Storm Forecast
Valid: Fri 15 Jun 2012 06:00 to Sat 16 Jun 2012 06:00 UTC
Issued: Thu 14 Jun 2012 21:00
Forecaster: TUSCHY

A level 1 was issued for parts of United Kingdom mainly for isolated large hail, strong to isolated severe wind gusts and an isolated tornado event.

A level 1 was issued for parts of E France mainly for an isolated large hail event.

A level 1 was issued for N-Algeria mainly for large hail (a very large hail event is possible) and severe downbursts.

A level 1 was issued for parts of the Ukraine, E-Belarus and W-Russia mainly for large hail and strong to severe wind gusts.

A level 1 was issued for N-Finland mainly for isolated large hail, strong wind gusts and an isolated tornado event.

SYNOPSIS

A deepening cyclonic vortex over Ireland shifts to the NE and approaches Scotland during the end of the forecast. In response to the more zonal flow pattern, geopotential heights increase over most of S-Europe. A well defined upper trough over E-Europe moves off to the SE with no weakening trend anticipated during the forecast period.

DISCUSSION

... UK, Scotland ...

Behind an E/NE-ward moving occlusion, a maritime air mass overspreads most of UK from SW to NE. With cold mid-level temperatures atop and placed beneath the left exit region of a strong 45 m/s 500 hPa streak, scattered deep convection is anticipated mainly between 9 and 21 UTC. Overlap of roughly 500 J/kg MLCAPE and 15-25 m/s DLS (increasing from NW to SE), a few well organized multicell events are forecast. Isolated large hail and strong to isolated severe wind gusts accompany strongest convection. Despite a somewhat marginal directional shear component in the lowest 1 to 2 km AGL, forecast 150 m^2/s^2 SRH-1 and any deviant storm motion provide adequate helicity for an isolated tornado event. Storms gradually diminish during the first night hours (also in line with faint warming of the mid-levels).

... France, Benelux and parts of Germany ...

An extensive warm sector evolves over France all the way into Germany, as a well structured warm front rapidly moves to the NE. Trailing cold front remains aligned nearly parallel to the background flow with marginal net motion to the south. We do not yet like the more SW-erly flow component regarding the BL recovery process, as dewpoints in the mid tens would be already present over far S-France. Latest thinking is that this airmass won't be incorporated (or only partially) into the evolving warm sector and therefore only dewpoints in the low to mid tens are forecast (in line with ECMWF and way below GFS). Weak lapse rates atop modest BL moisture only assist in weak MLCAPE in the order of 200 to locally 500 J/kg with highest values confined to S/SW France. Also, thick WAA cloud cover is expected mainly along the trailing cold front and further east into the warm sector, so diabatic heating may also be an issue (despite areas over S/SW France). Hence, despite rapidly improving kinematics (e.g. DLS in excess of 30 m/s), only a low to modest risk of severe thunderstorms is currently expected, mainly along a confluence zone from E-France arching to the SW. A marginal level 1 was issued for the region, where isolated large hail and strong/isolated severe wind gusts can't be ruled out with a strong storm or two. Otherwise a broad thunderstorm area was added, to reflect some elevated thunderstorm chances along the warm front and some activity along the cold front over NE France/Benelux. BL moisture recovery will be monitored in case of more robust CAPE build-up and higher severe probabilities.

... N-Algeria ...

Moisture-rich air covers most of the areas along and north of the Tell Atlas mountains. Surface dewpoint readings are in the upper tens/lower twenties. Very deep EML overspreads that area, so capped 2-3 kJ/kg MLCAPE are forecast. Numerous weak waves are embedded in the westerly flow regime, which may provide enough lift for a few thunderstorms during the afternoon/evening hours. 20-25 m/s DLS, deep and dry subcloud layers, very steep mid-level lapse rates and discrete storm structure should assist in a few severe hailstorms with very large hail possible. Uncertainties in magnitude and region of initiation preclude higher probabilities for now. A few models concentrate initiation mainly offshore along the northern fringe of the hottest air mass, with other models focusing more on the rough topography. We therefore issued a broad level 1 area, to reflect those uncertainties. Thunderstorms keep going during the night although mainly confined to offshore areas.

...Ukraine, Belarus and Russia ...

Scattered thunderstorms are forecast surrounding the SE-ward moving upper trough. Areas over SW Ukraine see an overlap of 25m/s DLS and 500 J/kg MLCAPE during the afternoon hours, adequate for a few organized thunderstorms with large hail and strong wind gusts.

More unstable air is situated further north with MCALPE aoa 1000 J/kg, but weaker DLS (15 m/s or less). Nevertheless, a few better organized multicells with large hail and strong to severe wind gusts are forecast. Given strengthening diffluent flow aloft during the evening hours onwards, storm clustering is possible with locally heavy rain the main severe risk.

Further south, over Romania and Bulgaria, weaker forcing and a stronger cap preclude more widespread convective initiation. A few isolated storms can't be ruled out which may produce marginal to isolated large hail and strong wind gusts. Coverage however remains too isolated for a broad level 1 area.

... N-Finland ...

A northward moving depression advects a warm and moist air mass far to the north with surface dewpoints in excess of 10 °C likely all the way to N-Finland. A 20 m/s 500 hpa jet crosses the forecast area during peak heating with DLS increasing to 15-20 m/s. Moderate mid-level lapse rates and adequate forcing assist in scattered thunderstorms, a few stronger mutlicells well possible. Isolated large hail and strong wind gusts will be the main risk. A small time frame for an isolated tornado event exists during the late afternoon/evening hours, as LL shear strengthens in response to the gradually deepening surface low, LCLs remain below 800 m with some LLCAPE forecast. The severe risk rapidly diminishes after sunset and so does the thunderstorm risk after 21Z.