Storm Forecast
Valid: Fri 07 Aug 2015 06:00 to Sat 08 Aug 2015 06:00 UTC
Issued: Thu 06 Aug 2015 23:08
Forecaster: PUCIK
A level 2 was issued for NE France, E Belgium, W Germany mainly for large to very large hail, severe wind gusts and excessive precipitation.
A level 1 was issued for S to Central France and NW Germany mainly for large hail, severe wind gusts and excessive precipitation.
A level 2 was issued for NE Spain mainly for large to very large hail and severe wind gusts.
A level 1 was issued for E Spain mainly for large hail and severe wind gusts.
A level 1 was issued for NW Russia mainly for severe wind gusts and large hail.
A level 1 was issued for S Italy, Greece, W Turkey mainly for excessive precipitation, severe wind gusts and marginally large hail.
SYNOPSIS
Synoptic situation will feature a trough over the Atlantic and a ridge stretching from the Mediterranean towards NW Russia with hot, tropical airmass covering much of SW, W and Central Europe. In between of these two features, strong SW-ly flow will be observed along the frontal zone separating tropical from polar airmass. This zone will become a focus for severe DMC during the forecast period. Another focus for DMC will be a very persistent, shallow mid to upper tropospheric low over the Ionian Sea.
DISCUSSION
... S France towards NW Germany ...
A plume of 7 - 7,5 K/km lapse rates will be advected N-NE wards from Iberia towards Central France, E BENELUX and NW Germany. With wave developing along the frontal system and shallow low travelling to the north, more abundant low-level moisture should also be advected northward with dewpoints over 16 deg C. Moderate CAPE values are forecast, with values locally above 1500 J/kg, decreasing northwards as low-level moisture becomes less pronounced. At the same time, strong SW-ly flow will contribute to DLS over 20 m/s and also enhanced SREH on the west and north side of the surface low. Combination of these conditions will favour well organised convection, including supercells, capable of (very) large hail and severe wind gusts. Very large hail threat will be prominent especially in case of isolated supercells. Later on, storm clustering may favour QLCS with more pronounced wind gust threat and perhaps one or two heavy precip events. Tornado threat should not be too pronounced given high LCL heights and not so strong LLS, but we do not want to rule out some event over NE France, E Belgium and NW Germany if isolated supercells persist into the late evening hours.
There are two major questionable factors:
1/ Degree of moisture return towards the north - perhaps limiting CAPE values
2/ Storm coverage
Lvl 2 is introduced for area where the best CAPE/shear overlap plus the highest confidence in CI exists.
... NE / E Spain ...
Zaragoza and Barcelona Thursday 12 UTC soundings show very steep lapse rates and considerable CAPE. CAPE / strong DLS overlap will not be perfect, with 20 m/s of DLS being displaced a bit more west of the high CAPE values. However, surface obs show Tds over 20 deg C, which in combination with steep lapse rates will likely yield moderate to high CAPE even inland. Well organised multi and or supercells are forecast over the area especially late in the afternoon and evening, with threats of (very) large hail and severe wind gusts. Lvl 2 is introduced for an area, where the highest threat of supercells producing very large hail will exist.
... NW Russia ...
Ahead of the cold front and subtle short-wave trough, NWP agrees on low to moderate CAPE values overlapping with around 15 m/s of DLS. Some well organised multicells are forecast, capable of large hail and/or severe wind gusts. As much of the shear is confined to the lower troposphere, a QLCS form of convection is most likely which will move fast eastwards. Thus, in this scenario, it seems that severe wind gust threat will dominate over the hail threat.
... S Italy, Greece, W Turkey ...
Although in weak shear regime, diurnally driven pulse storms in moderately to strongly unstable conditions and steep lapse rates (thanks to the cooler temps at mid troposphere) may be capable of marginally large hail, downbursts and some excessive rainfall events. Severe weather threat will quickly diminish after the sunset as storms decay.
