CONVECTIVE FORECAST
VALID 06Z SAT 21/08 - 06Z SUN 22/08 2004
ISSUED: 20/08 23:54Z
FORECASTER: DAHL

There is a slight risk of severe thunderstorms forecast across the N Balkans ... Romania ... W Ukraine.

General thunderstorms are forecast across central Europe ... N Europe ... E Euroope ... central Mediterranean regions.

SYNOPSIS

Main actor this period will be vigorous upper trough which is expected to sweep across the central Mediterranean regions and the Balkans on Saturday. Upper low over the North Sea is FCST to phase with the Mediterranean trough and move across S Scandinavia. In the wake of these features ... weak mid/upper ridging will overspread SW/W Europe. At low levels ... slowly weakening ... large-scale SFC low should persist over the Nordic Sea/N Scandinavia ... supporting low-level warm advecion deep into NE Europe/NW Russia. Main frontal boundary should extend from extreme NW Russia across the E Baltic States and E-central Europe into the central Mediterranean by Saturday 12Z ... with the SRN segments expected to leap eastwards upon approach of the upper trough.

DISCUSSION

...N Balkans ... Romania ... W Ukraine ... Italy...
Friday's 12Z and 18Z soundings from the central Mediterranean and the W Balkan States indicate substantial variability in boundary-layer moisture depth ... yielding CAPEs ranging from a few 100 J/kg to 2500 J/kg. There are no indications that this will change ... and mesoscale regions of ample instability should be present on Saturday across the Balkan States and S Italy ... maybe spreading into the Ukraine late in the day. Elsewhere across E Europe ... Friday's profiles are nearly neutral with no indications of locally enhanced BL moisture.

Deep-layer shear will be sufficient for severe storms with 20 to 25 m/s 500 hPa flow ... and about 10 m/s at 850 hPa. Majority of models assume falling pressure over the N Balkans ... spreading into S Poland ahead of the vigorous upper trough ... supporting 0-3 SRH in excess of 200 J/kg over Romania late in the day per GFS 12Z. Also ... complex terrain may locally support large SRH values.

Best chances for convective development will likely exist over the N Balkans late in the day where pressure falls should be maximized. Models support this scenario by producing maximum precip in this region. However ... although net QG upward forcing will likely be quite weak over Italy ... apparently owing to strong CAA forcing at low levels ... an isolated TSTM or two could initiate ... though confidence is limited ATTM given no precip signals in the 12Z/18Z model runs.

Convective mode in the SLGT and SEE TEXT regions may vary from rather high-based outflow dominant bowing lines to tornadic supercells given variability of thermodynamic profiles. Best signal in the model fields supportive to supercells exist late in the day over Romania where SRH is expected to be maximized.

...central Europe ... Scandinavia...
Diurnally driven TSTMS should form in uncapped and weakly unstable polar air mass over much of central and northern Europe in DCVA regimes and beneath thermal-trough axes where air mass/convective mixing should be deepest ... shear is quite marginal and severe TSTM threat should be rather low.

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