Storm Forecast

Storm Forecast
Valid: Fri 09 May 2014 06:00 to Sat 10 May 2014 06:00 UTC
Issued: Thu 08 May 2014 20:31
Forecaster: TUSCHY

A level 1 was issued for parts of the E-UK mainly for an isolated large hail/tornado risk.

A level 1 was issued for the Netherlands and NW Germany mainly for an isolated large hail and tornado risk.

A level 1 was issued for N/NE Germany and NW Poland mainly for an isolated large hail risk.

SYNOPSIS

A Rossby wave over the N-Atlantic streaks NW-Europe. Attendant polar front jet along its eastern fringe steers numerous mid-layer waves over N-Europe from west to east. A pronounced wave is forecast to evolve over the British Isles with a rapid motion towards Germany and Poland until the end of the forecast. Despite a weakening trend of the mid/upper jets, winds at 500 hPa still exceed 40 m/s while passing over NE France. Models offer consistent timing of that wave but its amplitude and final location remain a bit uncertain with ECMWF prefering a smaller amplitude (displaced a bit to the north) compared to GFS.
A large-scale upper low over SE Europe won't move a lot throughout the forecast, so unsettled conditions persist over Romania, Bulgaria, Greece and Turkey. Rising mid-level heights bring quiet conditions to the Iberian Peninsula (excluding orographically induced convection).

DISCUSSION

... E-UK, Ireland ...

Not much BL modification is needed to achieve thermodynamic profiles, which support deep updrafts. Already during the later morning hours, probabilities for electrified storms increase and scattered thunderstorms are forecast beneath the left exit of a powerful 45 m/s jet streak. 10-15 m/s DLS assists in a few better organized/longer lived storms with an isolated large hail/strong wind gust threat. LL shear with similar strength and 100 m^2/s^2 SRH-1 next to LCLs lower than 1 km point to an isolated tornado risk. The mid-level wave and attendant thunderstorms exit the area to the SE until noon. Spotty postfrontal non-severe convection occurs until sunset.

A depression approaches Ireland from the west between midnight and 06Z. Another favorable setup for isolated CI evolves beneath the left exit of a powerful 45 m/s mid-layer jet during the passage of strong synoptic forcing. CI is possible along the eastward racing cold front. Strong wind gusts will be the main hazard for most of Ireland.

... The Netherlands and NW-Germany ...

Early initiation is forecast...probably as early as 9-10 UTC. Either BL dewpoints in excess of 10 C or lowering 500 hPa temperatures over the Netherlands support the idea of weakly capped 200-600 J/kg MLCAPE. From noon onwards, additional insolation and approaching strong synoptic forcing result in more widespread initiation and stronger thunderstorms over the Netherlands and NW Germany. DLS oscillates around 15 m/s, so updrafts could gain some temporal organization and a few longer-lived thunderstorms are possible. An isolated large hail and strong wind gust event is expected. The main focus however will be the evolution of the low-tropospheric wind field. LL shear increases during the afternoon/evening hours, so an overlap of increasing LL shear and slowly decreasing CAPE could yield an isolated tornado risk . Thunderstorms decay rapidly after sunset as a stable nocturnal BL evolves.

... N-Germany and NW Poland ...

Overlap of 500-800 J/kg MLCAPE and 20 m/s DLS result in organized pulsating storms with an isolated large hail and strong wind gust risk. Storms weaken after sunset.

... Alps ...

Isolated to scattered afternoon storms occur with marginal hail and gusty winds. We lean more towards ECMWF with better handling of (lower) CAPE. Nevertheless, 20 m/s DLS could support an isolated large hail risk with stronger/more persistent updrafts (e.g. in Carinthia, SE Tyrol and S-Salzburg [Austria]). Discrepancies of CAPE between WRF/GFS and EZ remain too substantial for a level 1.

...Rest of the lightning areas ...

Diurnal driven non-severe storms with peak activity during late afternoon /evening are forecast. Marginal hail and gusty winds occur. Rapid decay is anticipated after sunset.

... W-Turkey and the W-Black Sea into S-Ukraine ...

Elevated convection with 20 m/s DLS could bring an isolated hail threat. Latest indication however keep more robust CAPE to the east of our forecast area. We won't add a level 1 for now.

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