Storm Forecast
Valid: Wed 11 Apr 2012 06:00 to Thu 12 Apr 2012 06:00 UTC
Issued: Wed 11 Apr 2012 05:43
Forecaster: PUCIK

No threat levels have been issued.

SYNOPSIS

A broad mid and upper level trough is forecast to cover western half of Europe, with its axis stretching from the Northern Sea / British Isles towards Northern Italy. A distinct short-wave is forecast to develop on the rear flank of the trough, affecting western Iberia in the evening hours. Strong flow surrounds the trough, but the major jet-stream is forecast to lie over the Southern Mediterranean / Northern Africa along with the main frontal zone. Another trough is forecast to cover zone from the Black Sea towards Central Turkey with eastward movement during the day.

Closer to the surface, a large area of shallow low pressure system is anticipated covering most of Western and Central Europe, including also the Central Mediterranean. Several distinct low centers are forecast, the most important being over the Norwegian Sea and Northern Italy. A wavy frontal boundary will stretch from the Southern Mediterranean all the way up to Southeastern Sweden.

DISCUSSION

... Italy ...

Ample synoptic scale forcing is forecast with the approach of the mid/upper level trough, its position in the exit region of the mid-level jet. 00 UTC soundings reveal steep mid-level lapse rates and very favorable wind shear profiles in the area. However, wind shear is forecast to weaken during the day, yielding only 15-20 m/s of DLS between 12-18 UTC period. LLS should stay generally below 10 m/s, apart from the northern part of Italy in proximity to the center of the surface low. Marginal instability is forecast with MLCAPEs reaching only few hundreds J/kg per GFS and ECMWF simulations. Moreover, as of 5 UTC, a dry intrusion from the trough has already overspread Northern/Central Italy. This is a bit in contrast with the model simuations that predict the passage of the axis of the trough along with the IPV enhancement during the early afternoon hours. A low end threat of severe will exist for the region - either in form of marginally large hail over western parts of Central Italy (moderate shear combined with low freezing levels) or in form of excessive precipitation over Northern Italy (enhanced low level flow, saturated conditions). For hail, marginal instability and for excessive precipitation, low dew points seem to be the most limiting factors. Attm Level 1 is not warranted, but an update might become necessary if instability/shear overlap prove to be better than anticipated.

... Central France ...

In the post-frontal airmass involving steep lapse rates, a marginal instability is forecast to develop. Under strong mid-level flow, more than 20 m/s of DLS is forecast, which is sufficient to well organised convection. However given the fact, that low level flow will be rather weak, 0-3 km bulk shear should stay below 10 m/s. This in combination with marginal instability giving rise only to low topped DMC along with no concentrated synoptic scale forcing will likely prevent any severe weather threat.