Storm Forecast
Valid: Mon 16 Nov 2009 06:00 to Tue 17 Nov 2009 06:00 UTC
Issued: Sun 15 Nov 2009 22:22
Forecaster: SCHLENCZEK
A level 1 has been issued for Portugal and W Spain mainly for excessive rainfall and severe gusts and to a lesser extent for tornadoes.
SYNOPSIS
An upper longwave trough over the central N Atlantic will shift eastward during the period. Ahead of this trough, a 90 m/s jet streak stretches from extreme NW Iberia towards N Germany. In the vicinity of this jet streak, a developing surface low centered NW of Scotland will move northeastward. The cold front of this low will cross western France in the early morning and move eastward while weakening. Most showers and thunderstorms on Monday should be associated with the upper trough / cold front. Further upstream, a plume of warm and moist air is advected from the Canary Islands towards SW Iberia with cyclogenesis expected on Tuesday near the Bay of Biscay. An upper cold core over the Norwegian Sea will lead to unsettled conditions in parts of Scandinavia.
Most parts of southern Europe will see quiescent conditions on Monday and Tuesday as an upper ridge overspreads the western / central Mediterranean. Remnants of an upper trough over the E Mediterranean / Black Sea are forecast to shift eastward, leading to thunderstorms over the Black Sea and the E Mediterranean.
DISCUSSION
... SE Ireland, Irish Sea, British Channel, SW North Sea...
A few hundred J/kg of CAPE are forecast along the northern parts of the cold front and also near the upper cold core of the surface low near Scotland. A narrow multicell line may develop along the cold front in a strongly sheared environment (DLS around 30 m/s, LLS around 15 m/s) and an isolated severe gust cannot be ruled out. A strong gradient flow may allow some strong / severe gusts in the wake of the cold front which are mostly non-convective. Overall threat is too marginal for a level 1.
... W Iberia...
Ahead of an upper longwave trough, warm and moist air is advected into Portugal and W / SW Spain. MLCAPEs in order of 500 J/kg are forecast and multicells may form in a strongly sheared environment (LLS around 15 m/s, DLS in order of 25 m/s). As most of the shear is confined to the lowest 3 km, storms will likely organise into small multicellular lines or a MCS. The main threat should be excessive rainfall as storms will likely affect the region for several hours. Intense winds at lower levels (20 m/s at 850 hPa) are forecast and severe wind gusts will be likely. Locally, SRH1 will reach values between 150 and 300 mē/sē and, combined with low-based storms, an isolated tornado is possible as well. The tornado / severe gust threat will decrease in the evening hours but the heavy rain threat may last until Tuesday morning.
...S Turkey, Cyprus...
Near the axis of the upper trough over the E Mediterranean, a strong upper vort-max will cross S Turkey and Cyprus on Monday morning. Low-end instability and 20 - 25 m/s deep layer shear are present in the jet's left exit region. Southerly / southeasterly SFC winds ahead of the trough will lead to locally enhanced values of SRH3. Most storms are expected to be well-organised multicells or supercells, posing a threat of isolated large hail / severe gusts. The hail / severe gust threat will diminish in the late afternoon / evening hours.
