Storm Forecast
Valid: Mon 26 Aug 2013 06:00 to Tue 27 Aug 2013 06:00 UTC
Issued: Sun 25 Aug 2013 19:38
Forecaster: TUSCHY

A level 2 was issued for parts of SE/E Spain and the Balearic Islands mainly for large to very large hail, excessive rainfall amounts, severe wind gusts and an isolated tornado event.

A level 1 covers most of the W/C-Mediterranean mainly for heavy to excessive rainfall amounts, large hail, strong to severe wind gusts and an isolated tornado event.

A level 1 was issued for N-Italy and S-Austria mainly for a few funnel/isolated tornado events and heavy rain.

A level 1 was issued for the N-Balkan States towards Moldova mainly for large hail, strong to severe wind gusts, heavy rain and an isolated tornado event.

SYNOPSIS

A quasi-stationary upper low over Switzerland and adjacent regions is forecast to continue its slow retrograde motion to the west with its center being placed over C/E France during the end of the forecast. This low is part of an extensive low pressure channel, which extends from France all the way to far W-Russia. Ridging over UK, Scotland, Ireland and the North Sea continues to build east, also affecting Sweden and Norway during the forecast. This blocking pattern however features numerous short waves, which circle those synoptic pressure anomalies and insert either unsettled conditions or enhanced probabilities for organized DMC (e.g. Mediterranean).

At lower levels, the main focus will be the weak front, which entered the W-Mediterranean 24h ago, steered by the strengthening and eastward expanding high pressure area over the Azores and a developing lee cyclogenesis over Italy. Both features pushed this front into the W-Mediterranean basin. This boundary is not well discernible in synoptic data as surface winds along that boundary show only diffuse/very weak convergence with already ongoing BL moisture recovery in the postfrontal air mass. NESIDS TPW analysis supports that idea, showing a drop of roughly 10 mm in PWs behind the front while the moisture gradient already starts to weaken. In VIS imagery , that boundary is only characterized by warm Sc with no real postfrontal pressure anomaly detected in latest surface data. Hence, we think this boundary will stall just south of Sardinia and over the C-Tyrrhenian Sea. It reveals a wavy structure, as numerous surface waves and a pronounced short-wave will modulate the position of that boundary.

Another extensive quasi-stationary boundary has established from NE France to the Czech Republic to SW Ukraine. No serious motion is forecast although some faint N-S oscillation is possible with regionally enhanced convective activity.

DISCUSSION

...E-Spain towards the Balearic Islands, Sardinia, Italy and parts of the Adriatic Sea ...

With the synoptic feature showing marginal to no net-motion through the forecast, numerous weak impulses dictate DMC chances during the forecast, as they eject out of the base of this main upper low. The strongest wave will leave NE Spain during the morning hours to the NE. It should affect NE Italy during the first part of the night, before weakening while moving into a meso-beta ridge to its east. Also, numerous impulses, which are embedded along the far N-fringe of the subtropical jet cross Algeria/Tunisia to the NE during the forecast. Hence, CI will be likely over a broad area, but different in regional coverage and intensity.

South of and along the quasi-stationary front, which runs from the Balearic Islands to S-Sardinia to C-Italy, thermodynamics will be good, given a very moist marine BL (dewpoints in the mid twenties) and steep mid-level lapse rates on the order of 7 to 8 K/km atop. TPW anomalies run between 100 to 150% with readings between 30-40 mm, so this moisture is also well shared in the lower to mid-troposphere. 1000 to 2000 J/kg MLCAPE is forecast along and south of that boundary. The Tyrrhenian Sea could see even higher values during the night, when BL moisture continues to increase/pool along the stationary front.

A brisk W/SW-erly flow keeps shear profiles high with 0-6 km bulk shear up to 25 m/s and 15 to 20 m/s shear in the lowest 3 km AGL. While shear gradually weakens over the Balearic Islands during the night, it even increases slightly over the Tyrrhenian Sea and adjacent areas.

Thunderstorms will already go on over the Balearic Islands and the coastal areas of E-Spain with the influence of the departing short-wave and weak capping. Soon, thunderstorms also initiate over SE to NE Spain with only some additional heating needed. This activity will continue well into the night ... probably all night long. Well organized multicells and supercells with large to very large hail, severe wind gusts and excessive rain are forecast. A concern is that 15 kt easterly winds over the Balearic Islands could offset 15-20 kt NW-erly storm motions with a long-lasting and repeatedly regenerating thunderstorm cluster sitting between València, Barcelona and the Balearic Islands. PWs in excess of 40 mm indicate an enhanced risk of damaging flash flooding both along the E-coast of Spain but also over the Islands. Other than that, an isolated tornado event is possible with mature supercells along the coast, where sea-breeze interaction and enhanced LL CAPE increase a spin-up.

From Sardinia to the east, CI probably awaits the mid-level impulse at 18 Z onwards. Isolated to scattered CI is forecast although models differ regarding strength of cap and the path of further and weaker impulses. Hence a broad level 1 area was issued with the lack of confidence in any CI bullseye precluding a level 2 for now. Shear and CAPE both support severe multicell/supercell events with all facets of severe. An update and upgrade might be needed if models have converged a bit regarding magnitude and placement of CI. The same applies to N-Algeria/Tunisia.

... N-Balkan States towards Moldova ...

Strengthening SW-erly winds and a leisurely southward pushing boundary over the far S-Ukraine/Slovakia create an environment with rich BL moisture beneath moderate mid-level lapse rates. 500 to 1000 J/kg MLCAPE are forecast with 15 to 20 m/s DLS. Long-lived multicells and isolated supercells with large hail and strong to severe wind gusts are well possible. Despite weaker shear profiles in the lowest 1 km AGL, low LCLs over Slovenia, E-Croatia and Hungary could assist in an isolated tornado event. Thunderstorms continue until the evening hours, before decaying CAPE induces a downswing of DMC. Ongoing but slowly weakening clusters however might offer an isolated flash flood risk until midnight...especially over Moldova, where PWs increase.

... N-Italy and parts of Austria ...

The level 1 was expanded far north despite weaker shear profiles. 10-15 m/s DLS, 400 to 800 J/kg MLCAPE and a passing impulse create a favorable set-up for scattered to widespread CI. Favorable BL conditions (low LCLs and augmented LL CAPE) increase the funnel and tornado risk which was the main reason for the upgrade. Otherwise, marginal hail and gusty winds accompany convection with isolated severe hail/wind events possible in case of mature multicells. Heavy rain could prouce a few flash flood events with PWs sticking in the upper twenties and storm motion vectors drop to 10-15 kt.