Storm Forecast
Valid: Sat 06 Apr 2013 06:00 to Sun 07 Apr 2013 06:00 UTC
Issued: Sat 06 Apr 2013 00:29
Forecaster: PISTOTNIK

A level 1 was issued for a belt from Tunisia via Malta to the Ionian Sea for severe wind gusts and large hail.

A level 1 was issued for NW-ern Algeria for severe wind gusts.

SYNOPSIS

A long-wave trough rests in place over large parts of Europe, crested by a small but intense upper-level low which moves from Catalonia into Tunisia during the forecast period. Along its rear flank, continental cold air continues to flow southward and covers most parts of Europe, even including the Iberian Peninsula. Low moisture and mainly anticyclonic influence near the surface preclude any deep convection. The only exception is a small area of coastal Norway, where maritime convection in the range of a cold-core vortex may become deep enough to produce a few lightning strikes.
Spring-like conditions are still confined to Southeastern Europe and the Mediterranean region, where a deep Southwesterly flow (30-35 m/s at 500 hPa from Libya to the Black Sea) and noteworthy dynamics allow thunderstorm activity in various regions. These are discussed in further detail below.

DISCUSSION

...Bosnia, Montenegro, Serbia, Romania, Moldova, Ukraine...

Ahead of the long-wave trough, a long and almost stationary frontal zone runs from Northeast to Southwest across these regions. Diurnal heating, converging low-level winds and repeated patches of large-scale lift support are supportive for the buildup of a few hundred J/kg of CAPE. Isolated thunderstorms are expected to form from the late morning hours especially over orographic features (mostly the Eastern Carpathian mountains) and to move northeastward. Forecast models agree on a wave-like disturbance to develop over Romania and to move into the Ukraine in the afternoon. This indicates a possibility of a higher storm coverage and upscale growth into a larger cluster in its path, if the cyclonic forcing finds a good phasing with the first developing storms. Apart from that, the degree of storm organization is expected to remain on the low side due to the limited CAPE supply and only weak to moderate deep-layer shear (10-15 m/s) at safe distance from the jet axis. Only the strongest storms may be accompanied by local, probably sub-severe hail. Activity will mostly decay again until sunset.

...Tunisia, Malta and surroundings, Ionian Sea...

The deep Southwesterly flow advects hot and dry Saharan air towards the Northeast. Most stations in Tunisia and Libya reported temperatures in excess of 30°C and blowing dust on Friday afternoon and evening, contrasted by the coastlines where sea breezes helped to maintain somewhat lower temperatures but much higher dewpoints. At Friday 18 UTC, the dewpoints of the coastal stations in Libya displayed the whole range from -6°C to 18°C, depending on the victorious wind regime. This obviously high variability of boundary layer characteristics and its poor coverage with observational data keeps the confidence of the forecast rather low. However, the maritime boundary layer beneath the elevated Saharan airmass can well act to create a "loaded gun" situation with a rapid increase of the boundary layer energy content under strong insolation, demonstrated by the Friday 15 UTC readings from Hons (Libya), which combined the inland heat (temperature of 38°C) with the high coastal moisture (dewpoint of 15°C). The Saharan plume will become elevated while it further overspreads the Ionian Sea on Saturday, where forecast models consequently agree on the buildup of moderate but strongly capped CAPE.
Like usual in these situations, convective initiation will probably be confined to the rear fringe of the hot air plume, where the strong cap becomes eroded by forced ascent, which originates both from a cold front from the West (situated near the Algerian / Tunisian border at the beginning of the forecast period) and the coinciding arrival of a pronounced upper-level short-wave trough from the Southwest. Consequently, a quite thundery cold front passage is expected over Tunisia in the noon to afternoon hours with a threat of severe wind gusts (fostered by very dry environmental air) and large hail (as soon as the sea breeze moisture invigorates the updrafts near the East coast). Storms will move offshore and will then likely start struggling to stay alive. Despite strong low-level cyclogenesis, forecast models are very reluctant to show further precipitation signals around Malta in the evening and over the Ionian Sea overnight, indicating that the cap is simulated to be very strong. Yet it was decided to extend the level 1 and the >15% thunder area all the way from Tunisia to the Greek West coast in order to play it safe. It is worth emphasizing that the persistent and remarkable overlap of CAPE (500-1000 J/kg), deep-layer shear (around 25 m/s) and powerful lift support would be sufficient for a level 2 scenario, but the doubts are too serious that convective initiation will happen widespread and timely enough for the storms to benefit from the departing strong shear regime.

...Western Mediterranean...

The upper-level low creates another complex setup over the Western Mediterranean region, while it moves from Catalonia to Tunisia. Widespread and plentiful precipitation is forecast near its path, but it is thought to be mostly stratiform. Only at its Southwestern flank, low-end CAPE may build up beneath the coldest upper levels and allow isolated postfrontal storms. Deep-layer shear remains weak over most of the region, before it will be boosted to 35 m/s near the Western fringe of the highlighted area with the arrival of a North-South-oriented jet streak. Hence a level 1 was introduced in Northwestern Algeria for a brief chance of severe wind gusts in the afternoon, before the depth of convection will soon be cropped by strong subsidence.

...Italy...

Omitted by the events described above, Italy may see isolated to scattered afternoon thunderstorms in an environment of weak vertical wind shear and limited CAPE. Severe weather is not expected.