Storm Forecast
Valid: Thu 08 Oct 2015 06:00 to Fri 09 Oct 2015 06:00 UTC
Issued: Wed 07 Oct 2015 22:01
Forecaster: GATZEN

A level 2 was issued for northern Morocco and northern Algeria mainly for large hail and excessive precipitation and to a lesser extent severe wind gusts and tornadoes.

A level 1 was issued for the southern Mediterranean and northern Tunisia mainly for large hail.

SYNOPSIS

Blocking high and two flanking polar troughs are situated across northern Europe. A strong zonal jet stream extends to the south of this blocking and affects the Mediterranean. An amplifying ridge will form along this jet, leading to rising geopotential from the central into the eastern Mediterranean. At lower levels, cold air advection is expected ahead of the building ridge across the east Mediterranean. Later on, warm air advection sets in across northern Africa and the south Mediterranean Sea. Low-level moisture will be pushed to the south with northerly surface winds where it overlaps with steep lapse rates. Additionally, some CAPE is expected across the western Balkans due to cold mid-level associated with a mid-level trough. Thunderstorms are forecast during the day, but organized storms are not expected due to weak vertical wind shear. Across central and northern Europe, lapse rates are weak and thunderstorms are not likely.

DISCUSSION

Northern Morocco, Algeria, Tunisia

A well-developed frontal zone forms across northern Africa ahead of an amplifying trough centred across Morocco. A deep southerly flow advects an EML northward. Ahead of this feature, easterly surface winds cause strong moisture return into northern Africa, where up to 14 g/kg mixing ratio is expected. The overlap with the EML can result in 5000 J/kg MLCAPE at the end of the period that will be strongly capped.

Best chances for thunderstorms are forecast at the northern flank of the warm air advection regime. Mid-level temperatures are relatively cold and CIN will be weak. Additionally, QG forcing is expected near the deepening trough. CAPE in the order of 500 J/kg is indicated by latest model output, and thunderstorms are forecast to develop during the day across the Atlas mountains, spreading across northern Morocco and Algeria.

With north-easterly to easterly surface winds and increasing southerly flow aloft, strong vertical wind shear will evolve, exceeding 15 m/s in the lowest 3 km. Therefore, supercells can be expected, capable of producing large or very large hail. Severe wind gusts and tornadoes are not ruled out. Especially across the mountain ranges, flooding is possible due to excessive rain. Overall threat is quite robust, and a level 2 is issued. Later in the period, storms may affect portions of the Mediterranean and Tunisia. Large hail is expected to be the main threat.