Storm Forecast
Valid: Sun 13 May 2012 06:00 to Mon 14 May 2012 06:00 UTC
Issued: Sun 13 May 2012 03:44
Forecaster: PUCIK

A level 1 was issued for NE Spain mainly for large hail and severe wind gusts.

A level 1 was issued for western coastline areas of Central to Southern Italy mainly for excessive precipitation and marginally large hail.

A level 1 was issued for Bulgaria to Romania mainly for excessive preciptiation.

SYNOPSIS

A significant CAA regime is underway over most of Central Europe on the forward flank of a large high pressure system, ridging into Central Europe from the Atlantic. To the north of the high a deep cyclone will affect Scandinavia. Main frontal boundary will progress slowly to the east across Russia and Ukraine, detaching from the mid and upper level jet-stream associated with the weakening trough. More complicated development is expected over Italy and SE Europe - Alps and Carpathian mountains will likely deform the frontal boundary with lee cyclogenesis likely, rendering the front a wavy structure.
A mid-level ridge will protrude from NW frica into Iberia and S France while Turkey and adjacent areas will be under a weak mid-level cyclone with cooler airmass at that level aiding in the development of latent instability during the daytime.


DISCUSSION

... NE part of Spain ...

There are major discrepancies regarding the NWP of CAPE values or convective initiation over this area. Instability is likely generated thanks to EML over the area (as nicely displayed by Tuesday 12 UTC or Wednesday 00 UTC Barcelona sounding) and weak onshore flow advecting moister airmass from the Mediterranean. As storms have already formed over the region during Tuesday and situation will not undergo a major change, it is likely that at least some degree of instability will exist over the area. Strengthening geopotential gradient between mid-level ridge and approaching trough will contribute to the moderate to strong DLS, with regions south of Pyrennees having DLS values around 30 m/s in the late afternoon hours. Degree of wind shear should decrease towards south. Especially close to the coastal areas, slightly enhanced SREH is expected with weak onshore flow as opposed to (north)westerly mid level flow. Lack of synoptic scale forcing, high LFCs and a capping layer will likely be the inhibiting factors in the storm coverage. Still, local topographic lift might aid in a spotty, isolated development of DMC, which can become well organised in the aforementioned conditions. Supercells and strong multicells are well possible once DMC initiates, capable of large hail or severe wind gusts.

... Western coastline of Central to Southern Italy ...

Cyclogenesis behind the Alpine zone with strongly enhanced flow at the lower levels has established over the Northern Alps in the night hours with a well organised MCS with training pattern of the cells. This situation might happen again as the surface cyclone, along with cold front progress to the south and encounter marginal to moderate instability over Central Italy. Towards the evening, approaching mid level trough should offer enhanced synoptic scale forcing, as well as improving vertical wind shear, which should reach over 20 m/s at the 0-6 km layer in the northern part of Level 1. Mesoscale convergence along the coastlines could initiate another MCS with training pattern - especially if the local convergence zone would align with the prevailing flow. Especially towards the evening hours when dropping LCLs and increasing saturation of low to mid levels would enhance precipitation efficiency, an isolated threat of excessive precipitation seems to be possible. Threat will shift to the south later on towards the night hours as trough progress southeastwards. Moreover a marginally large hail is not ruled out completely with stronger convection in the afternoon hours in the northern half of the Level.

... Bulgaria to Romania ...

Frontal wave is expected to develop along the cold frontal boundary that will cross Carpathians during the day. Low pressure center will likely back the low level flow to southerly/southeasterly with models being quite consistent in displaying a tongue of CAPE values reaching up to the Central Romania. Very weak mid and upper level flow will limit the DMC organisation. Movement of storm cells should be very slow, especially over Romania, where weak upper level northerly flow will be opposed by a southerly/southeasterly low level flow. This factor, along with the moderate values of mixing ratios and low LCLs point to the hazard of excessive precipitation. Training effect within slow moving thunderstorm clusters will be quite possible especially where low level flow will impinge on the local topograhic features. From this perspective, South-Central Romania looks like the area with most likely occurrence of excessive precipitation.