Storm Forecast
Valid: Wed 01 Jun 2011 06:00 to Thu 02 Jun 2011 06:00 UTC
Issued: Wed 01 Jun 2011 06:08
Forecaster: PUCIK

A level 1 was issued for Southwestern to Central Finland mainly for severe wind gusts and tornadoes.

SYNOPSIS

A complex synoptic scenario evolves over Europe as two mid-level ridges, one situated over Eastern Atlantic and another one over Eastern Russia, are forecast to connect, as a mid-level cut-off low, at first situated over Southern France retreats slowly to southwest. More to the north, strong westerly to southwesterly flow with several disturbances dominates the weather above Scandinavia. Closer to the surface, a similar process should take place, involving the merger of two surface highs, with the dominant one being centered firstly over the Atlantic and stretching into Great Britain, BENELUX and further to the east. The only low pressure system worth talking about will be centered over Iceland with slow northeasterly transition during the day. A main frontal zone will lie at 12 UTC across Finland and stretching towards south-southwest across Western Poland, Western Czech Republic, then strongly deformed by the Alpine range also over the Western Mediterranean.

DISCUSSION

... Finland ...

Ahead of the surface cold front, models tend to agree on the development of latent instability, with most typically simulated CAPE values in the range from 500 to 1000 J/kg. 00 UTC sounding from Jokioinen does show very similar degree of elevated instability as GFS advocated, making its run more trustworthy. With quite strong flow especially in the lower levels of troposphere, parallel to the frontal boundary, more or less unidirection shear profiles, with slight curvature ahead of the frontal trough close to the surface level, are forecast, with the most of wind shear being confined to the lowest 3 km. Bulk 0-3 km shear values of 15-20 and 0-6 km values of 20-25 m/s will likely overlap with the tongue of instability, albeit 1-8 km wind shear should reach only around 10 m/s. Moreover, region will not be in a favorable position related to the mid and upper-level tropospheric features, making the initiation closely tied to the cold front. The above-mentioned facts point to the scenario involving quick clustering of cells, very likely merging into MCS traveling along the frontal boundary. This MCS, if oriented perpendicular to the low level flow, will probably result in severe wind gusts and this should be the most significant threat of the day. Due to the rather strong wind shear at lower levels (above 10 m/s), moderate LCLs and enhanced SREH values, tornadoes can occur as well, albeit the MCS mode of the convection might limit this threat. With stronger individual cells, marginally large hail can not be discounted and so can not be the possibility of back-building MCS with some excessive precipitation threat, although these threats are very conditional regarding to the situation development.

... Large portions of Central and Eastern/Southeastern Europe ...

Models agree on the development of weak to moderate degree of latent instability across a very broad region with no apparent synoptic scale forcing mechanism. In the very weak vertical wind profiles, disorganised convection if forecast, being initiated mostly by the local triggers, such as anabatic flows, convergence lines, etc... Especially in the mountainous areas, very slow storm movement might result in the locally high precipitation sums and at the same time, stronger "pulse" cells might briefly produce marginally severe hail or wind gusts. However it is difficult to choose a spot, where mesoscale conditions will become more favorable for such threats and no Level is therefore introduced with the expectation of only very isolated severe weather, especially over Poland, where models simulate the highest CAPE values.