Storm Forecast
Valid: Sun 06 Mar 2016 06:00 to Mon 07 Mar 2016 06:00 UTC
Issued: Sat 05 Mar 2016 16:22
Forecaster: TUSCHY

A level 2 was issued for the coasts of Bosnia and Herzegovina, Montenegro and W-Albania mainly for excessive rain.

A broad level 1 surrounds the level 2 mainly for excessive rain, large hail, severe wind gusts and an isolated tornado event.

SYNOPSIS

Blocked pattern persists over most of Europe as strong trough continues to dig south towards Tunisia. Although 100 hPa maps indicate ongoing weakening of the branch of the polar vortex, no weakening trend is yet seen at mid-tropospheric levels.
By 00Z, the upper trough extends from Norway to N-Africa with the main center being placed over Benelux. Another wave enters the western fringe of this trough and crosses the Bay of Biscay during the overnight hours from NNW to SSE. Else, numerous ripples circle around the main upper trough along its border areas:

a) A weak wave exits Austria to the north during the start and affects the Baltic Sea during the overnight hours.

b) Another impulse lifts from N Africa to the NE and will be the main player of today's outlook over the S-Adriatic Sea.

c) A third wave leaves Spain to the SE and will impact N-Algeria during the night.

Wave a) will suffer from the absence of low-tropospheric moisture including meager mid-level lapse rates. Not enough for CAPE build-up. However, b) and c) will have to be discussed in more detail including the disturbance over the Bay of Biscay.

At low-levels, complex pressure configuration continues with falling pressure over the W-/CNTRL Mediterranean and the Bay of Biscay. No substantial change of intensification (up and down) is anticipated however.

DISCUSSION

... S-Bosnia and Herzegovina, Montenegro, Albania and extreme NW Greece including the Strait of Otranto and the Gulf of Taranto ...

A wavy low-tropospheric front sets up over the CNTRL Adriatic Sea towards Sicily as upstream pressure continues to fall (e.g. a diffuse vortex evolves over the Ligurian Sea as the main trough axis approaches from the W). Hence, the front will push to the SSE on a slow rate and probably exits the far S-Adriatic Sea not before midnight.
Despite frontal intrusions in the past, increasingly confluent low-tropospheric flow over the Ionian Sea causes a deepening of the moist marine layer, which becomes advected to the NE by a 15-20 m/s LLJ. 0-1 km average mixing ratios exceed 10 g/kg and placed beneath 7-8 K/km mid-level lapse rates, 500-900 J/kg MLCAPE is forecast. CIN weakens to the north as 850 hPa temperatures decrease and hence a moist/unstable inflow persists during the forecast, pointing towads the S-Adriatic Sea.

Right entrance of a 50 m/s 300 hPa jet and upper-level divergence will be placed over the area of interest until the night and wave b) offers adequate deep lift for repeatedly developing deep convection. Predominant CI will be along SE-ward sagging front, but with approaching synoptic-scale lift, enhanced CI is also expected further to the south (e.g. far N-Ionian Sea), where the unstable air mass becomes more or less uncapped.

DLS of 25-35 m/s is more than enough for organized DMC and this also applies for the 0-3 km shear (15-20 m/s). Quite unidirectional 500-700 hPa flow blows near parallel to the front with stronger ageostrophic flow below 850 hPa along the front itself. Excessive rain will be the main hazard due to repeated thunderstorm development offshore/along the coast. Latest model data keeps region of CI offshore which limits confidence in widespread excessive rainfall reports in the area of interest. However, MUCAPE plume extends well inland, so even onshore, thunderstorms are forecast, which could locally increase the rainfall amounts. Upslope flow will also increase the rainfall amounts onshore.
Fow now we issued a small level 2 area, where MCS remains quasi-stationary for a couple of hours, before gradually shifting to the SE.
W/SW Albania and extreme NW Greece can expect all kind of hazards with the SE-ward dropping MCS. A tornado/severe wind gust risk will be confined to the coastal areas and excessive rain extends more inland. Given MUCAPE of 500 J/kg well inland with 30 m/s DLS, large hail can't be ruled out well inland over Albania during the night. We also added W-Albania to the level 2 due to the enhanced supercell risk as the MCS approaches from the NW.

Beside the rainfall risk, any discrete cell will be capable of producing large hail, a few tornadoes and strong to severe wind gusts. In case a V-shaped MCS builds to the SW all the way to SE-Italy, an enhanced tornado/large hail risk may extend that far to the SW.

More isolated CI occurs towards Sicily but DLS in excess of 35 m/s and 500-1000 J/kg MLCAPE point to a supercell risk with any longer-lived thunderstorm. Large hail is forcast and even an isolated very large hail event can't be ruled out...next to an isolated tornado and severe wind gust risk. We therefore expanded the level 1 all the way to Sicily. We have to keep a close eye on degree of CI, as more widespread initiation is possible ... divergent model signals precluded an upgrade for now.

... W-Mediterranean ...

Scattered thunderstorms occur all day long as 500 hPa temperature drops to -30 °C or less. Weak shear precludes organized DMC, but a rather cold mid-troposphere should increase a graupel/isolated large hail risk next to gusty winds. LLCAPE is not high, but 50-100 J/kg LLCAPE and weak background flow, including numerous low-tropospheric convergence zones, point to an isolated waterspout risk.

Repeated shower/thunderstorm development over the Balearic Islands may assist in a few heavy rainfall reports.

... Bay of Biscay and far N-Spain ...

The approaching wave will be accompanied by intense forcing. A few thunderstorms may evolve along a southbound moving cold front, which approaches far N-Spain around midnight from the N. 20-25 m/s winds at 850 hPa point to a severe wind gust risk with this line, but confidence/ovrall model signals do not yet support a level 1.
Active postfrontal convection occurs over the Bay of Biscay with gusty winds and graupel until 06 UTC.

... E-France to NW-Germany and N-Italy ...

Meager BL moisture, 500 hPa temperatures below -30 °C and temporal diabatic heating result in a broad area with low-end weakly capped SBCAPE. Nearly non-existent shear precludes organized convection, although cold mid/upper levels may result in graupel/an isolated large hail event with stronger updrafts. Activity rapidly vanishes around sunset.

The same for N-Italy. Slightly better CAPE profiles increase the hail risk a bit, but current data does not justify an upgrade.