Storm Forecast
Valid: Thu 29 Apr 2010 06:00 to Fri 30 Apr 2010 06:00 UTC
Issued: Wed 28 Apr 2010 22:10
Forecaster: GATZEN

A level 1 was issued for southern France and northern Spain mainly for large hail.

A level 1 was issued for central and northern France as well as the Benelux countries mainly for large hail.

A level 1 was issued for southern Italy mainly for large hail.

SYNOPSIS

A broad ridge over Central Europe weakens and is shifted southward as a pronounced long-wave trough over the north-eastern Atlantic expands into northern Europe. A sharp short-wave trough over the Bay of Biscay shrinks during the forecast period as it is absorbed by the approaching jet. Its slightly negatively tilted axis is expected over Germany at the end of the period. From the low geopotential over eastern Europe, the southern portions cut-off over the Balkans as the axis of the European ridge tilts southward. While a tongue of warm air spreads north-eastward across France and Germany ahead of the short-wave trough, rather cool air masses will be present over eastern Europe and most of the Mediterranean. Cold air will also advect into the British Isles during the period.

DISCUSSION

Southern France, northern Spain

A warm air mass present over Iberia will spread further north-eastward ahead of low geopotential over the Bay of Biscay. Latest soundings do not indicate steep lapse rates or ample moisture over Iberia so far, so it is quite questionable that sufficient instability will develop. On Thursday, the jet streak of the mentioned shorts-wave trough will move north-eastward, leaving the Pyrenees at its right entry region, where some QG forcing becomes likely. Indeed numerical models predict mid-level cooling, while the low-level air remains warm. However, the boundary-layer will likely be rather dry and given the warm air above the boundary-layer, sufficient instability remains questionable. However, given orographic induced easterly winds to the north of the Pyrenees, moisture flux convergence becomes likely over southern France, where some instability may be released in the afternoon and evening hours. Storms that develop will likely ingest veering profiles at low-levels, and storm rotation becomes likely. Given the rather strong mid-level southerly flow, about 20 m/s deep layer vertical wind shear will assist for storm organization. Large hail seems to be the most significant threat with these storms that may cluster later on given the rather large cold pool strength due to the dry air mass. Storms are expected to decay as they move eastward and leave the region with low-level convergence.

Over northern Spain, a comparable situation evolves over the mountains, with some low-level convergence and moisture pooling during the day. While the lapse rates are forecast to be steeper compared to France, the deep-layer vertical wind shear as well as the low-level veering will be weaker. While the potential of storm organization seems to be weaker as over France, the coverage o storms may be higher given the higher due to the high temperature over the highlands. While the storms will likely move along the convergence to the north-east, they may produce strong precipitation especially over the Pyrenees, where excessive rain is not ruled out.

France and Benelux

Warm and rather moist air advects into France ahead of the approaching short-wave trough. Models indicate low-level mixing ratios up to 10 g/kg. The general set-up seems to be quite unfavourable for severe convection given that the low-level air spreads eastward quickly, leaving the short-wave trough to the west. Weak low-level convergence is expected over most regions, and initiation of storms is rather questionable. Every storm that manages to form will have a good potential to organize given 20 to 25 m/s deep layer vertical wind shear. Current thinking is that isolated storms are possible posing a threat of isolated large hail and severe wind gusts.

The storms are expected to weaken slowly in the evening and night hours, given decreasing instability but on the other hand adequate lifting ahead of the short-wave trough and along the low-level convergence line.

Southern Italy

The sharp and intense cut-off low that forms over the Balkans moves across southern Italy during the day. This will provide steepening lapse rates while the low-level moisture will remain in the range of 7 g/kg. Diurnal heating will likely lead to instability and initiation is expected along of sea-breeze convergence lines. Storms that develop will profit from a mid-level northern jet streak that intensifies along the western flank of the cut-off, resulting in multicells that will be capable of producing large hail. Late in the period the cut-off low moves across the Mediterranean, where the cool low-level air mass and weak convergence limit the potential of deep convection gradually.