Storm Forecast
Valid: Sun 03 Jan 2010 06:00 to Mon 04 Jan 2010 06:00 UTC
Issued: Sat 02 Jan 2010 23:57
Forecaster: SCHLENCZEK
A level 1 was issued for NW Turkey and the Bosporus region mainly for severe wind gusts and tornadoes.
A level 1 was issued for SW Portugal and SW Spain mainly for excessive rainfall and severe wind gusts and to a lesser extent for tornadoes.
SYNOPSIS
A large upper trough over the central and eastern parts of Europe shifts eastward during the period. The associated surface low over the Bosporus region will be one focus of convective activity on Sunday. Ahead of this low, rather cold but unstable air is advected into W / NW Turkey where an upper vort-max near the left exit of a 60 m/s upper jet streak will provide sufficient forcing for ascent. This feature will move eastward during the day while weakening.
The second area where organised deep moist convection should be most likely on Sunday is SW / W Iberia and the Bay of Biscay. Ahead of a progressive upper trough over the Azores, a 995 hPa surface low has developed and will reach S Portugal in the evening / night hours. Strong QG forcing will lead to convective initiation over the E Atlantic in an environment with several hundred J/kg MLCAPE and thunderstorms will move onshore during the night.
An upper low over N Norway moves southward and will lead to snow showers and weakly electrified thunderstorms over the Norwegian Sea. Strong to severe wind gusts are possible, but most of the wind should originate from the strong pressure gradient in the wake of the surface low. Some non-thundery snow showers are also possible in E / SE Germany, NE Austria and the Czech Republic in the vicinity of another upper cold core.
DISCUSSION
...NW Iberia, Bay of Biscay...
Decreasing LL winds due to the occlusion process and continuous weakening of the upper vort-max over NW Iberia are forecast on Sunday morning. Remnants of the nighttime convective systems will weaken during the morning hours but intense rainfall is still possible. Overall CAPE is quite marginal with 100 - 400 J/kg but 15 - 20 m/s deep layer shear could be sufficient for a small number of organised multicells. Locally, strong wind gusts are expected along the coastline and also offshore.
...SW Iberia...
During the late morning / early afternoon, a plume of unstable air reaches S Spain and SW Portugal. About 500 J/kg moderately capped MLCAPE are forecast and latest GFS runs indicate that the cap should break during the afternoon when the upper trough approaches the region, leading to strong QG forcing. In this environment with 15 - 20 m/s deep layer shear, storms will likely become multicellular, posing a threat of isolated severe gusts and intense rainfall. Especially near the Strait of Gibraltar, SRH3 in order of 300 mē/sē may allow rotating updraughts and a tornado may occur.
...Bosporus region, W Turkey...
As the surface low over the Bosporus region moves further eastward, cold air overspreads SE Europe. MLCAPE is forecast to decrease to around 100 - 300 J/kg but there is still a small region where 25 - 35 m/s deep layer shear are also in place. Storms that stay discrete will have a good chance to become supercellular and may produce strong to severe wind gusts. SRH and LLS are slightly enhanced along the coastline which may allow a tornado. This threat should diminish during the afternoon hours when instability decreases and the storms move into the central parts of Turkey.
