lundi 19 mai 2014

European Storm Forecast Experiment

Storm Forecast
Storm Forecast
Valid: Mon 19 May 2014 06:00 to Tue 20 May 2014 06:00 UTC
Issued: Mon 19 May 2014 05:20
Forecaster: TUSCHY
A level 2 was issued for parts of the Ukraine mainly for large to very large hail, severe to damaging wind gusts and a few tornado events. A strong tornado is possible.

A level 2 was issued for parts of S-Finland, Estonia and Latvia mainly for large to very large hail, severe to damaging wind gusts and a few tornado events. A strong tornado is possible.

A level 1 surrounds both level 2 for similar risks but with less confidence in extreme events.

A level 1 was issued for parts of Sardinia and Corsica all the way to far SE France mainly for excessive rain. An isolated tornado event over Corsica and NW Sardinia can't be ruled out.

A level 1 was issued for parts of N-Spain and W-France mainly for an isolated large hail, strong to isolated severe wind gust risk. An isolated tornado is possible.

A level 1 was issued for parts of UK mainly for excessive rain and an isolated tornado/large hail event.

A level 1 was issued for N-Algeria mainly for large hail.


The anticyclone over far W-Russia keeps its strength during the forecast with no forward motion expected. Its antagonist is an unseasonably strong trough over the E-Atlantic, which continues to amplify south. This trough evolves into an extensive upper low just west of the Bay of Biscay. 30-60 m/s mid/upper jets circle this vortex and start to affect far W-Europe during the forecast. In the meantime, a small-scale low over the Balearic Islands moves towards N-Italy and later on towards the N-Balkan States. Yesterday s channel of low geopotential heights, which covered a region from Denmark to the Black Sea dissolves and transforms into numerous smaller scale mid-level vortices. One over NE Germany lifts to the north and affects Norway and Sweden whereas another one crosses the Black Sea from west to east.

In the lower troposphere an extensive baroclinic zone remains anchored along the western fringe of the Russian high. It extends from the Ukraine to Latvia and further north towards Finland. Over Finland a developing LL depression modifies the boundary into cold and warm fronts with a broadening warm sector over S Finland.
A cold front enters the scene from the west and crosses the Iberian Peninsula and the Bay of Biscay until 06Z.
Numerous convergence zones over CNTRL Europe also play a role in today s thunderstorm outlook.

...Corsica and Sardinia until 15Z...

As the aforementioned small-scale upper low lifts to the NE it interacts with a plume of very moist low/mid-tropospheric air. Effective PWs exceed 30 mm. A 20-30 kt southerly low-tropospheric jet advects the moisture to the north. A cluster evolves east of the Balearic Islands and spreads east. It should approach Corsica and Sardinia around noon. Excessive and flash flood producing rain will be the main hazard (especially for Corsica), although 20 m/s DLS may support well organized tail-end storms with an isolated tornado threat. The risk diminishes during the afternoon hours as CAPE decreases and the upper low weakens/moves east. The level 1 was expanded far north to highlight an heavy rain risk also over far SE France/NW Italy.

...Ukraine, Belarus, Latvia, Lithuania, Estonia and parts of Finland...

Not much has changed compared to the past 24 h regarding CAPE and shear dispersal along the extensive baroclinic zone. Deep BL moisture due to persistent low-tropospheric convergence beneath mid-layer lapse rates in excess of 8K/km ensure another day with MLCAPE in excess of 1500 J/kg in the proximity of the front. CAPE gradually decreases further east as insolation and shallow BL moisture result in strong mixing of the BL. West of the front , 3 km and 6 km bulk shear significantly increase to 20 m/s or more.

For the Ukraine and Belarus the synoptic forcing once again remains diffuse and bound to weak short-waves or regional initiation along the orography. A somewhat stronger impulse affects the Ukraine during the evening and overnight hours, before moving slowly towards Belarus. Therefore, daytime activity should consist of isolated thunderstorms, already long-lived and severe. Scattered storms evolve over the Ukraine during the evening and rapid upscale growth into a large cluster is forecast. Spatiotemporal CAPE/shear overlap support large or very large hail with a few events larger 5 cm in diameter and a few tornado events with a significant event well possible due to SRH-1 in excess of 200 m^2/s^2 and 20 m/s 0-3 km shear. Strength of cold pools probably dictate forward speed of any cluster and a swath of damaging wind gusts will be possible.

Further north over Latvia, Estonia and S-Finland, GFS/EZ both agree in MLCAPE in excess of 2000 J/kg and 15-20 m/s DLS. Also, both models show a weak surface low development over the Baltic Sea, which moves towards S-Finland. Therefore confidence in CI is higher compared to yesterday.
Right now at 05 UTC, a long-lived cluster of storms crosses the area of interest from S to N and is faster compared to what models proposed. Some weakening is possible around sunrise over the offshore areas, but this system could re-strengthen during the forenoon hours as it crosses the N-Baltic Sea and W-Finland. Hence we can t rule out a substantial severe risk already before noon.
Next round of intense convection most likely awaits the developing surface low and enhanced convergence along the front. Scattered thunderstorm clusters evolve over Latvia and Estonia during the afternoon. These clusters then move to the N and affect S-Finland during the evening and overnight hours. However, do not focus too much on proposed timing as models had a mixed handling during the past 48h. Widespread severe accompanies those storms with large to very large hail. Hail in excess of 5 cm is possible. Cold-pool driven clusters may produce swaths of severe to damaging wind gusts. Depending on the final strength of the LL depression, the tornado risk could be enhanced significantly and a strong tornado event is possible. This activity spreads to the NE during the night with an ongoing severe risk.
Due to the displacement of strongest forcing to the west and forecast subsidence (on the synoptic scale) over parts of S-Finland, final coverage of storms over SE Finland remains a bit unclear. We expanded the level 2 and 1 boundaries more to the east as thunderstorms from the morning could push an outflow boundary far east.

...Extreme N-Spain, W-France and UK...

Seasonable prefrontal moisture resides ahead of the approaching Atlantic front from the west. A plume of steep mid-layer lapse rates from Spain spreads north and assists in an area of 300-600 J/kg MLCAPE. Regionally enhanced BL moisture could boost CAPE to 800 J/kg mainly over W-France. DLS increases from CNTRL France/E-UK to W-France/W-UK from 10 to 20 m/s and scattered CI is forecast over UK around noon and further south during the afternoon hours.
For France and extreme N-Spain: A few large hail events are possible. Storms grow upscale into numerous clusters with heavy rain and an ongoing hail threat...especially over W-France. Bowing storms may produce swaths of strong to isolated severe wind gusts over W/NW France. A small window of opportunity for an isolated tornado threat exists during the evening hours, where LL shear increases.
For UK: Scattered and early CI is forecast. Initial storms may produce a few large hail events, strong wind gust and an isolated tornado. Betimes, clustering storms may cause heavy to isolated excessive rain due to PWs of more than 20 mm and slow storm motions.
Thunderstorms continue during the night next to the synoptic front over W-France. Elevated and weakening clusters however may spread all the way to Benelux until 06Z.


Overlap of 800 J/kg MLCAPE and 20 m/s DLS cause organized storms with isolated large hail. Heavy rain becomes the dominant risk betimes.

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