Storm Forecast
Valid: Thu 19 Jun 2014 06:00 to Fri 20 Jun 2014 06:00 UTC
Issued: Wed 18 Jun 2014 22:52
Forecaster: GATZEN

A level 2 was issued for north-western Turkey mainly for very large hail, tornadoes, and to a lower extend excessive rain and severe wind gusts.

A level 2 was issued for eastern Bulgaria and north-eastern Greece mainly for excessive rain.

A level 1 was issued for the central Balkans, northern Greece, and southern Romania mainly for large hail and excessive precipitation.

A level 1 was issued for eastern Spain mainly for large hail and to a lower extend severe wind gusts.

SYNOPSIS

A long-wave trough is placed over southern Europe, centred over Italy. A strong mid-level jet streak at its southern periphery is directed towards the Aegean Sea region on Thursday morning. The long-wave trough will accelerate east at the southern flank of a new polar outbreak into northern and central Europe. Mid-level ridging will affect the west Mediterranean and east Iberia therefore. Western Iberia is affected by an Atlantic cut-off low. At lower levels, an axis of warmed polar air has developed from southern France across the Alps into the Ukraine. Another plume of warm air evolves ahead of the cut-off low over Spain. Cold air advection is dominant from northern Europe into Germany as well as over south-east Europe.

DISCUSSION

North-western Turkey

A dangerous weather situation evolves over parts of north-western Turkey on Thursday. A strong mid-level jet streak curves around the Mediterranean long-wave trough. It affects the Aegean and Black Sea region together with an intense negatively tilted short-wave trough. An ageostrophic low-level flow will advect steep lapse rates from western Turkey northward into the Black Sea area. Low-level flow will remain from the east near the Black Sea due to cyclogenesis over the Aegean and western Turkey. A cold front will enter western Turkey until noon and will continue eastwards in the afternoon.

Rich low-level moisture is present over the Black Sea, but advection of drier air behind overnights storms may occur. On Thursday, easterly surface winds will likely lead to moisture pooling below the elevated mixed layer, and models agree on at least 12 g/kg low-level mixing ratio. Daytime heating may be slightly limited as high and mid-level clouds may travel north-east ahead of the short-wave trough. Nevertheless, robust CAPE of 1500 J/kg or more is forecast.

Current thinking is that storms initiate over north-western and western Turkey during the noon hours. Convective activity will spread north-east until the evening. Strong vertical wind shear (25 m/s 0-6 km bulk shear) will allow for rapid storm organization, with supercells most likely over northern Turkey where favourably curved hodographs are forecast. Very large hail and damaging wind gusts are possible with every supercell that forms. Additionally, tornadoes are forecast, woth the focus along the sea-breeze convergence, where moisture pooling and low-level vertical wind shear is strongest. A strong or even violent tornado is not ruled out. Storms are expected to be rather isolated so than a level 2 was issued rather than a level 3.

Storms are expected to move into the Black Sea in the evening hours, where a cluster or MCS may develop in response to the strong QG and frontogenetical forcing. High winds and excessive rain will be the main threat during the evening and night hours, but large hail and tornadoes are not ruled out as well.

South-eastern Bulgaria and surroundings

Strong synoptic-scale forcing persist through-out the day due to the approach of an intense negatively tilted trough axis. Low-level moisture advection is forecast while mid-level lapse rates will be relatively steep, creating moderate CAPE up to 1000 J/kg. Widespread thunderstorms are forecast that will become slow-moving during the period near the trough center. The main threat will be excessive precipitation and flooding. Additionally, storms may produce large hail, especially in the morning hours when vertical wind shear will be stronger. In the evening hours, the threat will gradually decrease due to low-level stabilization.

Italy to central Balkans and Greece

Near the through center, weakly-sheared storms are expected to develop. These will pose a threat of excessive precipitation over the Balkans, whereas drier air will limit the threat further west. Along the southern portions of the area, stronger deep layer vertical wind shear will be present on Thursday morning. Multicells or supercells may develop with the focus in the morning and noon hours over Greece. Large hail and excessive precipitation are forecast, together with an isolated wind gust and tornado threat.

Eastern Spain

Low pressure near the axis of warm air over central Iberia will cause an easterly upslope flow over eastern Spain. Rich moisture from the Mediterranean will overlap with relatively steep lapse rates in response to diurnal heating. Thunderstorms are forecast to develop in the afternoon hours. Given a moderately strong westerly mid-level jet, deep layer vertical wind shear will exceed 15 m/s, and multicells and supercells are forecast. Main threat will be large hail, but severe wind gusts are not ruled out due to local inverted-v profiles. Storms will weaken after sunset.