evolve with all severe weather modes possible, especially severe to damaging wind gusts . The more difficult part of this forecast will be to pinpoint when dry desert airmass, blown offshore by strong Sirocco event, gets mixed enough for deep convection. SSTs in the upper tens should support quite rapid moistening and hence we decided to extend the highlighted area far towards the south.
A broad level - 1 area was issued as final degree of instability release and strength of the surface depression are still uncertain but an upgrade of parts of the level - 1 area may be possible.
... France, the Low Countries and parts of Germany ...
At the time of writing ( 24th May ), GFS is already running 2 - 3 °C too high regarding surface dewpoints and current thinking is that this trend extends well into the extended forecast time - frame, where spots of near 20 °C area forecast by GFS. Regarding the moisture pool in the western Mediterranean / surrounding areas with readings in the mid tens, we think that widespread 14 - 17°C will be the most probable scenario, so we disregard the 1kJ/kg and more CAPE islands e.g. over SE Germany / NW France, expecting only low - end instability values in the range of 100 - 400 J/kg. Shear would be strong, especially over France, but for now we will stick with the BOLAM output, showing only a few convective signals. The main area which has to be monitored for initiation will be S / central / E - Germany, where mid - level lapse rates are steep and DLS more than adequate for a large hail risk. Latest model output keeps BL too cold / 850 hPa layer too warm for initiation.
During the night hours, an area of stratiform rain with embedded thunderstorms spreads northward from SE / S France but severe weather risk should be limited due to marginal instability release.
... SE Europe ...
A few diurnal driven and non severe thunderstorms are forecast. Moderate DLS and some veering could support an isolated large hail report with stronger pulsating thunderstorms but coverage is too low for warranting any level areas.
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