Storm Forecast
Valid: Thu 30 Sep 2010 06:00 to Fri 01 Oct 2010 06:00 UTC
Issued: Wed 29 Sep 2010 15:46
Forecaster: GATZEN

A level 2 was issued for the northern Black Sea mainly for excessice rain, tornadoes, and severe wind gusts.

A level 1 was issued for southern Turkey mainly for tornadoes.

A level 1 was issued for the west Mediterranean Sea mainly for tornadoes and large hail.

SYNOPSIS

Low geopotential over the north-eastern Atlantic causes a rather southerly jet stream that expands into the Mediterranean as low geopotential is centered over eastern Europe. Short-wave troughs are forecast to move eastward with this jet. Over northern Europe, a high is located. Cool and dry air masses have spread across most of Europe in the wake of a cold front that has moved into the east Mediterranean, so that the potential of deep moist convection is low.

Southern Turkey

The severe weather episode is forecast to end during the forecast period, as low-level advection of cooler air is indicated by latest models. The potential for some organized storms capable of producing tornadoes, large hail, severe wind gusts, and excessive rain will only gradually decrease on Thursday, though. Especially along the axis of a very strong mid-level jet streak that crosses Turkey in the morning hours, severe storms are forecast given the rich low-level moisture and strong dynamic forcing. Given more than 25 m/s 0-6 km vertical wind shear, well organized storms are forecast including supercells due to favorably veering profiles. The potential for tornadoes is expected to be highest along the southern coasts of Turkey, where weak low-level winds and rich low-level moisture will be present initially.

Later in the forecast period, chance of new storms decreases from the north-west.

Northern Black Sea region

Explosive cyclogenesis is forecast during the night hours over the western Black Sea due to the approach of an intense negatively tilted mid-level trough and associated strong mid-level jet streak as well as warm air advection. At 06 UTC, models indicate that a 20 m/s southerly jet will accompany the occlusion at the 700 hPa level over the northern Black Sea region. The low-level profiles will likely be close to the moist-adiabatic lapse rate, but weak instability will be likely as the low-level moisture is quite rich.

Current thinking is that strong precipitation is likely along the occlusion. Embedded thunderstorms are forecast to pose a threat for excessive precipitation and also tornadoes given the strong low-level vertical wind shear in excess of 10 m/s in the lowest kilometer. Severe wind gusts may also occur due to the strong synoptic wind field. Later in the period, the potential of severe convection decreases strongly as the low moves north-eastward.

Balearic Islands region

Latest Palma de Mallorca sounding indicates that some low-level moisture is still present to the east of the Iberian Peninsula. With weak low-level winds, this moisture may increase slightly during the following hours. On Thursday, diurnal heating will likely lead to falling pressure over the Iberian Central Plateau. Superimposed to the developing land-sea-breeze south-easterly surface winds will likely evolve. A frontal boundary is expected to the north of the Balearic Islands due to cold air advection to the south of France. Along this frontal boundary, some low-level convergence is likely.

Above the boundary-layer, initially stable lapse rates will gradually improve in the morning hours as the Atlantic trough extends into France and Iberia. Together with the westerly mid-level flow, a vort-max will enter the west Mediterranean and QG forcing is expected to strengthen. Weak cyclogenesis is expected by numerical models to the east of the Iberian Peninsula in the evening and night hours and will likely be associated with increasing low-level forcing.

Current thinking is that increasing lapse rates will lead to weak CAPE on Thursday. Some showers or thunderstorms are not ruled out over the Balearic Islands in response to diurnal heating and low-level convergence. Best chances exist during the noon hours when the mid-level trough axis will provide some QG forcing. Bulk shear will reach values of 20 m/s in the lowest 6 km given the strong mid-level westerly flow and weak low-level winds. Forecast hodographs and increasing SRH values indicate some potential of supercells capable of producing large hail and severe wind gusts. Tornadoes are not ruled out near the coasts where low-level SRH and convergence are concentrated as indicated by latest forecast models.

Later in the period, several vort-maxima will still migrate eastward. Given the more concentrated low-level forcing to the north-west of the Balearic Islands, deep moist convection becomes more likely over the sea in the evening hours. Although the vertical wind shear is quite strong, the potential of marginally severe hail and severe wind gusts is expected to decrease gradually due to the approach of high geopotential from the west.