Storm Forecast
Valid: Sun 26 May 2013 06:00 to Mon 27 May 2013 06:00 UTC
Issued: Sat 25 May 2013 23:21
Forecaster: PUCIK

A level 1 was issued for Central Spain mainly for large hail.

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

A level 1 was issued for Serbia, Eastern Poland and Western Romania mainly for large hail and to the lesser degree for severe wind gusts.

A level 1 was issued for Northeastern Romania, Ukraine and Belarus mainly for large hail, severe wind gusts and tornadoes.

SYNOPSIS

A deep cyclonic vortex at mid and upper troposphere centered initially over the Czech Republic and Southwestern Poland will undergo only very little change with respect to its position. Short-wave trough associated with it will translate from the Adriatic Sea towards Romania. Strong flow will be observed especially at the southern and eastern flank of the low. A relatively stagnant short-wave trough is forecast to cover especially Northwestern Iberia. An extensive ridge will hover over Russia and Finland as the warm air advection regime is underway. Closer to the surface, a shallow, but large low pressure system will cover most of Central to Southeastern Europe. Thus, cold airmass will have spread across most of Western / Central Europe with cold front advancing across Balkans towards east. On the other hand, wavy nature of the frontal boundary will allow for warm air and moisture advection over Ukraine, Belarus and also eastern Poland.

Most of the thunderstorm activity should stay confined to Southern and Eastern parts of Europe, elsewhere low level moisture will be too scarce to support any considerable DMC risk.


DISCUSSION

... Spain ...

With the approach of the short-wave trough, geopotential heights as well as mid-level temperatures are forecast to drop, yielding quite steep mid-level lapse rates over the region. On its forward flank, around 15 m/s of westerly-southwesterly flow at 500 hPa is simulated. As the surface low will form over the western part of Iberia, low level winds might shift to southerly or southeasterly direction ahead of the local convergence zones. CAPE values will likely be limited by insignificant dew points values close to the surface, but, very steep lapse rates along with the moderate vertical wind shear and colder mid-level temperatures might result in a slight risk of large hail in case that well organised multicells (supercells not being ruled out) manage to form in this environment.

... Serbia to Romania, Ukraine, Belarus and Eastern Poland ...

This region should experience strengthening of the mid to upper tropospheric flow as the trough approaches the region. Perhaps the strongest forcing is expected over Serbia and Western Romania - here, also the strongest flow is forecast ahead of the trough and DLS values may exceed 25 m/s. These two factors may compensate for the lack of CAPE, which should stay rather marginal due to the poor low level moisture. Nevertheless, this could suffice for a few low-topped, large hail producing supercells in environment of strong wind shear and steep mid-level lapse rates.

As we go more to the east and northeast, flow will become weaker, but low-level moisture prospects should be better and thus, CAPE values could be more pronounced. Falling surface pressure in the frontal trough could result in backing low level wind field and enhance degree of wind shear with DLS values mostly between 15 and 20 m/s. Moreover, it looks like there might a belt of enhanced SREH values, just to the north / northeast of the surface low advancing along the frontal zone to the north. Perhaps the best conditions for severe weather (regarding shear and instability overlap) will exist in the belt from NE Romania towards Central Ukraine and Belarus.

Thunderstorms will likely initiate near the frontal zone or close to the local convergence zones. In the beggining, scattered well organised multicells, or even few supercells, could pose the risk of large hail. Towards the evening, low level shear will increase, as well as the storm coverage. Thus, isolated severe wind gusts will be possible in case of forward-propagating MCS. Tornadoes are not ruled out as the LLS values could approach 10 m/s, but this threat will likely depend on the amount of "isolated supercell" convection mode during the evening.

... Algeria ...

In the strong westerly mid-level flow, surface low should be situated well inland with frontal system running over N Algeria. Along this system, significant easterly component of low level flow should exist, thus DLS values might reach over 25 m/s as a result. Steep lapse rates should contribute to enhanced CAPE values, especially over the western part of the Level 1. Albeit shear would certainly allow for well organised DMC, including supercells, confidence in initiation remains lower due to the lack of mid to upper tropospheric forcing. Some widely scattered storms might form over the front, posing risks of large hail thanks to the strong shear and steep lapse rates and/or severe wind gusts due to the deep and dry boundary layer.