Storm Forecast
Valid: Tue 03 Apr 2012 06:00 to Wed 04 Apr 2012 06:00 UTC
Issued: Mon 02 Apr 2012 22:07
Forecaster: TUSCHY

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

SYNOPSIS

A broad branch of the polar vortex remains in place over NE Europe. Along its fringe, numerous more or less defined upper disturbances round that feature. A pronounced one, featured by a strong cold-core low, is placed just west of the English Channel at 20Z (2nd April). WV imagery still highlights an healthy low (e.g. tight moisture gradient at high-levels) and is captured well in model data. This progressive low races eastwards during the forecast and opens up into a wave during the traverse of N-France (until 18Z) and Germany (until 06Z). No major changes are forecast with the positive tilted trough over SW-Europe, so expect another day with active showers and thunderstorms over Spain. Strenghtening WAA affects areas downstream, including CNTRL/E-Mediterranean with increasing convective precipitation probabilities.

At the surface an ill defined thermal low over SW-France gradually transforms into a dynamically driven low pressure area downstream of the progressive upper wave and covers parts of Germany, Benelux and France.

DISCUSSION

... Portugal and Spain ...

Positive tilt of the upper low shows no sign of structural change with another shot of cold air along its western fringe, resulting in the development of a new upper wave just west of Portugal. Downstream WAA remains weak, but marginal mid-level thickness increase is expected over the Iberian Peninsula with a warm-up of the mid-levels by 1 or 2 K. So eventually the overall set-up for thunderstorms becomes slightly worse compared to the past 48 h and confined mainly to the central and eastern part of Spain. MLCAPE will be lower compared to yesterday with 200-500 J/kg forecast. Also, forcing will be weaker compared to the past 24 hours, so expect another round of mainly orographically forced thunderstorm development and pulsating storm activity. Again, yesterday's radar data revealed locally intense updraft cores, capable of producing hail and we expect this again mainly within the 50-% thunderstorm area. However with near zero DLS and weakening CAPE, we did not issue a level 1 this time, although an isolated large hail event remains in the realm of possibility, given cool lower troposphere and a well mixed sublcoud layer. Strong wind gusts also accompany that activity.

During the night, the thunderstorm activity gradually diminishes onshore but slowly increases offshore as the lower column of the troposphere cools down a bit atop roughly 14°C SST. No severe risk is anticipated with expected weak shear/moderate CAPE environment.

... N-Algeria, parts of Italy, Greece, the Adriatic Sea and the Ionian Sea ...

As a response of the quasi-stationary and positive tilted upper trough over the Iberian Peninsula, a strong 40 m/s subtropical jet works its way northeastwards. Numerous upper waves are embedded in this jet and affect the highlighted areas. This results in a messy looking picture of locally enhanced showers and thunderstorms over a broad area.

The main severe risk again evolves along the Tell Atlas region over N/NE Algeria and N-Tunisia. Dewpoints along and north of the mountainous area further increased in the past 12h with values in the lower to mid tens. Briskly SW-erly flow affects the area so DLS remains in the 15 to locally 20 m/s range, which is adequate for a few well organized multicells. An EML plume grazes the forecast area, so 300-500 J/kg MLCAPE over N-Algeria increase to up to 800 J/kg over N-Tunisia. A few large hail events are forecast with a maximized threat over NE-Algria and N-Tunisia, where a level 1 was issued. Next to that risk, strong wind gusts accompany that activity. Similar to the past 2 days, thunderstorms gradually decay after sunset, while moving offshore. The activity may last well into the night over NW-Algeria due to the proximity to the upper trough.

Isolated to scattered thunderstorms occur within the WAA regime over S-CNTRL Mediterranean area mainly after sunset due to the approach of a stronger mid-level impulse and good diffluence aloft. Nothing severe is anticipated with that activity.

... France and Germany ...

Yesterday's anticipated moisture return over S-France was evident in HVIS imagery with organized cumuli streamers south of the Massiv Central and enhanced convection during the evening hours atop the Massiv Central. However, WLA was elevated with surface dewpoints still in the single digits and therefore a bit lower what models advertised.

Return flow over SE France increases during the forecast period as the progressive upper wave over N-France approaches from the NW, so we expect a plume of deepening moisture to rapidly expand from SE-France all the way to SW-Germany. This moisture creeps in an increasingly colder environment with 500 hPa below -20°C over SW-Germany/E-France. Forecast soundings reflect slower BL moisture recovery at LL with a pretty dry subcloud layer and large near surface T-Td spread, which may limit initiation away from complex topography. Isolated daytime driven initiation along the Vosges, Jura and Black Forest is forecast around noon onwards with more concentrated/widespread development during the later afternoon hours, as the upper wave interacts with the aforementioned moisture tongue....e.g. from Massif Central to Vosges. Showers and thunderstorms move E/NE-wards. Significant spread in model QPF over S/SE Germany still exists. We don't want to ignore sporadic activity at 21Z (2nd April) over the Jura and hence the more bullish GFS was incorporated in the final lightning area dispersal.

Roughly 300 J/kg MLCAPE is forecast with peak values in the 50-% lightning area. The wind field in all levels increases during the afternoon hours in response to the approaching upper wave, reaching 15-20 m/s within the 0-6 km layer. Current weak CAPE signal in model fields limits the overall severe risk although a deep and dry subcloud layer and augmented LCLs may assist in an isolated large hail and strong wind gust event mainly in the 50-% thunderstorm area. No level 1 will be added due to the marginal thermodynamic set-up. The activity diminishes after sunset with constant clustering of showers/storms.

During the night, the activity becomes more elevated, as the eastward racing upper wave pushes the moisture plume towards CNTRL/E-Germany. Some MUCAPE next to moderate forcing may assist in isolated and elevated thunderstorm activity within a messy complex of showers or stratiform rain. Accordingly the 15-% thunderstorm area was expanded a bit to the NE.