Storm Forecast
Valid: Mon 13 Jun 2011 06:00 to Tue 14 Jun 2011 06:00 UTC
Issued: Sun 12 Jun 2011 22:55
Forecaster: DAHL

A level one threat has been issued across the eastern Ukraine ... eastern Belarus ... and extreme northwestern Russia, mainly for excessive convective rain and large hail.

A level one threat has been issued over Switzerland ... S Germany ... Austria ... and N Italy mainly for large hail and excessive convective rain.

A level one threat has been issued for NE Spain and extreme S France, mainly for large hail.


SYNOPSIS

A rather broad, meandering upper westerly flow extends from the Atlantic across the southern portions of Europe. N of this regime,
there is an extensive quasi-stationary upper low S of Iceland, affecting the NW and N parts of the forecast region. At low levels, the flow pattern is rather stagnant, except for weak and rather small low-pressure systems associated with upper perturbations at the periphery of the upper low, and those imbedded within the broad westerly upper flow. It seems that there will be a rather extensive region of positive CAPE across much of Europe, though again the GFS seems to be overdoing the boundary-layer moisture over parts of Europe, so e.g., the CAPE maxima over eastern Europe are probably not as pronounced in reality. Unfortunately, no sounding data are available from this region, so the degree of instability remains uncertain.

DISCUSSION

... E Ukraine into extreme NW Russia ...

Although the magnitude of CAPE is unknown at the moment, Sunday's convective evolution included several large clusters, and
this should happen again on Monday. Shear is quite weak, but as storms coagulate into clusters, there is a chance of locally excessive rainfall as well as large hail.

... Switzerland ... S Germany ... Austria ... N Italy ...

The inter- and pre-Alpine regions will likely be a focus for convective development, aided by weak large-scale ascent. Though indications are that instability and shear will be weak, the elevated terrain and/or local augmentations of the environment owing to the complex terrain could support isolated large hail. Moreover, local heavy rain/flooding could occur.

... NE Spain ...

Shear over NE Spain should be marginally supportive of organized convection capable of large hail and perhaps damaging winds. Expect at least a few isolated cells to develop during peak-heating hours, but to decay in the evening hours with the loss of daytime heating and increasing mid-level ridging.

Elsewhere ... QPF signals in the models and environmental parameters do not look supportive of severe convection. However, serendipitous coincidence of local augmentations of the thermodynamic/kinematic fields might locally/briefly result in large hail/damaging winds. Also local flooding could occur. However, there are no clear signals that would point towards a preferred location, and the severe-weather probabilities relative to the entire region where convection is expected are well below LVL1 limits.