Storm Forecast
Valid: Fri 19 Nov 2010 06:00 to Sat 20 Nov 2010 06:00 UTC
Issued: Thu 18 Nov 2010 19:54
Forecaster: SCHLENCZEK
A level 1 was issued for extreme NW Iberia mainly for severe wind gusts and tornadoes and to a lesser extent for marginally severe hail.
SYNOPSIS
High pressure over Scandinavia and Russia will lead to stable conditions over the eastern parts of Europe. Most convection on Friday should be associated with a developing low pressure system west of Iberia. Low geopotential heights are also located over the north-central Mediterranean. Overall temperature and pressure gradients are rather low and therefore significant severe weather is not expected.
DISCUSSION
...Scottish Sea and Scotland / N Ireland...
Ahead of the upper cold core of a low pressure system west of the British Isles, marginal deep layer shear and a few hundred J/kg CAPE are forecast. EL temperatures should be in a range of -10 to -20°C and lightning is probably too sparse to issue a low-probability thunderstorm area.
...Channel region, Bay of Biscay, NW Iberia...
Near the southern tip of the upper trough, cyclogenesis is expected over the E Atlantic close to the left exit of a 50 m/s upper jet streak. Some hundred J/kg CAPE ahead of the upper vort-max may allow showers and thunderstorms that may produce isolated severe wind gusts as low level winds reach 20 - 25 m/s over NW Iberia. GFS shows T/Td around 14/12 °C over NW Iberia near the cold front which leads to LCL heights around 500m AGL. Combined with locally enhanced SRH1 (150 - 250 m²/s²) and LL shear in order of 12 - 15 m/s, a few tornadoes cannot be ruled out. Instability tends to decrease towards the south where shear is somewhat stronger and therefore only a small level 1 region is forecast.
...Balearic Islands, N Mediterranean, Tyrrhenian Sea...
Some hundred J/kg CAPE may be created over the Mediterranean near the upper jet streak in an environment with weak upper level forcing / LL convergence. In the evening / night hours, multicells may develop and could produce some isolated hail and wind gusts. Overall threat is probably too marginal for a level 1.
...Adriatic, Balkans, Ionian / Aegean Sea...
A similar situation is also expected for the regions further east during the afternoon / evening hours. Locally, 15 - 20 m/s deep layer shear and 200 - 400 J/kg CAPE will overlap and some storms may become more organized. An isolated severe hail / wind gust event cannot be discounted but a threat level is not warranted.
