Storm Forecast
Valid: Wed 01 Jul 2020 06:00 to Thu 02 Jul 2020 06:00 UTC
Issued: Tue 30 Jun 2020 20:24
Forecaster: PUCIK

A level 2 was issued across southern France and western Switzerland mainly for large to very large hail, excessive rainfall and severe wind gusts.

A level 1 was issued across Northern Spain mainly for large hail and severe wind gusts.

A level 1 was issued acros the Alps and southern Germany mainly for excessive rainfall, large hail and severe wind gusts.

A level 1 was issued across central Germany and western Poland mainly for severe wind gusts and tornadoes.

A level 1 was issued across Estonia, Latvia and northwestern Russia mainly for tornadoes and to the lesser extent for severe wind gusts.

A level 1 was issued across southwestern Russia and southeastern Ukraine mainly for excessive rainfall and to the lesser degree for severe wind gusts and large hail.

SYNOPSIS

A broad belt of westerly to southwesterly flow will have established across much of Europe by the beginning of the forecast period in between of the cyclonic vortex over the Atlantic and a ridge stretching from the Mediterranean into the Balkans. This flow pattern will yield warm air advection regime across much of central Europe, followed by a cold front that will move from western France towards Germany and NW Poland by Thursday morning. At the same time, a plume of steep mid-tropospheric lapse rates will be advected from Iberia through SE France towards the Alpine range and southern Poland.

Towards north, a short-wave is forecast to cross the Baltics and Finland during the day, resulting in scattered deep-moist convection. Further southeast from here, a dryline will cross parts of southern Russia with storms forming along and ahead of it.

DISCUSSION

... Northern Spain towards Germany and Poland ...

A complex forecast scenario is foreseen across this area. On Wednesday morning, elevated storms may be ongoing over southwestern France, northern Italy and perhaps also along a warm front extending over northern Germany. In the morning hours, surface heating, in combination with the lapse rates reaching 6.5 - 7 K/km and abundant lower tropospheric moisture, will yield build-up of CAPE over the area. CAPE values will increase from north to south, with the highest values simulated over southern France and in the vicinity of the Alps, where the the best overlap of moisture and lapse-rates is forecast. Degree of vertical wind shear will increase from south to north and from east to west. The highest 0-6 km bulk shear values are forecast over central to northern Germany, reaching 20 - 25 m/s. Over southern France and the Alpine area, 0-6 km bulk shear will be mostly around 15 m/s, locally enhanced as topography modifies the low-level flow.

Storms are forecast to initiate along the convergence zone over southern France, Massif Central and the Alpine range in the early afternoon hours. These storms will pose a risk of large hail, severe wind gusts and excessive rainfall. Degree of storm organisation will increase with increasing shear in the late afternoon to evening hours over France and western switzerland / Italy. Subsequently, very large hail will be possible with the transient supercells.

Convective initiation in the afternoon hours over Germany is uncertain. It is possible that the first storms will initiate along the boundary over central to northern Germany. Storms will pose isolated severe weather risk with tornado threat increasing towards north with increasing low-level shear and decreasing LCLs. Severe wind gust risk will increase towards south with increasing depth of well-mixed boundary layer. Increase in convective coverage can be expected in late evening to overnight hours as the cold front crosses Germany.

Another uncertain area is northern Spain, where some NWP simulates spotty convective initiation over the high terrain in strongly sheared environment that could support supercells. However, stable layer near 500 hPa with very dry airmass and rather skinny CAPE profiles may make initiation particularly difficult. Nevertheless, a low-end Lvl 1 is kept in case that one or two supercells capable of large hail and/or severe wind gusts develop in the late afternoon to evening hours.

... Estonia, Latvia, northwestern Russia ...

As the trough crosses the area, synoptic-scale lift will contribute to build-up of marginal CAPE on the order of several hundreds J/kg by 09 UTC. Marginal and skinny CAPE will limit the large hail and severe wind gust threat. Simulated hodographs reveal strong lower tropospheric shear, with 0-1 km bulk shear reaching 10 - 15 m/s and 0-1 km SRH around 150 m2/s2. Combined with low LCLs, low-topped supercells that form in this environment will be capable of producing tornadoes. The highest threat will exist between 09 and 12 UTC and will diminish later on as the trough moves further inland of Russia.

... southwestern Russia, eastern Ukraine ...

A dryline is forecast to move across the area with drier air advecting from northwest. Pronounced low-level flow convergence along it will initiate scattered to widespread storms. Forecast profiles reveal skinny CAPE profiles with EL tops near 200 hPa and rather low LCLs. Primary threat will be excessive rainfall, but a couple of hail and/or wind gust reports will be possible in stronger storms.