Storm Forecast
Valid: Sun 26 Sep 2010 06:00 to Mon 27 Sep 2010 06:00 UTC
Issued: Sat 25 Sep 2010 20:36
Forecaster: SCHLENCZEK
A level 1 was issued for NE Italy towards N Albania including Serbia / Montenegro mainly for excessive precipitation.
A level 1 was issued for S / E Hungary and W Romania mainly for severe wind gusts and to a lesser extent for large hail and excessive precipitation.
A level 1 was issued for the E Aegean and SW Turkey mainly for large hail and severe wind gusts.
SYNOPSIS
A cut-off process took place on Saturday and led to a system of two upper lows over the central parts of Europe. The stronger one over Benelux is forecast to move to the southwest while the second one over N Italy tends to move to the northeast. This upper level feature will lead to heavy rainfall in large portions of south / central Europe. According to GFS, ECMWF and local WRF model, most precip on the north side of the Alpes should be stratiform and is therefore not covered by any threat level!
Ahead of the upper low over Italy, some unstable air is advected towards the Dinaric Mountains and intense convectively-enhanced rainfall is expected for this region. Some parts of SE Europe will also be affected by this system. A plume of very unstable air (simulated MLCAPEs well above 2 kJ/kg) over the E Mediterranean should reach the SW coast of Turkey during Sunday. This should be the place where severe convective weather is most likely in this forecast period.
DISCUSSION
...Benelux countries...
Not much cloudiness is expected in the vicinity of the upper cold core over the Benelux countries which will lead to lapse rates in order of 7 K/km from 0-4 km which is fairly enough for diurnally driven showers and weakly electrified thunderstorms. GFS suggests T / Td in order of 14 / 10 °C in most places and calm winds near the surface. It is questionable whether the 100 - 300 J/kg CAPE are really uncapped or not. Vertical profiles from GFS 06Z suggest a layer of poor lapse rates around 850 hPa over Belgium during the afternoon and therefore the possibility of short-lived tornadoes is too low to be mentioned.
...NE Italy, Slovenia towards Albania...
Recent GFS / ECMWF / WRF outputs show some hundred J/kg CAPE ahead of the upper vort- max in a region with deep layer shear in order of 10 - 15 m/s. Current thinking is that scattered showers / thunderstorms will develop over the Dinaric Mountains which tend to stay almost stationary, and some storms over the Adriatic which will make landfall. The persistence of storms will lead to excessive rainfall in some places (WRF shows locally rain amounts exceeding 50mm / 3h with accumulated sums over 100 mm / 24h). Other types of severe weather should be rather unlikely as the upper level wind fields are quite weak and lapse rates are not very steep.
...Hungary and Romania...
Some hundred J/kg CAPE and 20 - 25 m/s deep layer shear should overlap over parts of Hungary, Romania and N Bulgaria. Given some 15 - 20 m/s at 700 hPa and some almost linear QG forcing, a linear MCS with isolated severe wind gusts and marginally severe hail may form. Although storms should translate to the north / northeast rather quickly, isolated heavy rainfall with flooding is not ruled out.
...Black Sea region...
Low-end shear but a long persistence of 500 - 1000 J/kg MLCAPE is expected for the western Black Sea region. Thunderstorms will likely develop through the whole period and the coastal areas may get some heavy rainfall. An isolated hail / wind gust event is not ruled out but overall probability is too low for a categorical threat level.
...E Aegean, SW coast of Turkey...
In a very warm subtropical airmass, some 1,5 - 2 kJ/kg MLCAPE should be in place with only about -50 to -100 J/kg CIN. Deep layer shear in order of 15 - 20 m/s and locally enhanced SRH may allow a MCS and / or some well-organized multicells / supercells which should be capable of producing large hail and isolated severe wind gusts. An isolated significant hail / wind gust event is not ruled out but overall coverage will not be sufficient for a level 2. LL shear is not significant (below 10 m/s) and poor lapse rates around 900 hPa as seen in vertical profiles by GFS 06Z should preclude any spin-up. Therefore, tornadoes should be very unlikely.
