Storm Forecast
Valid: Tue 27 Apr 2010 06:00 to Wed 28 Apr 2010 06:00 UTC
Issued: Tue 27 Apr 2010 00:20
Forecaster: KOROSEC

A level 1 was issued for extreme NE Algeria and northern Tunisia mainly for large hail.

SYNOPSIS

An eastward building ridge from western Europe prevails on Tuesday. On its edge, two disturbances will be the focus for some convective activity. A weakening short-wave trough/low moving from Poland into Belarus/Ukraine and an upper low moving south across west-central Mediterranean into northern Tunisia by Wednesday morning. Diurnal driven showers and thunderstorms can be expected across parts of southern Europe.

DISCUSSION

... eastern Poland and Slovakia towards southern Belarus and western Ukraine ...

As the weak upper level disturbance/vort-max continues its progress eastwards and destabilization seem likely along the surface convergence/front once the diurnal heating overtakes the weak capping. A few hundreds J/kg of instability should release. Despite the weakening mid-level jet, up to 15m/s of deep-layer shear could favour storm organization, especially across the western Ukraine. The strongest cells could pose a threat for marginally large hail locally. However, no widespread coverage or severity can be expected to warrant a severe threat level.

Ahead of an expanding high pressure area across central Europe, another small upper low disturbance will move southeastwards in the evening/overnight hours which could release some marginal instability and therefore convective activity along the remnants of the weak surface front. For this reason a low probs thunder line has been extended into eastern Hungary as well.

... extreme NE Algeria into northern Tunisia ...

An upper low approaches northern African coast during the day. Once the high falls occur, destabilization should occur and MLCAPE exceeding 500 J/kg seem likely to release over extreme NE Algeria and northern Tunisia. With mid-level jet bringing moderate 15-20m/s deep layer shear, an organized storms are likely. Therefore, a large hail seem to be the main threat. Given the expected larger coverage, a level 1 has been issued for these areas. Storms may cluster into a larger convective system in the evening hours/overnight.

... central Italy and western Balkans ...

A high probability thunder line has been placed across central Italy and western Balkans where convective initiation seem most likely - mostly along the mountainous terrain. A rather rich BL moisture and cooler mid-levels seem enough for initiation of mostly diurnal driven showers and thunderstorms. Locally, marginally large hail and excessive rainfall are possible due to slow moving cells, especially over central Italy where highest instability is expected (500-800 J/kg). However, no severe storms are expected given the weak shear in place. Storms should rapidly diminish after sunset as instability vanishes.