Storm Forecast
Valid: Mon 07 Apr 2008 06:00 to Tue 08 Apr 2008 06:00 UTC
Issued: Sun 06 Apr 2008 21:51
Forecaster: GROENEMEIJER

SYNOPSIS

Around a mid-level low filled with very cold air over the British Isles rotate several vorticity maxima. One moves eastward just north of the Alps. Another one, that stretches westward over Ireland strongly amplfies. Over the Mediterranean Sea region exists a strong westerly flow. A shortwave ridge, over central Iberia at 06:00 UTC on Monday morning moves eastward and is followed by a shortwave trough that reaches south Portugal at the end of the morning.

DISCUSSION

... Iberian Peninsula ...

A number of elevated storms are expected to be ongoing over central Spain on Monday morning. It is expected that these will increase in coverage during the day as large scale upward motion increases as warm air advection intensifies on the approach of a trough. As rather strong deep-layer shear is present, it is possible that some of the more isolated storms become rather well-organized and produce some small, perhaps marginally severe hail. Gradually, the elevated CAPE should be consumed per GFS and stratiform precipitation will become the most dominant precipitation type over Spain.

During the second half of the morning a second area of convective activity reaches SW Portugal and spreads northeastward during the day. The convection is associated with a shallow moist zone in the wake of an ill-defined frontal system. Strong low-level shear is expected within this air-mass as winds at 850 hPa are forecast to be near 20 m/s. Strong insolation between the storms and the presence of a moist low level air-mass (T/Td 15/12), creates an environment rather supportive of mesocyclone development and possibly a few tornadoes. Additionally, strong gusts and some marginally severe hail will be possible. The area of highest threat is forecast to translate northeastward together with a postfrontal surface trough ahead of which storm-relative helicity is maximized. This region moves from southern Portugal (around local noon) to Castilia in the evening hours.

... E Austria, NW Hungary, E Czechia, S Slovakia ...

Strong frontogenesis is expected over northern Austria in the morning as a pool of cold low-level air travelling eastward along the northern Alps meets relatively warm air moving northward over the NW Balkans. The combination of frontogenesis and the influence of a strong upper-level trough moving eastward will likely result in the development of some strongly-forced linear convection along the front as it moves east to northeastward over Austria, Hungary and southern Slovakia. Given that wind speeds in the prefrontal low-level jet are expected to be inthe 20-25 m/s range and near 250m2/s2 0-3 km storm-relative helicity is forecast, it cannot be ruled out that an isolated rotating updraft will develop. Present thinking is however that very weak instability will strongly limit the overall threat of severe weather. Hence, a threat level is not issued at this moment. More poorly organized convection will be possible further north into Poland.