Storm Forecast
Valid: Wed 16 Mar 2011 06:00 to Thu 17 Mar 2011 06:00 UTC
Issued: Wed 16 Mar 2011 00:46
Forecaster: PUCIK

Level 1 has been issued for Northern Tunisia/Northeastern Algeria for marginally severe hail and wind gusts.

SYNOPSIS

The major macrosynoptic feature will be a large cyclonic vortex at mid-levels situated initially over Northeastern Spain, shifting gradually further northeast towards Central Europe. Several vorticity maxima rotate around the vortex and a strong southwesterly jet-stream is observed at the forward flank and the base of the vortex. This feature will be the primary player in the interesting scenario evolving over the Central Mediterranean region. Otherwise, stable conditions are expected with a blocking pattern developing over Eastern, Northeastern Europe, mostly due to the large high pressure system at lower levels centered over Finnland. Two main frontal zones are forecast - the first one over the Mediterranean - in the form of a cold front in the association with a trough of a surface low centered over the Northwestern Mediterranean. The second one in the form of a slowly moving frontal system over Poland, Ukraine and Southern Russia, separating cold airmass advected from the forward flank of the surface high and warmer airmass advected from the Mediterranean.

DISCUSSION

...Central Mediterranean...

Synoptically speaking a very favorable situation for well organised convection has unfolded over this region with a series of vorticity maxima simulated by both GFS and ECMWF moving in the belt of a strong southwesterly flow which overspreads the warm sector of the low. However, meager instability is simulated within the warm sector, with both GFS, ECMWF and WRF models simulating spotty, low-end CAPE values, with local maxima reaching just few hundreds of J/kg. On top of that, this instability will likely be elevated in nature, further reducing the severe risk. Per rawinsonde data from Tuesday 12 UTC this is likely due to the rather weak lapse rates and stable layer in the temperature profiles around 850 hPa. Higher lapse rates and better defined CAPE fields are forecast behind the cold-front, with the decrease of mid-level temperatures, but in this case, weak vertical wind shear should preclude severe threat.

It seems that the only region with some better developed CAPE/shear overlap will be Northern Tunisia and Northeastern Algeria. By Wednesday noon, a vort. maxima is predicted to pass over the region, allowing for thunderstorm initiation. Instability should be rather weak but with favorable forcing, storms might persist in the strong shear environment with the subsequent formation of a few low topped supercells, which could pose a risk of marginally severe hail or wind gusts. Threat should not be very high but nevertheless deserves a low-end Level 1.

A broad stratiform rain shield is forecast in the warm sector of the cyclone with embedded convection likely contributing to the rainfall, albeit the degree of this contribution should be low, as revealed e.g. by convective vs. overall precipitation sums simulations of GFS. Even though locally excessive precipitation will occur, it is not expected that it will be mostly the result of a deep-moist convection and no level is introduced at the moment. Two areas that should be monitored for the excessive precipitation are western coasts of Central Italy (stratiform, then convective precipitation by Wednesday afternoon) and eastern Adriatic coastlines (by Thursday morning).