Storm Forecast
Valid: Thu 23 Apr 2015 06:00 to Fri 24 Apr 2015 06:00 UTC
Issued: Wed 22 Apr 2015 15:40
Forecaster: GATZEN

A level 1 was issued for northern Algeria and Tunisia mainly for large hail.

SYNOPSIS

High-over-low blocking is present across the NE Atlantic, yielding a southern branch of the westerly jet that extends across the northern Iberian Peninsula into the Mediterranean. The northern portion of the westerly jet is located across Scandinavia into Russia. In between, a cut-off low is located over the eastern Alps. Pretty marginal low-level moisture can be expected over most areas. Locally enhanced moisture exceeding 7 g/kg in the lowest 500 m is indicated by latest GFS model along the southern flanks of the Alps, across the south-west Mediterranean, and from the Bay of Biscay into south-western France. Low-level moisture partly overlaps with steeper lapse rates evolving near the Alpine cut-off low and across the south-west Mediterranean area.

DISCUSSION

Northern Africa

In the morning hours, some elevated storms are forecast over the Mediterranean Sea between northern Africa and Sardinia within a plume of steep mid-level lapse rates pushed away by a cold front. These storms will decay later on as lapse rates weaken.

Further south, low-level moisture increases along the northern slopes of the Atlas Mountains during the day due to north-westerly upslope flow behind the cold front. At higher levels, westerly to south-westerly flow will remain, subsequently advecting steep mid-level lapse rates from the Atlas to the north again, where they overlap with the moisture. Diurnal heating is associated with CAPE in the order of a few 100s J/kg in the afternoon hours, capped by a rather strong inversion. QG forcing will be relatively weak, although some lift can be expected at the right entry region of a mid-level jet, and some warm air advection evolving late in the period along the northern flank of a frontal boundary.

Current thinking is that chance for new initiation increases in the afternoon and evening hours with increasing WAA and due to upslope flow/convergence at the northern flanks of the Atlas Mountains. Thunderstorms can evolve into supercells given relatively strong 0-3 km vertical wind shear of 15 m/s, together with favourable veering (0-3 km SRH around 200 m²/s²). Large hail is forecast to be the main threat with every supercell that forms. Weakening instability during the night hours limits the severe potential gradually, and storms may become elevated over night.

Northern Adriatic into eastern Alps

At the southern edge of the cut-off low, cold mid-level air is located across the eastern Alpine region. A mid-level jet streak is expected across the central Alps, pointing towards northern Italy. QG forcing is expected as this jet streak moves southward into northern Italy until the end of the period. Additionally, some warm air advection is indicated by latest GFS to the south of the Alps.

At low levels, a southerly flow advects some moisture from the Mediterranean towards the Alps, where convergence results in increasing low-level moisture. Diurnal heating will be associated with improving lapse rates that overlap with the moist boundary layer. Thunderstorms are forecast through-out the period that will gradually expand southward into northern Italy. Although CAPE will be pretty marginal, around 15 m/s 0-3 km vertical wind shear can assist in the development of multicells of mesocyclones, capable of producing locally large hail. Overall threat is too weak for a threat level, but continues during the night hours when QG forcing and instability spread into the northern Adriatic.

Central France

A convergence zone is expected between dry easterly flow across north-eastern France and a moist westerly flow that enters France from the Bay of Biscay in the range of an old cold front. Lapse rates are pretty weak in the cool maritime air mass, but some moisture near the convergence may overlap with steeper lapse rates of the prefrontal air mass. In response to diurnal heating, CAPE is expected in the afternoon, and thunderstorms are forecast to initiate especially over the north-western Massif Central. Weak vertical wind shear keeps the potential of severe storms low, and no threat level is issued.