Storm Forecast
Valid: Fri 21 Mar 2008 06:00 to Sat 22 Mar 2008 06:00 UTC
Issued: Thu 20 Mar 2008 17:49
Forecaster: TUSCHY
SYNOPSIS
Nordhemispheric streamline pattern has a quite significant wavy pattern over the US - Atlantic -Europe region as stout ridge over the northern Atlantic is framed by two longwave troughs. The trough over Europe is the dominant and most significant one and airmass modification is underway from north to south over Europe as a cold and well mixed airmass spreads southwards. Thunderstorm chances should be limited and mainly diurnal driven.
DISCUSSION
... The Netherlands, Belgium, Germany and NW Poland...
An unusal strong decrease of geopotential heights over the area of interest is already underway during the start of the forecast period, with a decrease of 240 gpm / 12h. Atmosphere especially at mid-levels cools down with readings at 500hPa of about -35°C and a broad area will see some low end instability release with higher values over the Netherlands and N-Germany. We decided to split the extensive outlook area into two areas:
... The Netherlands, N-Germany and NW Poland...
Thunderstorm activity will be mainly daytime driven with peak intensity between 14Z - 17Z. This area is nicely placed under a vertically stacked surface-upper-level cyclonic circulation and hence no significant moisture modification at the surface can be expected as airmass circulates around the center. Combined with the cold environment, scattered showers and isolated thunderstorms will be the result . Wind shear stays very weak and hence pulsating storms can be expected with gusty winds and sleet / small hail during the mature phase. Dependant on insolation and local moisture content, a very isolated large hail report is not out of the question, but current parameters, both instability and shear, look too meager to reflect this in the risk levels. Also, LL instability , non existing helicity values at lowest 1000m and missing convergence zones should preclude a more robust funnel / tornado risk although an isolated one can't be ruled out completely.
The activity will diminish around sunset as instability vanishes.
... NE-France, Belgium, central and southern Germany...
The focus for a slightly stronger hail threat arises mainly over SE Belgium and W-central Germany, as cold front moves southeastwards. Moisture gradients become a little bit diffuse over central Germany and a 13Z - 17Z time frame will be the main focus for enhanced convective activity along this boundary or just along its postfrontal edge as mid-level cooling approaches the boundary from the NW. We are not sure if updraft strength will be strong enough to support electrified convection but EL temperatures fall below -20°C, which is a sign for at least some deeper updraft potential. Shear is strong with 0-3km shear rapidly increasing to 20m/s and if indeed a stronger pulsating shower / thunderstorm can manage to develop, hail could become a topic with even an isolated large hail report possible. Very slim instability values preclude any higher probabilities and a lot also depends on strength of diabatic heating and available moisture content .
We decided to include those areas also in the general thunderstorm area although probabilities drop down rapidly over NE France.
... N-Italy...
A developing surface depression could be the focus for a few isolated thunderstorms during the late night hours but confidence in initiation is not high enough for issuing a general thunderstorm area. Adjustment may become necessary later on.
... W-Turkey...
Forecast soundings indicate the main inhibiting fact for a more robust thunderstorm threat are the mid-level lapse rates, which struggle to reach 6K / km. In fact, some low-end instability release is possible and speed shear is very strong but current thinking is that instability release will be too meager for a more sustained updraft strength. EL temperatures stay warm and no deep convection is anticipated right now. We highlighted the most favorable area and expanded that area well towards the SW as some instability also exists over / just north of Crete. An isolated large hail / severe wind gust risk will be possible if stronger updrafts can develop but otherwise the threat will be too low for a level-1.
... N-Spain...
A few lightning strikes are also possible as cold front reaches N-Spain during the morning hours, but slim instability values should preclude a more active cold front passage regarding thunderstorm activity.
