Storm Forecast
Valid: Wed 04 Jul 2018 06:00 to Thu 05 Jul 2018 06:00 UTC
Issued: Tue 03 Jul 2018 21:37
Forecaster: PUCIK
A level 2 was issued across N Spain and S France mainly for large to very large hail, severe wind gusts and excessive precipitation.
A level 2 was issued across N Italy mainly for large to very large hail, severe wind gusts and excessive precipitation.
A level 1 was issued across France, W Germany and Switzerland mainly for excessive precipitation and large hail.
A level 1 was issued across the Alpine range, Slovenia and Croatia mainly for excessive precipitation and large hail.
A level 1 was issued across Bosnia, Serbia and Bulgaria mainly for large hail, excessive precipitation and severe wind gusts.
SYNOPSIS
A quite complex synoptic-scale scenario evolves over Europe. At 06 UTC, Wednesday, a trough will stretch from NW France towards NW Spain. The trough is forecast to lift NE-wards through S France towards the Alpine region during the forecast period. S and E of the trough, a strong flow at mid to upper troposphere is observed, so that much of the W Mediterranean will be strongly sheared with DLS generally exceeding 20 m/s. A belt of the enhanced shear will also cover N Spain, S France and N Italy, where abundant lower tropospheric moisture is observed ahead of an advancing cold front that will cross France during the evening hours. During the day, moisture advection is forecast over much of N France, S BENELUX and W Germany.
A belt of enhanced lower tropospheric moisture has spread also towards the Balkans, residing south of the old boundary. Towards north, dry air covers much of E-Central Europe with too stable conditions for DMC.
Isolated to scattered DMC is forecast also over parts of Russia, underneath a cold mid-tropospheric airmass with steep lapse rates. This activity should be mostly non-severe though in the regime of weak CAPE and only modest vertical wind shear.
DISCUSSION
... N Spain to S France ...
As strengthening mid-tropospheric flow advects steeper lapse rates atop moist boundary layer, moderate to high CAPE values, locally exceeding 2000 J/kg are forecast by models. By late afternoon, synoptic-scale lift will erode much of the CIN. At the same time, forecast hodographs suggest DLS around 20 m/s with SRH0-3 km values of up to 200 m2/s2. In strongly sheared and highly unstable environment, a considerable risk of severe and well-organized convection, including supercells, is forecast. This is supported by a number of updraft helicity tracks simulated by high-resolution, convection allowing models.
Current thinking is that isolated to scattered storms will form in afternoon, particularly over Pyrenees and Massif Central. These storms will be capable of large hail and severe wind gusts. Thanks to the steep lapse rates and considerable buoyancy in hail growth zone, any sustained supercell may produce very large hail. Towards the evening hours, in virtually uncapped airmass and with the lift from the cold front, widespread initiation is forecast. Supercells and clusters will likely congeal into one or more MCS with dominant threats of excessive rainfall and severe wind gusts. The threat of extremely severe wind gusts will be diminished partly by the lack of strong lower tropospheric shear. Storms will spread NE-wards in the late evening and night hours, possibly reaching W Germany or S Belgium.
... N Italy ...
Similarly to S France, a favourable overlap of CAPE and strong DLS will exist over the region as the westerly flow induces cyclogenesis over the region with moist E-ly flow from the Adriatics. Forecast hodographs are linear, with DLS above 20 m/s, suggesting splitting supercells and well organized line segments. Given the environment, supercells would be capable of large to very large hail and severe wind gusts.
However, it is questionable to what degree is initiation possible over the lowlands before the night hours, when the trough approaches the region. It is perhaps more likely that during the afternoon, storms will be confined mostly to the mountains, while the storm coverage over lowlands will increase only during the night hours. In such case, stabilizing boundary layer may lessen the severe wind gust threat.
... S Austria, Slovenia, Croatia, Bosnia, Serbia, Bulgaria ...
In the abundant lower tropospheric moisture, moderate CAPE values, locally between 1000 and 2000 J/kg, will overlap with 10 - 15 m/s of DLS. Some well organized multicells are forecast that may be capable of large hail and severe wind gusts, particulary over Serbia and Bulgaria, where higher CAPE and drier boundary layer is forecast. On the other hand, moist profiles to the north, close to the Alpine range and over Croatia, suggest enhanced heavy rainfall threat. Initiation will likely be tied to the local orographic features and storms are forecast to spread towards the lowlands later on.
