Storm Forecast
Valid: Thu 25 Mar 2010 06:00 to Fri 26 Mar 2010 06:00 UTC
Issued: Wed 24 Mar 2010 23:42
Forecaster: TUSCHY
A level 1 was issued for SW-France mainly for large hail and isolated tornadoes.
A level 1 was issued for parts of Portugal and NW-Spain mainly for severe wind gusts, large hail and an isolated tornado.
A level 1 was issued for SW-Turkey mainly for an isolated tornado risk.
SYNOPSIS
An intense cyclonic vortex is situated NW of the Bay of Biscay, steering the flow over W-Europe. Embedded in this SW-erly flow are numerous disturbances, assisting in shower/thunderstorm development over W/SW-Europe. A weakening upper low over SE-Europe also fosters a few thunderstorms, whereas high pressure dominates in-between and over east-central Europe.
DISCUSSION
... France (daytime hours) ...
EZMWF and GFS agree well in a progressive SW-erly flow pattern over France during the forecast. A short-wave over the Bay of Biscay around noon affects the central/northern parts of France thereafter during peak time heating. Attendant UVV maximum affects mainly NW France/S-UK and fades somewhat to the SE. Also, best isentropic lift with this feature will be confined to the sharpest cyclonic height curvature to the north, so no thick cirrus shield is forecast over central France during the afternoon. Therefore, temporarily insolation is probable. Bound to the best mid-/upper energy, a surface cold front races to the north over NW-France and S-UK, to the NE over north-central France and trails more or less to the east/southeast over central France. The following storm coverage is therefore forecast for
SW-UK:
Main focus will be the shallow warm sector, which covers SW-UK between 12-16Z.LL mixing ratios are modest at best, but enhanced IPV values and modest mid-level lapse rates, plus crossing short wave yield some low-end MUCAPE/SBCAPE with an isolated thunderstorm risk. In respect of ingredient-based forecasting method, no organized thunderstorm is expected with gusty winds and marginal hail the main hazard. However, degree of insolation has to be monitored for better LL CAPE build-up and an isolated tornado risk due to good LL shear.
Central France:
As cold front aligns more parallel to the strengthening background flow, some insolation, good LL convergence along the quasi-stationary front and moisture pooling may yield a fetch of enhanced thunderstorm probabilities, running from SW-France to NE-France. 30m/s DLS over SW-France ought to cause a few more organized storms with an isolated large hail threat and better forcing over NE-France may support an organized, probably shallow convective line along the cold front with strong to isolated severe wind gusts, approaching Belgium after sunset, but SBCAPE decays rapidly with a decreasing risk for organized convection. In-between both areas (central France), showers/thunderstorms with marginal hail and strong wind gusts are forecast, maximized during the afternoon hours. For now, we stick with a level 1 over SW-France (until sunset) for large hail. The rest of that activity seems to be too limited in coverage or too weak (warm ELs, modest CAPE at best) for long-lived storms. However the level 1 may need an expansion to the NE, if better SBCAPE manages to develop ahead of the cold front, especially as LLCAPE is augmented over central France, indicating an enhanced risk for isolated funnels/tornadoes beneath stronger updrafts.
... Portugal and NW-Spain...
Moist onshore flow affects the NW-Iberian Peninsula with cold mid-/upper levels atop, as numerous short waves cross the area during the forecast. Therefore, showers and thunderstorms occur all day long with an isolated severe wind gusts, large hail and tornado risk, maximized along the coast.
... SW/W/NW-Germany ...
A combination of moist upslope flow (SW-Germany) and the outer verges of the French short wave help to spark a few showers/thunderstorms in the highlighted regions. Due to the surface based nature, rapid weakening after sunset will occur. Shear remains weak, so no hazard is expected with this activity.
... SW-France / 03Z-06Z...
A strong short wave exits the SE-Bay of Biscay after midnight with an intensifying onshore flow ahead of this wave and an eastward moving, developing cold front affecting SW-France after 03Z. Mid-levels cool down rapidly with steepening lapse rates, so SBCAPE of 100-400 J/kg are a reasonable guess. A level 1 for an isolated tornado/large hail event was issued with higher tornado probabilities along the coast. This risk continues until 06Z and thereafter, covered by the next outlook.
...Crete and SW-Turkey ...
Beneath a weakening upper low, a cold front crosses the Aegean Sea southwards, affecting Crete until the afternoon with isolated storms. An isolated waterspout is possible with no other hazard forcast. Very steep mid-level lapse rates over SW-Turkey and good LL CAPE hint on an isolated funnel/tornado risk with diurnal driven thunderstorms. A low-end level 1 was included, to cover that risk. Thunderstorms wane after sunset.
