Storm Forecast
Valid: Sat 04 Apr 2009 06:00 to Sun 05 Apr 2009 06:00 UTC
Issued: Fri 03 Apr 2009 23:33
Forecaster: SCHLENCZEK
SYNOPSIS
A weakening upper low over the western Mediterranean will travel eastward, affecting the Tyrrhenian Sea and parts of Italy on Sunday morning. Over the North Sea, an upper trough near southern England moves northeastward towards Denmark. Ahead of the trough, a 35 m/s jet streak stretches from Wales towards Iceland on Saturday 06 UTC. Two well-defined frontal boundaries are present in the southern parts of Europe: A cold front over the Bay of Biscay is forecast to cross western Iberia during the period and another frontal zone over the eastern Mediterranean moves towards Egypt. In most places where instability is present, there is almost no vertical shear. Best chances for thunderstorms are expected over the central parts of Europe. In NE and SW Europe, high geopotential heights will likely suppress convection.
DISCUSSION
...Balearic Islands, NE Algeria, N Tunisia...
In the wake of the trough, upper level cooling will lead to a few hundred J/kg of CAPE in a weakly sheared environment. Overall forcing should be weak and a few thunderstorms may develop. Given quite steep LL lapse rates and a weak LL flow, an isolated waterspout is not ruled out in the sea region between the Balearic Islands and NE Algeria.
...S-central Italy, N Balkans...
Conditions for deep moist convection will be quite similar as described in the western Mediterranean region. Diurnal heating may lead to a few hundred J/kg of CAPE and some weakly organized thunderstorms will develop. As the storms move rather slowly, some of them could lead to heavy precipitation in some places.
...Aegean / Ionian Sea, S Greece...
There is a small region near Crete where about 15 m/s deep layer shear as well as some hundred J/kg of CAPE are in place. An upper vort-max will move from Libya towards Greece and SW Turkey in the afternoon / evening hours and forcing should be sufficient for convective initiation. Most of the shear is within the higher levels and therefore some forward-propagating multicells or perhaps a small MCS should be most likely. The background flow around 700hPa is rather weak but nevertheless a few severe gusts are not ruled out. Most parts of the region are not shown in the forecast map and therefore a general thunderstorm area is drawn (the LVL1 area would be further south).
...SE France, W Switzerland, parts of Germany, W Czech Rep...
Over Central Europe, a warm and moist airmass with no well-defined frontal boundaries around the tongue of 40°C Theta-E is present. As there is not much vegetation right now, the estimation of the boundary layer moisture is quite tricky. In some parts of western Germany, dewpoints already exceeded 10°C on Friday afternoon. GFS predicts dewpoints around 10°C for parts of eastern and central Germany on Saturday with maximum temperatures around 20 - 24°C. This should be enough for some marginal CAPE in eastern-central Germany. As there is no real upper level feature that could provide some forcing, convective initiation will depend on mesoscale convergence zones which are often associated with orographic features. GFS shows some CAPE over almost the entire eastern half of Germany whereas WRF has a small maximum over northern Germany and a second one over W Czech. Probably, the approaching cold front from the North Sea is slower than anticipated by the 12 UTC runs of GFS and WRF which will shift the best conditions for thunderstorms further to the west. Except for an isolated severe gust / hail event, severe weather is not expected.
