Storm Forecast
Valid: Sun 04 May 2008 06:00 to Mon 05 May 2008 06:00 UTC
Issued: Sat 03 May 2008 19:56
Forecaster: GROENEMEIJER/PUCIK

SYNOPSIS

The dominant feature of the day will be an amplifying ridge of high pressure at 500hPa stretching from southern France to southern Norway. A mid-level low filled with cold air over the Norwegian Sea early on Sunday is should reach southwestern Finland on Monday morning. A similar feature moves slowly southward from southwest Poland to eastern Austria during the forecast period. A through slowly over the Bosporus slowly moves onto the Black Sea. At the surface, large area of high pressure with center above Scandinavia dominates the low level circulation. During the afternoon and evening a weak shortwave trough moves into Iberia.

DISCUSSION

North Spain...

On the approach of the aforementioned trough, forcing for upward vertical motion and strong surface surface heating are expected to yield up to 1000 J/kg of mixed-layer CAPE as the day progresses. Convective initiation is expected during the day with aid of local, orographic circulations before the storm coverage increases further during the evening hours. Deep level shear will be moderate, between 10 to 15 m/s in the 0-6 km layer, resulting mostly multicellular storm organisation. However, the combination of moderate shear, instability and quite significant lapse rates provide favourable conditions for isolated large hail. Due to the drier air mass underneath the storms, some marginally severe wind gusts cannot be ruled out either.

France...

Moderate instability with CAPE values up to around 1000 J/kg are expected over central and southern France, where low-level dew points should climb to 14-15 C in many places. This is less than the GFS predicts, which has shown a tendency to overestimate moisture during the last few days. Windshear shoud be rather weak low, on order of 10 m/s in the 0-6 km layer, so that multicell clusters are the most likely storm mode. For now, we are not forecasting severe weather in France, although if moisture is higher than anticipated, isolated large hail would be a distinct threat. Therefore we will monitor the area for a possible update.

Belt from North Finland to Romania...

The combination of a relatively cold air mass aloft and surface heating will result in the development of at least marginal instability. A lack of vertical wind shear and low instability will preclude a risk of severe weather. But, due to the cold air mass, some stronger storms can produce hail, locally between 1 and 2 cm in diameter. The risk of hail seems to be quite high especially across the Finnish part of the thunder area.
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