Severe storms to be expected across Europe and N Tunisia



  • Valid: Thu 29 Aug 2019 06:00 to Fri 30 Aug 2019 06:00 UTC
    Issued: Wed 28 Aug 2019 23:47
    Forecaster: GATZEN

    • A level 1 was issued across Lithuania, Poland, west Belarus and Ukraine, Czechia, Slovakia, east Austria, west Hungary, Slovenia, north Croatia, and north Bosnia and Herzegovina mainly for excessive rain and locally large hail and severe wind gusts.
    • A level 1 was issued across Sweden and surroundings mainly for severe wind gusts and locally large hail and excessive precipitation.
    • A level 1 was issued across the central Mediterranean mainly for waterspouts, excessive precipitation and large hail and to a lesser extent severe wind gusts.
    • A level 1 was issued across the Alps and Germany mainly for excessive rain and severe wind gusts and locally large hail.
    • A level 1 was issued across southern France and northern Spain mainly for large hail, severe wind gust, and excessive precipitation.
    • A level 1 was issued across northern Tunisia mainly for large hail, severe wind gust, and excessive precipitation.

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    Synopsis

    A weakly-forced situation has established across most of Europe. Mid-level ridging and moist low-level air extend from the west Mediterranean to central Europe and into Scandinavia. Steep lapse rates overlap with the eastern portion of this air mass, and moderate to high MLCAPE and weak CIN are forecast across the broad area. Stronger synoptic forcing evolves across north-western Europe ahead of an approaching short-wave trough. Here, strong deep layer shear overlaps with weak to moderate MLCAPE locally.

    Discussion

    Lithuania, Poland, west Belarus and Ukraine, Czechia, Slovakia, east Austria, west Hungary, Slovenia, north Croatia, and north Bosnia and Herzegovina

    MLCAPE around 1000 J/kg is expected during the day due to moist low-level air and steep mid-level lapse rates. In a well-mixed boundary-layer with weak CIN, storms will likely develop over mountains, as well as along outflow boundaries and sea breeze convergence zones. Stronger storms will pose a threat of severe downbursts due to moderate DCAPE and high water loading since some updrafts will be strong and the low-level mixing ratio and precipitable water are high.

    Storms may cluster along their outflow boundaries but further organization is unlikely given weak vertical wind shear. Storm coverage may be highest over north-east Poland and Lithuania near a weak-defined warm front. Here, veering profiles can support some briefly rotating updrafts with a threat of large hail together with severe wind gusts and excessive precipitation.

    Sweden and surroundings

    Moist and unstable air masses will be affected by an approaching short-wave trough during the day. Ahead of a cold front, storms are expected to develop in weak CIN across the widespread area. While MLCAPE is highest in the south-east, vertical wind shear increases in the north-west, where models predict up to 20 m/s DLS overlapping with low MLCAPE.

    Main storm mode will be multicells in association with straight line hodographs and DLS around 12 to 15 m/s. Lines of storms may form capable of producing severe wind gusts. Additionally, excessive precipitation is not ruled out. Storms may organize into an MCS crossing the Baltic Sea during the night affecting Finland and the Baltic States in the morning. Due to the stable boundary layer, potential of severe gusts will decrease during the night.

    Central Mediterranean

    Moist air masses will locally become unstable close to the mid-level trough that slowly moves east. Convective storms are most likely along the land-breeze convergence zones over the Tyrrhenian Sea in the morning with a threat of waterspouts given weak vertical wind shear. Later in the day, new storms are forecast along the sea breeze and over the mountains of Italy and Sicily as well as Sardinia and Corsica.

    Main threat is local flash flooding in a moist air mass and due to the slow storm motion. Large hail and severe wind gusts are not ruled out. Late in the period, storms will again move offshore and are forecast to go on through-out the night.

    Alps, Germany

    Moderate MLCAPE, weak vertical wind shear, and weak forcing are expected. As CIN is also weak, scattered storms are forecast over the mountains and ahead of a weak cold front that moves eastward during the day. Here, storms can merge to a linear cluster given storm motion vectors aligned with the cold front. Excessive rain is the main threat. However, locally, severe wind gusts and large hail are not ruled out. Storms will spread into south-eastern Germany late in the period.

    Southern France and northern Spain

    In a weakly-sheared environment, storms are expected due to rich low-level moisture and steep lapse rates as well as some lift over the mountains and ahead of an approaching trough. Excessive rain is the main threat, together with some large hail and severe wind gusts.

    Northern Tunisia

    Scattered storms are forecast during the day. Moderate vertical wind shear will support some multicells and supercells, capable of producing excessive rain, severe wind gusts and locally large hail. In the evening, storms are expected to decay.

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