Storm Forecast
Valid: Sun 20 Apr 2008 06:00 to Mon 21 Apr 2008 06:00 UTC
Issued: Sat 19 Apr 2008 22:01
Forecaster: SCHLENCZEK
SYNOPSIS
Blocking pattern with an upper high over Iceland and an upper low pressure complex over southwestern Europe will likely continue for some more days. Farther downstream of this low, an impressive EML with mid level lapse rates around 8 K/km and temperatures between 15 and 25°C at 850 hPa is advected towards the Aegean Sea / Greece / western Turkey but an upper ridge over the eastern Mediterranean should suppress convection in this hot airmass. Ahead of a 60 m/s upper level jet streak, warm and moist air should overspread the western Mediterranean Sea and southern France. A shortwave trough over the eastern Ukraine will move northeastward, leaving the forecast area in the first half of the period.
A frontal boundary is stretching from eastern Poland via Belarus towards western Russia, separating warm and moist air to the south from cold and rather dry air to the north. Over the eastern Iberian Peninsula, a rather weak cold front should move eastward during the day.
DISCUSSION
...Eastern / southeastern Spain... western Mediterranean... Balearic Islands...
Ahead of the upper trough over the Iberian Peninsula, a strong southwesterly low-level flow with more than 15 m/s at 850hPa has established. GFS 12Z predicts some hundred J/kg of CAPE along the cold front and in the trough region as well as strong QG forcing near the left exit region of the 60 m/s jet streak over southern Iberia. Convection should start to develop in the morning / afternoon over eastern Spain. Storms will profit from a good kinematic environment with 25 m/s deep layer shear and a veering profile up to 700hpa which results in 150 - 200 J/kg SRH3. As storm relative upper flow is almost parallel to the front, a severe MCS may evolve over Spain with severe wind gusts and isolated large hail being the main threat. Ahead of the front, cells will likely stay more discrete. Organized multicells and a few supercells may produce large hail and severe gusts. Steep low level lapse rates, good LL buoyancy, low LCL heights and more than 10 m/s low level shear may allow a few tornadoes.
Storms will continue eastward over the Balearic Islands towards Sardinia with a likely decreasing threat of severe weather as the overlap of CAPE and strong deep layer shear is forecast to disappear farther east.
...Southern and central France...
On the eastern flank of the surface low centered over the Bay of Biscay, some hundred J/kg of CAPE should be available over southern and central France around midday. An upper level vort max will provide strong QG forcing and some multicells and possibly a few supercells could develop in an environment with 20 m/s deep layer shear and SRH3 between 200 and 300 J/kg. Given low LCL heights, strong LL shear and good low level buoyancy, a few tornadoes are not ruled out. Main threat should be large hail and isolated severe gusts. During the late afternoon, low level moisture inflow should increase while shear decreases and storms will merge into one or two large systems with decreasing severe threat.
Storms may persist for several hours over some regions and torrential rainfall will lead to an enhanced flash flood threat.
...eastern Ukraine...
Ahead of an upper trough, warm and moist air is advected northward, leading to a few hundred J/kg of CAPE. Some thunderstorms may evolve in the late afternoon / evening. Deep layer shear around 15 m/s and weak forcing may allow a few multicells. Low level shear around 10 m/s and good LL buoyancy suggest that a brief tornado is not ruled out. Isolated large hail is not ruled out either but a threat level is not warranted at the moment.
