Storm Forecast
Valid: Sun 18 Apr 2010 06:00 to Mon 19 Apr 2010 06:00 UTC
Issued: Sat 17 Apr 2010 23:15
Forecaster: SCHLENCZEK
A level 1 was issued for NE Algeria / N Tunisia mainly for large hail and tornadoes and to a lesser extent for excessive rainfall and severe wind gusts.
A level 1 was issued for parts of the Tyrrhenian Sea and Sardinia / Sicily / S Italy mainly for excessive rainfall and to a lesser extent for tornadoes and severe wind gusts.
SYNOPSIS
A low pressure system centered over N Scandinavia will move northeastward during the period. Between this low and an upper high over the North Atlantic, cold air is advected southward towards the North Sea / British Isles. The surface high over eastern Europe shifts eastward towards SW Russia. Some showers and thunderstorms may evolve in a slightly unstable environment over N Sweden and N Norway and also in the warmer airmass over E France and SW Germany. Severe weather is very unlikely as shear and instability are marginal.
The Mediterranean region should be more active on Sunday as an upper trough affects the SW / S Mediterranean from the Iberian Peninsula towards central Italy. In the vicinity of a cold front, a 40 m/s upper jet streak stretches from SW Portugal towards Sicily. Moderate instability is forecast over Tunisia and Sicily in a region with enhanced deep layer shear, and also over the Iberian Peninsula in a weakly sheared environment.
DISCUSSION
... NE Algeria, N Tunisia, Tyrrhenian Sea, SE Sardinia, Sicily...
Ahead of the approaching upper trough, advection of warm and moist air will create moderate instability ( ~ 1000 J/kg MLCAPE) during the morning / afternoon hours. Some QG forcing near the upper vort-max should be sufficient for initiation and thunderstorms will develop in an environment with 20 - 25 m/s deep layer shear. SRH3 should be locally augmented and a few mesocyclones may develop which will increase the large hail threat. LCL / LFC heights around 800m AGL and locally enhanced 8 - 12 m/s LLS may allow an isolated tornado. Further south, the cloud base should be significantly higher and large hail / severe wind gusts will be the main threat. The area with the greatest threat of severe weather will move to the east- northeast, reaching Sicily in the late evening / night hours and parts of southern Italy on early Monday morning. On their way to the east, storms will tend to merge into one or two large systems which may lead to heavy convective rainfall over parts of Sicily, S Italy and Sardinia.
...Spain and Portugal...
In the calm region of the upper cold core, diurnal heating will lead to some 500 - 1000 J/kg almost uncapped CAPE and thunderstorms will develop during the day. The absence of significant vertical wind shear should preclude severe weather but an isolated large hail / tornado event is not ruled out. Peak activity is expected in the early afternoon and most of the storms will dissipate rapidly after sunset.
...E France, SW Germany, Switzerland...
Cold upper air (T500 around -25°C) is still present over parts of France and Germany. During Sunday afternoon, some slight WAA and diurnal heating may lead to a few hundred J/kg CAPE which will support isolated showers and thunderstorms with some marginally severe hail and gusty winds. Locally, heavy rainfall is possible as the storms will remain almost stationary. After sunset, storms will likely dissipate.
