CONVECTIVE FORECAST
VALID 12Z FRI 13/08 - 06Z SAT 14/08 2004
ISSUED: 13/08 11:54Z
FORECASTER: GATZEN

There is a slight risk of severe thunderstorms forecast across NErn Italy, Balkans, Bulgaria, Romania, western Ukraine

General thunderstorms are forecast across Central Europe, England, southern Scandinavia, eastern Europe, southeastern Europe

SYNOPSIS

Intense upper trough is present over western Europe with a major axis reaching from western Great Britain to northern Germany. Strong upper jet stream is analyzed at the southern flank of this trough from central France to northern Italy and further to eastern Europe. Over the Mediterranean ... upper long-wave ridge id present. On Friday ... western trough will separate into two parts as eastern vort-max will merge to northeastern European long-wave trough and southern upper high ridges into Great Britain. At lower levels ... cold airmass over Central Europe spreads into northern Italy as well as into eastern Europe west of a cold front. Over southeastern Europe ... warm airmass will remain during the day.

DISCUSSION

...Northeastern Italy and Balkans ...
Over northeastern Italy ... warm and unstable airmass is still present ... yielding CAPE values in the order of 700 J/kg indicated by latest GFS model output. Latest observations show rich low-level moisture ... with surface dewpoints in the range of 20°C. During the day ... cold airmass should spread into northern Italy from the west and north. However ... cyclogenesis over central northern Italy creates easterly surface winds over northeastern Italy. Rather strong low-level convergence is expected in the range of the developing low. Aloft ... very strong jet streak centered over the Alps ... and strong deep vertical wind shear is present over northern Italy. During the day ... approaching vort-max of European trough will move SE-ward ... and UVM is expected. Thunderstorms should initiate during the next hours and move eastward. Strong deep-layer wind shear ... with westerly winds up to 40 m/s at the 300 hPa level ... should be favorable for organized thunderstorms and supercells. Large hail and severe wind gusts will be possible with any supercell that forms. Rather low LCL heights and locally enhanced SRH values may be favorable for tornadic supercells as well. It is not ruled out that one or two tornadoes may be strong. Thunderstorms may merge into a MCS moving eastward into northwestern Balkans. Severe wind gusts and isolated large hail should be possible as well as a few tornadoes. To the south ... unstable airmass is present over the Balkans and Adriatic. Approaching trough and associated jet streak should lead to UVM and increasing deep layer wind shear. Thunderstorms are expected to initiated during the afternoon. Deep vertical wind shear may be favorable for mesocyclones and supercells are not ruled out. Large hail and severe wind gusts should be the main severe threat. There is also an enhanced potential for tornadoes indicated by high surface dewpoints and associated LCL heights. However ... it is not clear if low-level moisture is deep ... and locally LCL heights may be higher ... that should limit tornado potential.

...Eastern Europe ...
East of the cold front ... rather moist warm airmass is present over Romania and western Ukraine ... that should be instable due to insolation. Approaching vort-max and low-level convergence along the cold front may lead to sufficient UVM to initiate thunderstorms. Deep vertical wind shear may be sufficient for organized convection. Supercells should be possible capable of producing isolated large hail and severe wind gusts. Latest GFS output suggests relatively low LCL heights quite strong low-level (0-1km) vertical wind shear ... and a few tornadoes are expected if supercells will form. However ... QG forcing should be limited along the southern part of the cold front ... while thermodynamics are weak along the northern part. Best chances for tornadoes are expected over southwestern Ukraine. Thunderstorms may merge into a MCS over the Ukraine later on. Severe wind gusts should be the main threat if a MCS will form.

...Benelux, Germany, northern France ...
In the range of the upper trough ... cold airmass has destabilized due to daytime heating. Relatively rich low-level moisture is present and thunderstorms have developed during the last hour. Approaching vort-max is expected to cross Benelux and northern Germany during the next hours reaching southern Germany in the evening hours. UVM is expected and enhanced convection is forecast underneath the vort-max. Rather strong low-level wind shear is forecast by latest model output ... and thunderstorms may organize into small bows. There is also an enhanced chance for shallow mesocyclones given relatively large SRH values over central Germany. If supercells will form ... isolated tornadoes are possible. However ... QG forcing is expected to be rather weak and overall threat should be too low to warrant a SLGT RISK.

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