Storm Forecast
Valid: Tue 16 Oct 2012 06:00 to Wed 17 Oct 2012 06:00 UTC
Issued: Tue 16 Oct 2012 04:50
Forecaster: SCHLENCZEK
A level 2 was issued for S Balkans and Greece for excessive convective precipitation, severe wind gusts, large to very large hail and tornadoes.
A level 1 surrounds the level 2 for excessive convective precipitation, large hail, severe wind gusts and tornadoes.
A level 1 was issued for NW Iberia for excessive convective precipitation.
A level 1 was issued for SE Ireland and SW UK for severe wind gusts and tornadoes.
SYNOPSIS
A longwave trough with an intense cyclonic vortex at its southern tip is located over the E Atlantic. The low pressure system will affect Ireland and the British Isles on Wednesday. There is a chance of numerous non-convective severe wind gust reports, given a strong gradient flow with 850 hPa wind speeds above 25 m/s. Some shortwaves are embedded in the circulation of the longwave trough: One of them will affect NW Iberia with heavy convective rainfall and a second one is associated with a large MCS over the S Adriatic. Very unstable air and strong vertical wind shear will provide favourable conditions for convective severe weather of all kind. Yet another active severe weather day is forecast for SE Europe.
DISCUSSION
...S Balkans, Greece...
An airmass of high instability (MLCAPE in order of 3 - 4 kJ/kg) is advected towards the Adriatic coast. Southerly winds will aid the northward progression of this plume and thus maintain the supply of CAPE over that region, combined with 15 - 20 m/s of 0-6 km bulk shear. The main threat will be excessive convective rainfall over the Balkans and N Greece. Increasing wind speeds in the lower levels also enhance the risk of severe wind gusts along the Greek coast. Model data show also enhanced SRH1 / SRH3 (150 - 250 mē/sē). It is obvious that an organised MCS with possibly embedded supercells in such an environment of high CAPE and moderate deep layer shear will likely produce large to very large hail as well. Given locally enhanced LL shear and rather low LCL heights, tornadoes are possible as well.
After the passage of the first MCS, other convective systems will likely follow and the heavy rain threat will continue at least until Wednesday morning.
...S-central Mediterranean, Sicily, S Italy...
Lesser instability but still favourable vertical shear is forecast for this region and some isolated severe thunderstorms with large hail, severe wind gusts, heavy rain and tornadoes may develop during the first half of the forecast period. After that, instability is probably too meager for organised severe convection.
...Balkans, SE Hungary, W Romania...
Although instability is less pronounced in this region, some hundred J/kg of CAPE and 15 - 20 m/s of deep layer shear may provide favourable conditions for isolated severe thunderstorms with severe wind gusts, large hail and excessive precipitation. Due to locally augmented SRH1 and LL shear, an isolated tornado event cannot be discounted.
... NW Spain and N Portugal...
A plume of warm and slightly unstable air is advected towards the coastal regions of NW Spain and N Portugal. The precipitation along the cold front should be at least partly convective with total sums in order of 20 - 50 mm / 24h. It is likely that subsequent multicellular storms will make landfall and might easily exceed the mentioned rainfall amount in a time-scale of a few hours due to high values of effective PWAT and a continuous SWerly flow.
...SW UK and Ireland...
Some cold air convection is expected in the wake of the cold front of a low pressure system west of Ireland. Some hundred J/kg of SBCAPE in the lowest levels may point at a slight tornado chance as LL wind shear is in order of 10 m/s, locally even more. As common for intense extratropical cyclones with a strong gradient flow and convection in the wake of the trough, isolated convectively enhanced severe wind gusts are possible as well.
