Storm Forecast
Valid: Sun 13 Apr 2008 06:00 to Mon 14 Apr 2008 06:00 UTC
Issued: Sat 12 Apr 2008 21:37
Forecaster: SCHLENCZEK

SYNOPSIS

An almost stationary low pressure complex at upper levels is present over the North Sea, surrounded by a 40 m/s jet at 300hPa with some embedded shortwave troughs. One of these shortwaves will cross western France during the period, another one should rapidly move over northwestern Germany towards Scandinavia. Cold air convection with showers and weakly electrified thunderstorms is expected in the whole trough region.

Remnants of the low pressure system that led to thunderstorms over eastern Germany and Poland during the last two days should allow some thunder over northeastern Ukraine / southwestern Russia.

A frontal boundary that stretches from the Baltic States via western Ukraine and Romania towards southern Italy and Tunisia separates very warm and moist air to its east from a cold polar airmass to its northwest. 850hPa temperatures about 20°C are expected over the eastern Mediterranean and the Black Sea region.

DISCUSSION

...Eastern Aegean, Bulgaria and western Turkey...

An impressive EML has been advected into this region. Recent observations showed rich BL moisture with dewpoints up to 15°C but
the 12Z sounding of 16622 Thessaloniki indicates that the moist layer is probably very shallow. Nevertheless, GFS predicts some convective precip in the late afternoon / evening and some hundred J/kg of CAPE when an upper level vort max embedded in a 35 m/s jet streak at 500hPa will cross the region. Deep layer shear between 15 and 25 m/s as well as 0-3 km storm relative helicity in order of 300 J/kg in a highly capped environment are favourable for a few supercells, capable of producing large hail and possibly isolated severe gusts. Because of the huge cap, convective initiation is uncertain but a level one threat seems to be warranted nonetheless.

...southwestern and central France...

In the morning, the left exit region of a 40 m/s jet streak over the east Atlantic should approach western France and some convection will develop. In the vicinity of the jet streak, deep layer shear should reach values up to 30 m/s which is supportive for some organized multicells with a marginally severe hail threat. SRH3 in order of 200 J/kg and strong DLS may allow a few shallow mesocyclones and a funnel / brief tornado is not ruled out as low level shear around 12 m/s is expected and LCL heights are rather low. As CAPE is rather low and the exact location of the favourably sheared region is rather unpredictable given its small extent, a threat level is not warranted at the moment.

...northeastern Germany and Denmark...

Ahead of the upper trough, insolation should create a few hundred J/kg of CAPE in an environment with deep layer shear around 15 to 20 m/s and diurnally driven showers and thunderstorms are expected. An isolated large hail report and / or a brief funnel is not discounted but overall threat is too marginal for a categorical threat level.