Storm Forecast
Valid: Tue 19 May 2009 06:00 to Wed 20 May 2009 06:00 UTC
Issued: Mon 18 May 2009 17:06
Forecaster: TUSCHY
A level 2 was issued for W-Turkey mainly for large hail and severe wind gusts.
A level 1 was issued for S-Austria, Slovenia, Croatia parts of Hungary and Bosnia and Herzegovina mainly for large hail and severe wind gusts.
A level 1 was issued for S-CNTRL France mainly for large hail and severe wind gusts.
SYNOPSIS
A strong cyclonic vortex over NW-Europe moves slowly towards the south while transforming into a large, positive tilted trough just west of Europe. Downstream of this system, ridging persists for most parts of Europe as strong SW-erly flow spreads northeastwards. A cut off over SE-Europe moves slowly towards the east and brings unsettled conditions for those regions. Cool and stable conditions persist over NE-Europe.
DISCUSSION
... W-Turkey ...
A cut-off over the far eastern Mediterranean shifts towards the east during the forecast period and some strengthening is forecast. Eastward progress is quite slow which is reflected in a near vertical tilted axis of the depression throughout the atmosphere. Numerous short-waves rotate around this feature and affect the highlighted region during the following 24h. Beside those smaller scale features, complex topography will play an important role where initiation will occur.
Yesterday soundings revealed nice lapse rates all the way to 500hPa and those lapse rates persist during the forecast as hot and dry continental air advects towards the west over Turkey. In fact, GFS12Z has a tongue of 8-9K/km lapse rates all the way to the west coast of Turkey. Model guidance has a quite moist boundary layer mainly over NW/W-Turkey and it seems that GFS even underestimates the moisture slightly as dewpoints of 16°C were already present yesterday evening. Combined with those steep lapse rates, strong instability release likely results and .5-1.5kJ/kg MLCAPE is a reasonable guess with high fluctuations possible along the complex terrain.
As the eastward moving cut-off strengthens somewhat, the wind field along its northern fringe becomes stronger betimes with 0-6km bulk shear of 25m/s and 20m/s at lowest 3km. Directional shear remains strong with 200-400m^2/s^2 and interferes with strongest instability axis. Forecast soundings also have a strongly sheared hail growth zone with abundant instability, so the environment looks favorable for severe hailstorms. Hail in excess of 5cm is possible with the most dominant and long lived supercells. LCL heights stay above 1-1.5km and a well mixed/dry subcloud layer fosters strong downdrafts, which could locally exceed our severe criterion. Thunderstorms decrease in intensity and coverage during the night over the mountainous areas while the activity along the coast persists during the most of the night hours.
... S-Austria, Slovenia, Croatia parts of Hungary and Bosnia and Herzegovina ...
Upper air pattern becomes more diffluent with geopotential height gradients show a constant decreasing trend. This causes very weak wind fields over the highlighted area with diurnal driven thunderstorm activity. Initiation will start early in the morning hours either along weak convergence zones/old outflow boundaries or along the favored spots along the topography and therefore storms should first concentrate along the Dianaric Alps before moving erratic towards the E/NE.
Lapse rates at mid-levels are moderate but the boundary layer features high values locally in the upper tens. Like yesterday, MLCAPE release around 1000 J/kg is expected and weak shear ought to support rapid clustering of those storms. Nevertheless, a level-1 was issued as intense updrafts could still produce large hail in such a buoyant environment and GFS has strong DCAPE signals along the NE-coast of the Adriatic Sea, so an isolated severe downburst can't be ruled out. Thunderstorms continue during the night while moving slowly towards the east.
... S-CNTRL France ...
A quasi-stationary surface boundary extends from SW-France to near Luxembourg. Some moisture advection towards the north is forecast over S-France with moderate lapse rates atop that boundary layer, so up to 500J/kg MLCAPE ought to evolve over the level-1 area during the afternoon/early evening hours. ECMWF and GFS continue to show initiation during the late afternoon hours over the highlighted area so at least a few thunderstorms are forecast. 15-20m/s DLS and augmented SRH3km points to an isolated large hail and strong wind gust risk with those storms. Missing mid-/upper forcing enhances uncertainty regarding the coverage of storms, but we went with a level area as model persistence convinced us.
... Belgium, the Netherlands and NW-Germany ...
A more pronounced short wave crosses the highlighted area from the SW during peak daytime heating and some low-end instability release is possible. The airmass is weakly capped and current synop data reveal dewpoints around 10°C over N-France, so model output, ranging in the lower tens does not look too bad. However, GFS and ECMWF but also WRF had a hard time in developing precipitation constantly and over the same region during the past few runs. Uncertainty in thunderstorm coverage precludes any level area but if confidence increases in the next few runs, a level-1 may be needed. DLS near 25m/s ought to help thunderstorm to gain organization with an isolated severe wind gust risk.
... NE-Spain ...
EML covers most parts of Spain and the atmosphere remains capped during most of the day. However, as the upper trough draws near from the west during the night hours, a weak impulse crosses Spain from the SW. MUCAPE remains augmented during the night and an isolated, elevated thunderstorm is forecast. 20-25m/s DLS, strong directional shear and up to 500J/kg MUCAPE is more than adequate for an isolated large hail risk.
... General thunderstorm areas ...
Either shear or instability or both remain too weak for organized thunderstorms. Gusty winds and marginal hail are the main hazard.
