Storm Forecast
Valid: Mon 06 Jul 2009 06:00 to Tue 07 Jul 2009 06:00 UTC
Issued: Sun 05 Jul 2009 18:22
Forecaster: TUSCHY
A level 1 was issued for NE/E Spain mainly for large hail and severe wind gusts.
A level 1 was issued for parts of Italy, Slovenia and S-Austria mainly for large hail and excessive rain.
A level 1 was issued for NE-France, Belgium, the Netherlands and extreme NW-Germany for large hail and severe wind gusts.
SYNOPSIS
A large-scale trough over NW-Europe moves slowly towards the east with warm and unstable conditions over parts of the N-Mediterranean and SE-Europe. Cold and stable conditions are present over NE-Europe due to a northeastward moving upper trough.
DISCUSSION
... E/NE Spain and the Balearic Islands ...
Originally weak branch of the subtropical jet gains strength over the Iberian Peninsula as upper trough over UK approaches from the NW. Various forcing mechanisms in form of embedded short waves affect the highlighted region during the forecast period, resulting in a prolonged thunderstorm period (probably all day long). A weak front from France/Bay of Biscay becomes stationary along the Pyrenees and east, so airmass remains warm and moist south of it. Placed at the western fringe of an EML tongue, lapse rates remain steep throughout the day. 500-1000 J/kg MLCAPE are forecast in 15-20m/s DLS (increasing somewhat during the day), so large hail will be the main risk with those storms. Given magnitude of instability and temporarily oscillating directional shear peaks over NE-Spain, even an isolated extreme hail event can't be ruled out. Severe wind gust risk increases during the afternoon hours as 0-3km shear grows to 15-20m/s.
The main uncertainty is how far south thunderstorms expand towards E-Spain and the Balearic Islands. Both shear and instability increase significantly but most of the global models keep the Balearic Islands capped throughout the day. Despite the warm layer around 800hPa, eastward traveling short waves and a slight cool down in the layer below 700hPa could locally support initiation. Fow now we sticked with the models and just issued low probabilities, but if initiation looks more imminent, an upgrade has to be performed.
... Italy, Slovenia, S-Austria and parts of Croatia...
Placed in-between the subtropical and polar jet, the wind field is variable and very weak with DLS values at or below 10m/s until sunset. No frontal intrusion is expected, so boundary layer will stay hot and moist with surface dewpoints in general around 20°C. However, lapse rates at mid-levels become somewhat steeper during the day as a quite diffuse Spanish plume draws near from the west. This ought to push the magnitude of instability release up to 1-2kJ/kg MLCAPE with locally even higher values not beyond question. Dewspite the disorganized wind field, adequate anvil layer shear will cause thunderstorms to produce intense updrafts which could even be longer lived compared to what is expected in such a weakly sheared environment. Large hail or a huge amount of small hail, excessive rain and strong wind gusts will be the main hazard. Thunderstorm clustering is expected with a high flash flood risk, given the magnitude of available moisture and very slow storm motion.
After sunset, shear at all levels increases from the west/northwest and new initiation itakes place as a weak cold front pushes eastwards. A possibel scenario is an eastward moving thunderstorm cluster over N-Lombardy and Trentino with an isolated large hail and strong wind gusts risk. Excessive rainfall could cause isolated flash flooding.
Further east, over Slovenia, S-Austria and Croatia, a weak shear environment is present, too with moderate instability release but DLS is slightly better with up to 15m/s over Slovenia/S-Austria. Large hail is possible with strongest storms next to intense rainfall and strong wind gusts.
... Ireland, United Kingdom and Scotland ...
Upper trough slides eastwards and crosses the area from the west. Cold mid-levels and hence moderately steepened lapse rates at mid-levels, adequate boundary layer moisture, weakly capped atmosphere and numerous east/northeastward moving convergence zones will cause an active thunderstorm period with widespread initiation. Marginal hail and strong wind gusts with isolated high rain amounts are expected. Another concern is an augmented funnel/tornado risk next to convergence zones and over S/SE UK, where LL shear is slightly enhanced. Thunderstorms decrease in coverage and intensity after sunset due to the diabatically driven nature of those storms.
... N-France, Belgium, the Netherlands and NW-Germany ...
The general position and strength of the trough/surface depression is not uninteresting in respect of tornadoes as a 20-25m/s speed maximum at mid-levels curves around the base. However, the LL speed maximum is displaced to the west (SW-UK), so LL shear is not strong and initially increases over N-France after sunset, when instability is on the decrease. The main focus for more organized convection is over Belgium, the Netherlands and NW-Germany, as DLS icnreases to 20m/s with SBCAPE in excess of 500 J/kg. Isolated large hail and strong wind gusts are possible with stronger storms. Thunderstorms move offshore during the night hours with a decrease in intensity.
... Rest of the highlighted areas ...
Neither shear nor instability are adequate for a more significant severe risk. Isolated large hail is possible over central Poland and E-Germany/W-Czech Republic next to strong wind gusts.
