Storm Forecast
Valid: Thu 04 Jul 2019 06:00 to Fri 05 Jul 2019 06:00 UTC
Issued: Wed 03 Jul 2019 23:52
Forecaster: GATZEN

A level 1 was issued for southern Romania, Bulgaria, Montenegro and Bosnia and Herzegovina region, and Croatia mainly for large hail, severe wind gusts, and excessive precipitation.

A level 1 was issued for Italy and the southern Alps mainly for large hail and excessive precipitation.

A level 1 was issued for north-western Spain mainly for large hail and severe wind gusts.

A level 1 was issued for southern Sweden mainly for severe wind gusts.

SYNOPSIS

A trough has established across northern Europe with a mid-level jet across the North Sea towards Poland and farther into southern Russia. Cool polar air has spread into Europe behind a cold front that is now almost stationary across the north Mediterranean and southern Balkans. To the south, high lapse rates are present that overlap with rich low-level moisture along the frontal boundary. Although lapse rates are weakening, high CAPE will still be possible. On the other hand, vertical wind shear is expected to be weak over most places.

DISCUSSION

Central and southern Balkans

Along the frontal boundary, storms are also expected on Thursday. High CAPE is forecast across Bulgaria and around Bosnia and Herzegovina, where steep lapse rates and rich low-level moisture overlap. Farther north, lapse rates will be quite stable, whereas steep low-level lapse rates will evolve farther south in response to diurnal heating. Rather weakly large-scale forcing will cause storms to fire over the mountains given weak capping. Over most places, storms will be rather weakly organized due to deep-layer shear below 10 m/s. Clusters of storms are forecast capable of producing large hail in their initial stages and some severe wind gusts and excessive precipitation events during their mature stage. Potential of severe wind gusts increases in the southern portions due to better mixing of the subcloud air mass.

Stronger vertical wind shear is expected over the area of Bosnia and Herzegovina, straight line hodographs overlap with more than 1000 J/kg MLCAPE. Storms may develop rotating updrafts capable of producing large hail. Additionally, clusters may organize, posing a threat of severe wind gusts. Overall severe potential is limited, though. Stronger deep layer shear is also forecast over Romania, but CAPE is forecast to be rather weak. A few severe storms are not ruled out, with large hail and severe wind gusts the main threat. Storms are forecast to weaken rapidly in the evening.

Italy and southern Alps

Under weak north-westerly flow, an air mass characterized by rich low-level moisture and modest lapse rates will become unstable during the day with locally more than 1500 J/kg MLCAPE. Storms are forecast to form over the mountains and also along sea breeze boundaries. Most storms will be weakly organized and main severe potential is due to pulse type storms and associated large hail and excessive precipitation. Close to the Alps, vertical wind shear may be stronger locally, supporting one or two mesocyclones with a higher severe potential.

North-western Spain

A cut-off trough west of Iberia yields a mid-level southerly jet. Its cyclonically sheared flank affects north-western Spain. At lower levels, warm air advection evolves during the day. The affected air mass is characterized by steep lapse rates but limited low-level moisture so that MLCAPE will be below 1000 J/kg over most places. Additionally, deep layer shear will be rather weak as well, with about 10 m/s in the lowest 6 km. Therefore, storms will be rather weak. Large hail is not ruled out, as well as severe wind gusts, in particular where the boundary-layer is well-mixed.

Southern Sweden

An intense polar trough rapidly digs southward across Scandinavia, associated with a PV intrusion and increasing low-level lapse rates. Affected low-level air mass is rather dry, and only weak showers and thunderstorms are forecast initially. Severe wind gusts are not ruled out, though. Later in the day, the PV intrusion will partly overlap with a cold front and prefrontal moisture may overlap with improving lapse rates. Latest models indicate some weak CAPE in strongly-sheared environment (0-6 km shear between 30 and 40 m/s). Current thinking is that some storms can develop bow echoes with severe wind gusts the main threat. More widespread wind gusts are not expected at this time, however, if a longer convective line forms, severe wind gusts could occur rather widespread.