Storm Forecast
Valid: Fri 18 Aug 2023 06:00 to Sat 19 Aug 2023 06:00 UTC
Issued: Thu 17 Aug 2023 22:43
Forecaster: PISTOTNIK

Level 1 and level 2 areas are issued for central Italy, large parts of east-central Europe, the Baltic States, Belarus and W Russia for excessive convective precipitation and to a lesser degree for large hail and severe convective wind gusts.

SYNOPSIS

Most of Europe is under weak geopotential / pressure gradients and very warm to hot air. At 500 hPa, a faint mid-level high sits over NE Europe and slightly lower geopotential stretches from the Czech Republic to Albania.
The northern boundary of the warm airmass runs from England to Latvia and starts moving northward as a warm front on Friday. In the second half of the forecast period, the attendant cold front of an Atlantic cyclone crosses Ireland, Wales, England and N France from SW to NE.

DISCUSSION

... level 1 and level 2 areas ...

The warm airmass is characterized by moderately steep lapse rates and rich low-level moisture with 2m dewpoints often in the upper tens, which allows CAPE between 200 and 1000 J/kg across wide areas. Regionally, 2m dewpoints may even rise over 20C and can boost CAPE into the 1000-2000 J/kg range again, especially in the weak warm air advection regime over parts of Germany and Poland, as well as in a belt of maximized moisture over Belarus, Lithuania and Latvia.
Synoptic-scale vertical motion is rather neutral, as weak warm air advection towards the north is largely cancelled by weak negative vorticity advection. Convective initiation will therefore be rather erratic, both in space and time, and may fail in some regions but can turn quite widespread in others if a "snowball effect" of secondary initiation can be actuated. Apart from orographic features, also outflow boundaries or other convergence zones should therefore be closely monitored for signs of imminent convective initiation.
With deep-layer shear mostly below or at best shyly above 10 m/s, single cells and multicells are the expected storm modes. The main hazards are excessive rain, complemented by isolated large hail and cold-pool-driven downbursts in early stages. Due to the large uncertainties about the timing and placement of convective initiation, these hazards are encompassed by a large level 1 area. The two smaller level 2 areas are based on a fairly high CAPE magnitude (around or above 1000 J/kg) and reasonable model agreement on a high storm coverage.

... Ireland, Wales, England, N France, Belgium, Netherlands ...

In the periphery of the Atlantic cyclone, stronger SW-erly mid-level flow allows up to 15 m/s vertical wind shear across the 0-6 and 0-3 km layers, which also coincides with lift ahead of various short-wave troughs. The forecast model pool disagrees on the degree to which the fringes of the continental CAPE reservoir can be feed into these better kinematics and dynamics in the warm sector of the cyclone. Even if it should, CAPE may both be decoupled from the surface (especially in coastal areas) and cropped by rather low equilibrium level heights.
For the moment, a low-probability thunder area without a level 1 therefore seems to suffice. However, in case surface-based convection can yet form, heavy rain is well possible, and enhanced 0-1 km shear (especially in the evening and overnight) might allow one or two short-lived tornadoes. The hail and wind hazards are kept low by weak lapse rates and very moist, almost saturated air across all levels.