CONVECTIVE FORECAST
VALID Sat 28 May 06:00 - Sun 29 May 06:00 2005 (UTC)
ISSUED: 27 May 22:20 (UTC)
FORECASTER: Johannes Dahl
There is a slight risk of severe thunderstorms forecast across parts of the Balkans and the northern Black Sea region
SYNOPSIS
Deep and rather intense upper SWLY flow has established across WRN portions of Europe ahead of E-Atlantic long-wave trough ... with strong imbedded short-wave trough extected to lift from the British Isles into central Scandinavia during the period. Large-scale upper ridge will persist across central and eastern portions of Europe ... with weak upper low covering SE Europe. Fairly intense SFC low expected near Ireland at the beginning of the period ... will slowly move into the North Sea until Sunday morning ... while synoptically quiescent conditions continue over the rest of Europe. Main ... and quite intense ... low-level baroclinic zone should curve from W Spain across the Gulf of Biscay and W France into the North Sea towards the SFC-low center ... and from there stretch eastwards into the Baltic Sea by the beginning of the period. Mesoscale frontal waves and pre-frontal SFC-pressure troughs are expected to exist across western and central portions of Europe on Saturday.
DISCUSSION
...central and SW Europe...
According to radiosonde data from Friday 12Z ... ample instability has not managed to develop as of Friday afternoon ... though some profiles suggest that strong destabilization could easily occur once deeper boundary-layer moisture happens to be available. Latest SFC OBS indicate dewpoints in excess of 17°C across central France ... though indications are that this moisture is probably rather shallow.
Scenario will be somewhat complicated as plume of subtropical-Atlantic air mass will be advected across the WRN portions of the warm sector immediately ahead of the main cold front. It appears to be unlikely that this air mass will contain steep lapse rates ... and it may tend to be rather stable. However ... steep lapse-rates are present farther E ... and chances may exist that the moist Atlantic air underruns the EML ... creating moderate CAPE.
Altogether ... degree of instability ... and shape of thermodynamic profiles remains somewhat uncertain ATTM ... but confidence is rather high that at least weak CAPE will be present across much of WRN and central Europe.
Shear profiles across central Europe should be quite weak ... but improving towards the west ... with 15 to 20 m/s 500 hPa flow over Benelux and W France. Low-level shear looks to be rather marginal with 5 to 10 m/s 850 hPa flow ... but may locally be augmented by mesoscale features.
Large-scale forcing for UVM will be quite weak to nonexistent ... and TSTM evolution should generally be rather isolated ... being tied to mesoscale sources of ascent. Such sources may be a pre-frontal wind-shift line expected to extend from S Sweden across NE Germany into central Germany on Saturday afternoon/evening ... and a wave low FCST to develop in the afternoon hours over Denmark. Convergence along the main SFC cold front seems to be rather weak so that TSTM activity may tend to be confined to the pre-frontal areas.
TSTMS may produce some hail and gusty winds ... but severe threat seems to be rather limited given weak shear and thermodynamic profiles. Any severe threat will depend on meso-/convective scale augmentations of the profiles ... which is too conditional for a categorical risk.
Across Spain ... isolated TSTMS may occur late in the day. Thinking is that inverted-V type profiles will develop ... and that an isolated high-based TSTM or two may develop late in the day. Expect strong convective outflow winds with this activity ... but threat is too low for a categorical risk.
...Sweden and Denmark...
GFS 12Z and NMM suggest wave cyclone to develop over Denmark in the afternoon hours ... and track into S Sweden. Deep- and low-level shear should be enhanced in the vicinity of this feature. It looks that there will be much mid-level cloudiness/convection associated with the warm front ... but a window of few hours may exist in the afternoon/early evening hours where convection may develop along and ahead of the cold front. This activity would have the potential for becoming supercellular ... capable of producing large hail ... damaging winds and maybe a brief tornado. Threat is too conditional for a SLGT ATTM.
...North Sea...
At the beginning of the period ... weakening and slightly elevated convection may be present along the cold front over the WRN North Sea ... and possibly in the WAA regime farther N. Deep shear will be quite strong ... and a few severe TSTM events may occur. However ... lack of SFC heating over the water and weakening large-scale forcing for ascent with the lifting trough should limit allover severe threat.
...SE Europe...
At the periphery of the SE European upper low ... between 15 and 20 m/s mid-level flow is expected ... and 850 hPa flow in excess of 10 m/s. CAPE should be rather weak ... but nearly uncapped ... with profiles being rather moist throughout the troposphere. All this suggests that rather widespread TSTM development may occur again ... probably with isolated mesocyclones. Expect a few large-hail and severe-wind events ... though a brief tornado cannot be ruled out given moderate low-level shear and little CINH.
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