Storm Forecast
Valid: Wed 07 Jul 2010 06:00 to Thu 08 Jul 2010 06:00 UTC
Issued: Wed 07 Jul 2010 02:34
Forecaster: ESTOFEX

A level 1 was issued for Northern half of Portugal and Western/Northwestern Spain mainly for large hail and severe wind gusts.

A level 1 was issued for Southwestern Russia, Eastern Belarus, Central Ukraine and Moldova mainly for excessive precipitation and to the lesser extent for marginally severe hail or wind gusts.

SYNOPSIS

At midlevels, a ridge will become more amplified with its axis stretching from the Southwestern Mediterranean to Germany and Southern Sweden. To the west, a shallow cut-off low will slowly move to the north, influencing conditions over Western Iberia. To the north, an intense cyclonic vortex, surrounded by strong flow at mid to upper levels of troposphere, will be centered just to the south/southeast of Iceland. Another cut off low, centered over Hungary by Wednesday 12 UTC will move to the southeast.

At the surface, a deep low just south of Iceland will slowly fill during its progress to east-northeast. Over Central Europe, a high pressure system with its center over Germany will be connected to the Asorian high. A large region of shallow low pressure will be observed over Eastern Europe, with weak cold front associated with this feature. Over Iberia, a hot airmass will be advected from Northern Africa, with temperatures over 25°C at 850 hPa level.


DISCUSSION

... Portugal, Western/Northwestern Spain...

As a high Theta-E airmass will be advected into the region and with the approach of cooler mid-level temperatures associated with the cut-off low, a belt of very high lapse rates and high CAPE is predicted to form. Both GFS, ECMWF and WRF agree on moderate to strong instability build-up. GFS is less optimistic while ECMWF and WRF simulate over 2500 J/kg of CAPE especially over Northwestern Portugal/Spain. High CIN values are also anticipated, especially due to the very high temperatures at lower levels of troposphere.

On the eastern fringe of the cut off low, strong 500 and 300 hPa flow will overspread gradually this region, increasing DLS values to the range of 20-25 m/s. Almost 180° wind turn with height is forecast, with northerly/northeasterly surface winds and southerly flow aloft. Therefore, wind shear will be quite conducive to the well organised storms, including supercells, especially in the evening and night hours.

Initiation will be the most quesionable factor with ECMWF being the most optimistic regarding the coverage of storms. Nevertheless, with the approach of cut off low, favorable synoptic scale forcing might, during the late evening and night hours, provide necessary upward motion for CIN reduction. Complex terrain and simulated low level convergence over the region also point the possibility of the local initiation during the afternoon hours, especially in the highlands.

Level 1 is introduced for this region, as well organised storms might be capable of large hail or severe wind gusts (thanks to the deep, dry boundary layer and high lapse rates). Due to the questionable initiation and storm coverage an up or even a downgrade might become necessary during the day.

... Eastern Europe ...

In a weak flow regime, ahead of the surface cold front, in the region of the shallow low, abundant diurnally driven thunderstorm activity is forecast. GFS and WRF are quite optimistic regarding the instability while ECMWF keeps simulating only slightly above 500 J/kg of CAPE. In DLS less than 10 m/s, singlecells or weakly organised multicells are forecast. As of now, it is difficult to pick up a region with the best potential for severe weather, as local conditions, unforseen at the moment, might play a significant role in this case. If CAPE values manage to exceed locally 1500 J/kg, it will be quite probable that isolated pulse storms will form, capable of marginally severe hail or severe wind gusts. Slow storm motion might also lead to the local occurence of excessive precipitation, as high amounts of moisture will be present, as suggested by values of mixing ratio over 14 g/kg and surface dewpoints approaching 20°C. Low-end Level 1 is issued for the regions, where the combination of threat of excessive precipitation and marginally severe hail/gusts should have the highest coverage.