Storm Forecast
Valid: Wed 24 Oct 2007 06:00 to Thu 25 Oct 2007 06:00 UTC
Issued: Tue 23 Oct 2007 17:43
Forecaster: GATZEN

SYNOPSIS

A high over low situation affects most of Europe, with a well-developed subtropic ridge from eastern Atlantic to British Isles and southern Scandinavia and a surface high centred over western Russia. Cold and rather dry low-level air mass is located over most of Europe. Even the Mediterranean is flooded by cool and mostly stable air mass in the range of a broad low pressure area. A frontal boundary is present at the eastern flank of this low, affecting southern Aegean and Turkey during the period, where low-level moisture is more substantial and some instability is expected.

DISCUSSION

Southern Aegean and Turkey

In the range of the weak frontal boundary, latest soundings indicate rather steep lapse rates and rich low-level moisture, leading to CAPE in the order of some 100 J/kg. During the period, a 25 m/s mid-level jet streak is forecast to spread into southern Aegean/southern Turkey, providing QG forcing at its cyclonic flank. Convection that is likely due to instability and weak CIN will likely profit from this forcing, as well as from low-level forcing along the frontal boundary. Rather strong DLS around 15 to 20 m/s and some veering especially over Turkey with 100 J/kg 0-3 m SRH may assist organized storms including some rotating updrafts. These storms may produce large hail and severe wind gusts locally. Tornadoes are not expected at this time, as low-level vertical wind shear is not too strong.

Western Black Sea region

At the eastern edge of the south European trough, a strong vort-max/short-wave trough moves northward over western Black Sea in the morning hours. It will provide strong DCVA in the range of a frontal boundary that divides moist and rather warm air mass over the Black Sea from cool air mass over the Balkans. Thunderstorms, mostly embedded in stratiform rain, are forecast to go on until the afternoon. Given easterly low-level winds at the northern edge of a surface low and a southerly upper jet, strong vertical wind shear is forecast. Low-level SRH may reach more than 100 J/kg. Additionally, rich low-level moisture and mostly low LCL heights are likely that may be favourable for isolated tornadoes. Limiting factor is that thunderstorms are expected to be embedded and low-level buoyancy is also rather limited.

Southern/central Mediterranean

At the cyclonic flank of the upper jet over northern Africa, several embedded waves are responsible for some QG forcing during the period. Low-level buoyancy is weak and CAPE may be limited to a few 100 J/kg. Severe thunderstorms seem to be not likely. However, latest model output suggests that locally better low-level buoyancy may develop over the warm sea surface, and will not rule out a few tornadoes/waterspouts.

Southern Portugal

At the western flank of the south-European trough, a upper low moves southward. At lower levels, neutral to unstable lapse rates are expected over the relatively warm water surface of the eastern Atlantic ocean. Associated low pressure system also moves southward, and its cold front is expected to enter southern Portugal around noon. In the range of the cold front, maritime air mass spreads eastward into Portugal. Given strong vertical wind shear ahead of the cold front and favorable low-level veering profiles, a few rotating updrafts are not ruled out, capable of producing isolated tornadoes given low LCL heights and rather high low-level SRH values.