Storm Forecast
Valid: Wed 26 Oct 2011 06:00 to Thu 27 Oct 2011 06:00 UTC
Issued: Tue 25 Oct 2011 21:42
Forecaster: PUCIK

A level 2 was issued for Western Iberia mainly for excessive rainfall, severe wind gusts and tornadoes.

A level 1 surrounding level 2 was issued for the greater extents of western half of Iberia mainly for excessive rainfall and severe wind gusts.

A level 2 was issued for Northern Adriatics and Central to Southern Italy mainly for excessive rainfall.

A level 1 was issued for Italy mainly for excessive rainfall.

SYNOPSIS

From macrosynoptical point of view, Europe is divided into two regions - western and eastern half, both of which are dominated by the different synoptic features. The eastern one is covered by a large ridge, amplified in the meridional directions, stretching all the way from the Eastern Mediterranean into Northeastern Scandinavia. The ridge is overlying a large surface high centered over Ukraine/Belarus. To the west, a rather active weather pattern prevails with a large cyclonic vortex over Atlantic with two short-waves rotating around it.

The most significant short wave, amplifying in the exit region of a cyclonically curved jet-streak is associated with deepening low pressure system that is expected to affect Iberia especially during late evening and night hours of the forecast period. It will be this system, which is expected to provide the best conditions for the thunderstorm development. See discussion below.

Another focus for thunderstorm development is a baroclinic zone stretching over the Central Mediterannean. Mesoscale processes dominate will dominate its development, along with the progression of the mid and upper level trough.

DISCUSSION

... Western Iberia ...

Model outputs are consistent in developing a quickly translating low pressure system in the exit region of a jet-streak, in the forward flank of the short wave trough. In the warm sector of the low, strong LLJ, reaching over 25 m/s at 850 hPa level is simulated, transporting dew points over 16°C to the western coasts of Iberia. Low to moderate degree of instability is predicted, with MLCAPE values very likely exceeding 500 J/kg over the ocean. Questionable is, of course, the progress of unstable airmass inland, but the confidence remains high, that at least the elevated instability will be able to persist within the broad south-southwesterly LLJ.

Very strong wind fields will likely result in a high values of wind shear, especially that confined to the lower levels of troposphere (LLS inland over 15 m/s and SREH 0-1 km over 300 J/kg). Well organised convection is likely, albeit the more probable form, given the very strong forcing and widespread initiation, is one or more MCS. The strongest forcing and highest shear values will be observed over the SW Iberia during the late evening and night hours, in conjuction with the passage of the sharp trough.

Current thinking is, that the greatest threat will be posed by the excessive precipitation, especially in the northern reaches of Level 2. Several rounds of storms are possible in the area and forecast soundings revealed an environment characterised by strong LLJ and very humid profiles. With LLJ impinging on the coastal areas, strong convergence and upslope flow should result in widespread initiation with potential for cell training. Towards the south, tornado and severe wind gust threat should be more amplified, as the instability in this region might stay rooted in the boundary layer and also the best shear vertical wind profiles will exist here. If an isolated stronger updraft manages to develop, marginally severe hail can not be ruled out either, albeit this threat should be relatively meager compared to the other ones. The overall threat will be the greatest during the evening to night hours.

... Northern Adriatics ...

The aforementioned baroclinic zone is forecast to affect this region. 300 hPa level maps reveal that at the same time, divergence at the right entrance region of the jet might coincide with the cyclogenesis and subsequent strenghtening of the southerly low level flow. LAM show more instability and much higher precipitation sums than global models (GFS/ECMWF) and along with the fact of very moist vertical profiles and possibility of significant orographic contribution, severe rainfall event seems plausible in this region. The true outcome will likely very much depend on the development of marginal instability, the strenght of low level jet and its orientation to the orographic features.

... Central to Southern Italy ...

The baroclinic zone with history of producing very severe rainfalls over NW of Italy will slowly shift towards southeast. Very moist vertical profiles along with low instability values characterize the environment, along with the influence of the mid to upper level trough. It is difficult to pick up the zone with the highest excessive precipitation potential, as it will likely depend on the development of mesoscale factors during the day, such as the orientation of local forcing (e.g. convergence) zones to the mean flow. Regarding Central Italy, the highest threat will likely occur over western coasts. Level 2 denotes where the models best agree on the overlap of strong surface convergence, perpendicular onshore flow with the northern fringe of the tongue of instability. Threat will shift during the day from Central to Southern Italy.