Severe storm expected across Tyrrhenian Sea, Italy, Spain, Algeria and N Tunisia
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Valid: Thu 19 Sep 2019 06:00 to Fri 20 Sep 2019 06:00 UTC
Issued: Wed 18 Sep 2019 23:42
Forecaster: GATZEN- A level 1 was issued across Italy, the Tyrrhenian Sea, and surroundings mainly for excessive precipitation. Waterspouts are also forecast. Large hail is expected in the southern portions.
- A level 1 was issued across the eastern and northern Iberian Peninsula mainly for excessive precipitation. Large hail and severe wind gusts are forecast in the south-eastern parts of Spain.
- A level 1 was issued across northern Algeria and Tunisia mainly for large or very large hail, severe wind gusts and locally excessive precipitation.
Synopsis
High-over-low pattern across western Europe continues. Downstream amplification of low geopotential is associated with an intense short-wave trough that moves from Poland to the Black Sea. Within the base of the trough, a well-mixed polar air mass and low-level moisture will overlap to allow for some CAPE during the day. The southern flank of the trough affects the north Mediterranean Sea, where rich low-level moisture is present south of an advancing cold front.
Discussion
Tyrrhenian Sea, Italy, Adriatic
Lift is expected to spread southward ahead of the approaching cold front. It will affect a moist and unstable air mass with MLCAPE around 1500 J/kg. Deep-layer vertical wind shear will be rather weak and be mostly below 10 m/s. Together with skinny CAPE profiles, low cloud bases, and slow storm motion, excessive rain is the prominent threat. Waterspouts are also forecast, in particular in the morning hours with off-shore storms.
Storm organization is expected to be better farther south, especially southern Italy. Despite smaller storm coverage, multicells and supercells will pose a threat of large hail and excessive precipitation. Storms will go on through-out the day, with new storms forming across the land. With the progressing cold front, storms will decay from the north.
Northern and eastern Spain
Low mid-level geopotential is located across the Iberian Peninsula. Daytime heating results in improving lapse rates and moist onshore flow. Storms are expected to produce excessive precipitation given skinny CAPE profiles and slow storm movement. Large hail is forecast with some more isolated storms in the south-eastern parts of Spain, where vertical wind shear is stronger with about 15 m/s in the lowest 6 km.
North-eastern Algeria and northern Tunisia
Within a modest westerly mid-level flow, a frontal boundary remains across the Atlas. South-westerly winds at 850 hPa advect steep lapse rates to the shores, where Mediterranean moisture can progress onshore during the day. Upslope flow and weaker CIN, especially across northern Tunisia, results in convection initiation. Strong deep-layer shear supports multi- and supercells, capable of large or very large hail, severe wind gusts, and locally excessive precip.
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