Storm Forecast
Valid: Thu 22 Aug 2013 06:00 to Fri 23 Aug 2013 06:00 UTC
Issued: Wed 21 Aug 2013 22:13
Forecaster: GATZEN

A level 1 was issued for western Belarus mainly for large hail and excessive rain.

A level 1 was issued for western Turkey mainly for large hail.

A level 1 was issued for eastern Iberia mainly for large hail.

SYNOPSIS

Weak flow will be present across Europe in the range of a ridge extending into the North Sea and rather weak geopotential over eastern and southern Europe. The only exception is a cut-off low over Poland that slowly moves east as well as an intense short-wave trough moving across Finland.

DISCUSSION

Finland and north-west Russia

A well-developed short-wave trough moves eastward across Finland on Thursday. It will form a closed mid-level low at the end of the period. Cold air advection dominates the lower troposphere, but backing low-level flow in the north-eastern area of the forming cut-off will be associated with some warm air advection at the end of the period across north-west Russia. Given rather weak low-level moisture ahead of the trough and associated cold front and weak convergence along the cold front, initiation of stronger storms is questionable. Most showers and thunderstorms may be short-lived and rather weak, although vertical wind shear is relatively strong. Best potential for a stronger storm will be across north-west Russia in the late afternoon hours given some warm air advection and low-level convergence.

Poland into Belarus and Ukraine

An intense cut-off low moves eastward on Thursday. At lower levels, a frontal boundary extends from the Baltic States to western Belarus, Ukraine, and Romania. This cold front forms a wave ahead of the approaching cut-off low. Along this frontal wave, low-level moisture will likely overlap with steep lapse rates near the cut-off center. Especially in the afternoon hours, CAPE is expected to build. Showers and thunderstorms are forecast due to low-level convergence and QG forcing. Storms will likely merge along the cold front and move eastward as a squall line. Most storms will be likely not severe, however, around 15 m/s 0-3 km vertical wind shear is possible when low-level winds will turn to south-west ahead of the low pressure center. Therefore, a few better organized storms are forecast, capable of producing locally large hail and excessive rain. A tornado or wind event is also not ruled out. Ongoing QG forcing may keep the activity alive well into the night hours, but low-level cooling will lead to elevated storms and a weaker threat.

South-eastern Europe and Mediterranean

Weak geopotential with cold mid-levels and warm sea surface temperatures and diurnal heating will result in rather good lapse rates. Additionally, low-level moisture is rich across the sea as well as along the frontal boundary extending from Belarus into the Balkans. Rather weakly-capped CAPE will develop in a weakly-sheared environment. Mountain flow and sea-breeze convergence will be most important for convective initiation and storms are forecast during the day. Excessive precipitation is the most important threat given the slow storm motion. However, some waterspouts may be also possible in the morning hours along the coasts.

Over western Turkey, vertical wind shear will increase during the day. Weak northerly low-level winds will advect moist air masses from the Black Sea into the higher terrain, and upslope flow is expected to result in thunderstorms. These may organize given 15 m/s 0-6 km bulk shear. Large hail seems to be the most important threat.

Eastern Iberia

A weak mid-level trough moves into Iberia on Thursday. To the east of the thermal low, very moist upslope flow is expected over eastern Spain that overlaps with an EML. Large CAPE and at least 10 m/s deep lapse bulk shear are forecast. Storms are forecast to initiate over the mountains and slowly move eastward. Main limiting factor will be the strong capping inversion. Large hail and locally excessive precipitation is forecast.

British Isles

A mid-level low moves into the British Isles. Rich low-level moisture and increasing lapse rates result in CAPE during the day. Although vertical wind shear is weak, a tornado is not completely ruled out near sea breeze convergences. Additionally, excessive precipitation is possible with the slow-moving storms.