Storm Forecast
Valid: Mon 18 Nov 2013 06:00 to Tue 19 Nov 2013 06:00 UTC
Issued: Sun 17 Nov 2013 23:51
Forecaster: TUSCHY

A level 2 was issued for extreme NE Spain mainly for excessive rain.

A level 1 was issued for most of the W Mediterranean mainly for large hail, excessive rain, strong to isolated severe wind gusts and tornadoes.

SYNOPSIS

A vigorous cut-off over the W-Mediterranean feels the influence of a potent and positive tilted trough from the NW. In the course of this forecast, this cut-off becomes linked to that trough while weakening at mid/upper levels. Lower parts of that vortex act in a complete opposed manner with a gradual strengthening expected during the overnight hours. Initially ill defined surface low next to the Balearic Islands becomes stronger betimes and sends its frontal systems to the east (cold front) and to the west/southwest (warm front and occlusion).

This remains the only area of concern with strong high pressure over E-Europe and weakly sheared marine convection over the far NE Atlantic and the Norwegian Sea.

DISCUSSION

... Corsica, Sardinia and Italy ...

Tomorrow's daytime hours resemble a common pattern for scattered to widespread DMC activity along an eastward surging cold front. This front affects Sardinia around noon with a diminishing eastward/northward push thereafter as this boundary outruns stronger dynamics. A similar picture with expected forcing, which peaks around noon and weakens thereafter. A complex wind pattern is forecast, which results in various more or less defined wind/shear maxima along and ahead of this front. Most data however confirm a supportive environment for organized DMC along this cold front until well into the night.

Expect clustering storms to move east towards Sardinia around noon. There, strong LL/DL shear and up to 1 kJ/kg MLCAPE create a nice set-up for multicells and isolated supercells. Low and mid layer flow will be near parallel to the boundary, so discrete storm mode will be hard to realize. However, embedded supercells and supercellular tail-end storms are well possible. Severe winds, large hail and an isolated tornado are forecast. This risk spreads north towards Corsica and east over the Tyrrhenian Sea (during the evening hours onwards). There, conditions deteriorate for organized activity with a bimodale scenario:
a) still adequate forcing but rapidly decreasing CAPE should favor a solid line of storms to approach NW/W Italy from the W/SW during the night. Heavy rain and and an isolated tornado (LL CAPE in excess of 200 J/kg) next to strong wind gusts remain probable.
b) rapidly weakening forcing but better CAPE (aoa 1kJ/kg MLCAPE) support decelerating line of thunderstorms, which approach Sicily from the W. Lack of adequate shear should result in a messy appearance of that storm activity and the main risk looks like to be heavy rain and gusty winds. Again, an isolated tornado event along the coasts can't be ruled out with better LLCAPE and enhanced LL shear. This activity spreads east during the night and also affects S-Italy with a similar risk.

We thought about adding a level 1 to the N-coast of the Adriatic Sea, as conditions improve for back-building showers/isolated thunderstorms between 00-06Z. Limited CAPE and diffuse QPF signals however kept our confidence below a level 1 threshold for now.

... Balearic Islands, SW-France and NE Spain ...

A quasi-stationary low results in scattered to widespread shower/thunderstorm activity all day long. During the daytime hours, two foci exist. The first one will be prolonged period of onshore moving showers and thunderstorms along the SW coast of France and NE Spain. Effective PWs in the upper 20s (mm) and constant influx of moist air from the SE point to a risk of heavy rain over a broad area of S/SW France and NE Spain. Main concern will be orographic enhancement along the S-Massif Central and along the E-Pyrenees. Especially NE Spain could see rain sums in excess of 100 mm/24h with embedded convection locally enhancing that risk significantly. Hence a level 2 was added. The second risk will be slow moving storms over the Balearic Islands, which could produce excessive rain and numerous tornadoes (enhanced LL CAPE and convergent flow along numerous boundaries).

During the evening and overnight hours, most models show a decent deepening trend of the lower part of that vortex although phase diagrams keep this low without any warm core structure. Wamer air/moisture, wrapped around the center and constant latent heat release probably insert that deepening phase. Exact track of that vortex remains a bit uncertain as little deviation to the east could keep the most serious convection offshore off the E-coast of Spain. However, current thoughts support an ongoing excessive rainfall risk for the E-Pyrenees with a slow offshore trend beyond midnight. DLS over the Balearic Islands increases a bit to 20m/s, which enhances the risk for a few organized storms with isolated large hail. The main hazard however will be heavy to excessive rain and an ongoing waterspout/onshore tornado risk over the Balearic Islands. A broad level 1 area was issued although on a regional scale, level 2 rainfall conditions with a flash flood risk could occur.