Storm Forecast
Valid: Wed 05 May 2010 06:00 to Thu 06 May 2010 06:00 UTC
Issued: Tue 04 May 2010 21:36
Forecaster: PUCIK

A level 1 was issued for Sicily, Southern, Central Italy, Ionian, Adriatic Sea mainly for large hail and damaging wind gusts.

A level 1was issued for Northern and Northwestern Italy mainly for excessive precipitation and to the lesser extent for tornadoes or severe hail.

A level 1 was issued for Northeastern Italy, Northern Adriatics and Slovenia mainly for severe hail.

SYNOPSIS

The main feature concerning the synoptic situation will be a large cut off low at mid and upper levels, centered over the Northwestern Mediterranean Sea by Wednesday morning. On the forward flank of the low, a jet-streak is forecast with windspeeds over 35 m/s at both 500 and 300 hPa. Low will progress northeastwards during the day. To the north, yet another cut off low will be centered over Scandinavia, also having strong jet-streak at its forward flank. Both features are embedded in a large scale trough, covering most of Europe. To the west a high-amplitude ridge will stretch towards Iceland.

At the surface, models forecast deep low pressure system centered over Northern Italy, with minimum pressure below 1000 hPa. Its warm front will stretch from Austria to Western Slovakia and Poland. During the day, it will slowly progress northwestwards. Diffuse cold front will cross Italy and enter the extents of Adriatic and Ionian Sea during the day.

DISCUSSION

... Sicily, Southern, Central Italy, Ionian, Adriatic Sea...

Current satellite loops (as of 20 UTC Wednesday) give hints of a strong jet-stream existence, curving around the forward flank of the cut-off low. Behind the diffuse cold front, heigh falls will likely lead to the generation of few hundred J/kg of CAPE during the early morning hours. Dry intrusion overspreading the region might clear the clouds and surface heating might enhance the degree of latent instability, especially over Southern Italy and Sicily. Very strong wind shear is predicted, with DLS values over 30 and MLS over 20 m/s so that high chance of supercells would exist given that sustained updrafts formed. Most of the models are very modest in initiating the convection with only spotty precipitation simulated (which might be a result of high CIN values due to the warm temperatures around 850 and 700 hPa). Strong potential vorticity advection and a weak DCVA along with surface convergnce are, however, indicative of at least isolated storm formation. Given the favorable vertical wind shear conditions, very well organised multicells and supercells are possible, which might pose risk of large hail and severe wind gusts. Level 2 is precluded at the moment due to the questions regarding the storm initiation.

... Northern/Northwestern Italy ...

ICAPE fields, compared to the fields of MLCAPE suggest that most of instability in this region might remain elevated during the day. High storm coverage is expected with the proximity of the surface low and the exit region of cyclonically curved jet-streak. Strong, easterly low level jet is simulated by models with windspeeds up to 25 m/s at 850 hPa level. High moisture content, high RH in the lower troposphere and the presence of low level jet suggest the increased chances of excessive precipitation. This threat will be the highest on the southern slopes of the Alpine range, where a back-building system of thunderstorms might develop during the day. With strong wind shear in 0-6 km, its values over 20 m/s, well organised storms are possible. Due to the presence of low level jet, low layer wind shear (over 10 m/s) and storm relative helicity values in 0-1 km layer (locally over 150 J/kg) will be supportive of tornadoes. Questionable presence of CAPE in the boundary layer limits the confidence in this threat, even though one or two tornado events are not ruled out.

... Northeastern Italy, Northern Adriatics and Slovenia...

Overlap of strong wind shear and CAPE values between 500 and 1000 J/kg along with favorable forcing might be conducive to isolated well organised multicell or low topped supercells capable of producing marginally severe hail. Threat might be limited due to the abundance of cloudiness limiting the surface heating, especially in the northeastern parts of Level 1. The area might be expanded more to the east during the day (especially in case of Slovenia and Croatia) if conditions will prove to be more favorable also over this area.

... Hungary to Slovakia ...

Behind the warm front, humid airmass is expected to destabilise during the day with isolated spots of MLCAPE over 1000 J/kg expected, especially over Hungary. Bulk wind shear values should stay around 10 m/s, increasing up to 15 m/s towards evening. In case that wind shear values will increase before the degree of instability dimnishes considerably, stronger multicells might be capable of marginally large hail. This area is left open for a possible update during the day.