Storm Forecast
Valid: Fri 20 Jan 2012 06:00 to Sat 21 Jan 2012 06:00 UTC
Issued: Thu 19 Jan 2012 22:20
Forecaster: TUSCHY

SYNOPSIS

An upper trough, centered over CNTRL Europe, keeps growing in amplitude with a slight negative tilt of its axis. Active NW-erly flow along its western fringe spreads SE-wards with lee low development S of the Alps anticipated during the forecast. A branch of the Azores high still affects SW/W-Europe with stable conditions. The same with a robust 1050 hPa high over Russia where stable conditions are anticipated (regarding DMC).

DISCUSSION

...Germany, Austria, Italy, Slovenia, Croatia, Adriatic/Ionian/Tyrrhenian Sea and parts of the North/Baltic Sea ...

There are no striking indications that a weak cold front passage over S-Germany (until 09 Z) and Austria (until 12Z) will be accompanied by an organized line of convection. Some support may exist by available weak upper diffluence beneath a left exit region of a 40 m/s 500 hPa streak and some positive pressure tendencies behind the front. Also, modest BL moisture flux convergence is expected along the upstream side of the Alps, which my assist in adequate BL moisture for low-end instability build-up. Given a temporarily increasing isallobaric gradient along the front, short line segments of better organized convection may still affect S-Germany and E-Austria during the cold front passage. The main hazard will be strong wind gusts and sleet.

The general thunderstorm area was expanded northwards due to another weak wave, embedded in this NW-erly flow regime, which crosses Germany during the daytime hours from NW to SE. Some weak moisture advection ahead of that feature can be detected and hence marginal CAPE will be available over most parts of central/S-Germany. Only sporadic thunderstorm activity with sleet and strong wind gusts is anticipated with a peak activity between 9-15 Z.

After noon, the focus also shifts downstream of the Alps as a cold front with orographically forced split front character moves southbound. Similar to past events, GFS has nice CAPE signals (up to 500 J/kg MLCAPE) with the rest of the models indicating slim values. Cross sections also show strong downslope component with subsidence and hence we exclude thunderstorm probabilities for areas just south of the Alps. All models show some flare-up of convection atop the Adriatic Sea during the afternoon hours onwards and Ionian and Tyrrhenian Sea during the evening hours onwards. Westward facing coasts of the Adriatic/Ionian Sea may see heavy rainfall but due to the progressive nature of the low/trough, no confined excessive rainfall risk is expected.

... Scotland and N/CNTRL-UK after 00 Z ...

A NW-SE-ward moving cold front starts to affect those areas with indications of a narrow cold-frontal rainband event. This front seems to be placed beneath the intense mid-/high level jet and also beneath warm mid-level temperatures. Hence, only shallow convection is expected right now with no real lightning chances. However, isolated severe wind gusts may accompany that line due to 25-30 m/s winds at 850 hPa, which may mix down to the surface. Overall conditions don't justify a level area.