Storm Forecast
Valid: Sat 28 Apr 2007 06:00 to Sun 29 Apr 2007 06:00 UTC
Issued: Sat 28 Apr 2007 00:17
Forecaster: DAHL
SYNOPSIS
High-amplitude upper flow pattern persists across Europe ... with a further amplifying upper ridge over the North Sea and the Norwegian Sea, and a deep upper trough off the European W coast over the E Atlantic. Downstream of the upper ridge, a longwave trough exists over Scandinavia/NE Europe/NW Russia, which is expected to dig slowly southeastwards during the period. Two upper cut-off lows, associated with a southern branch of the upper frontal zone, are present over the Iberian Peninsula and the Aegean Region, respectively. These latter features will be accompanied by weak/small SFC lows. Except for an extensive SFC high pressure area over the northern part of the forecast region, quiescent SFC conditions prevail. Weakly unstable, warm/moist air masses are present over SW and W-central Europe, while a southward advection of polar air persists over NE Europe and NW Russia at the E periphery of the SFC high.
DISCUSSION
... Iberian Peninsula ... France ... Benelux ... Switzerland ... W Germany ...
Scattered TSTMS should form in weakly unstable and weakly capped air mass over France, Spain, Benelux, the W Alpine region, and extreme W Germany, mainly in response to diabatic SFC heating. Some profiles from France and Spain from Friday afternoon featured rather dry/deep boundary layers, which suggest that some potential for very gusty outflow winds existed. Similar environments, and an attendant wind threat should exist also on Saturday. However, given minimal vertical shear, minimal CAPE, and only marginally favorable thermodynamic profiles for downbursts, a categorical risk is not necessary at the moment.
Better large-scale support for upward vertical motions should overspread southern Spain and move into the SW Mediterranean ahead of the upper cut-off low. The accompanying, comparatively strong shear profiles are expected to remain offset from the regions of positive CAPE however, and it does not seem that a significant severe thunderstorm threat exists. However, storms may be quite numerous, and deep CBLs may be supportive of cold-pool formation and an attendant downburst threat. Again, threat should be too low to justify a level-one area.
... Turkey ...
Isolated high-based storms may occur over W Turkey ahead of upper vort max in the late afternoon hours. Thermodynamic profiles should be more supportive of strong downdrafts than those over W/central Europe given deeper/dryer CBLs. Also, deep-layer shear should be on the order of 15 to 20 m/s. However, signals in the prognostic CAPE fields are quite weak, and confidence in widespread TSTM development is somewhat limited at the moment. Should the storm coverage turn out to be greater than currently anticipated, an upgrade to level one may be warranted with later outlooks.
... Balkan States ... parts of east-central Europe ...
It does seem that isolated late-afternoon thunderstorms may also develop over the N Czech Republic and N Slovakia, as well as over the mountainous terrain of the Balkans. Simulated instability and QPF fields are quite weak at the moment, but a TSTM area is introduced nonetheless. Should storms survive into the night over the Czech Republic and Slovakia, some hail/wind threat would exist as deep shear is expected to increase in response to an approaching upper vort max. This feature may also help to maintain a favorable thermodynamic environment for deep convection. However, this scenario is somewhat unlikely, and will thus not issue a level one threat at this time.
