Storm Forecast

Storm Forecast
Valid: Mon 03 Mar 2014 06:00 to Tue 04 Mar 2014 06:00 UTC
Issued: Sun 02 Mar 2014 18:27
Forecaster: TUSCHY

A level 1 was issued for the SE Bay of Biscay mainly for severe wind gusts.

A level 1 was issued for SW/W-France and parts of N-CNTRL France mainly for an isolated tornado event, marginal to isolated large hail and strong to severe wind gusts.

A level 1 was issued for the N-Pyrenees mainly for excessive rainfall.

Two level 1 areas were issued for Corsica, extreme SE France and NW Italy mainly for excessive rain.

SYNOPSIS

Predominant high-latitude trough over Europe weakens and breaks open into numerous smaller scale troughs and mid/upper tropospheric waves. One trough over Italy and the Ionian Sea moves to the east towards Greece but gets disrupted by a eastward moving 40 m/s mid-layer jet streak along its southern fringe and an approaching major cold-core vortex with falling pressure/geopotential heights from the west. Also, a compact cyclonic vortex over the far NW Black Sea lifts north and crosses far E-Ukraine from south to north.
The major synoptic feature however will be an extensive cold-core vortex, which covers Ireland, Scotland, UK and France during the forecast. During the afternoon hours, surface pressure and height fields decrease over the NW Mediterranean, resulting in a rapidly consolidating vortex over the Ligurian Sea during the overnight hours.
Both, ridging and predominant dry and cold air keep conditions for deep and organized convection very hostile over Scandinavia and Russia.

DISCUSSION

... Ionian Sea, the Aegean Sea and Crete ...

Pronounced PV/NV couplet along the eastward moving but weakening trough affects the area. Strongest lift resides south of Crete with only moderate upper diffluence forecast during the day over the forecast area. The UVV field at mid/upper levels covers a broad area, so the synoptic setup favors widespread shower and thunderstorm development. Focus for more active/concentrated thunderstorm chances will be the eastbound moving cold front, which affects Crete around noon and the Aegean Sea and SW Turkey during the late afternoon/early evening hours.

Forecast soundings and model data indicate only modest mid-layer lapse rates of 7K/km or less. A shallow marine layer with seasonable BL moisture is forecast in the warm sector and forecast soundings show veering (until 700 hPa)/backing wind profiles from Crete to the E/NE, not supportive for long lived and well organized thunderstorms. Still, 15-20 m/s 0-6 km shear and 200-600 J/kg MLCAPE likely support temporarily organized pulsating storms or multicells with heavy rain, isolated large hail and gusty winds. Excessive rain is most likely over Crete due to the prolonged period of cross-mountain flow and over SW Turkey, where the eastbound moving cold front gets squeezed along the rough orography. In general, heavy rain with isolated embedded thunderstorms is possible over the complete Aegean Sea and adjacent areas. Confidence in level-1 rainfall amounts is not very high for now, so only lightning areas were issued. CAA atop SSTs of 16-19 C also results in widespread LL CAPE build-up with an isolated waterspout risk present along any mesoscale/synoptic boundary.

Thunderstorms exit the area of interest to the east during the night, although isolated storms along the coasts of SW Turkey probably continue well into the night.

... France and W/C-Mediterranean ...

Similar to last Friday, a potent 60 m/s 500 hPa jet streak along the eastern fringe of a strong E-Atlantic ridge dives to the southeast towards the W-Mediterranean. This time, the lead impulse with strongest dynamics is about to enter the NW-Mediterranean during the start of the forecast, where a vortex evolves/intensifies. Therefore, strongest QG forcing affects an area from the Balearic Islands to the Ligurian Sea. Upstream forcing pattern showes a mixture of weak PVA/NVA maxima/minima, mainly bound to SE-ward racing short-waves. Very intense mid/upper-level NW-erly flow along the frontal zone crosses the Pyrenees and most models agree in the development of deep tropospheric lee waves. This affects mainly the Balearic Islands to the S/SW, where convective initiation remains uncertain.

France and UK:
The most active area of thunderstorm activity will be found over the CNTRL-/S-Bay of Biscay and over SW-France. Long fetch over 14-16 C SSTs results in a tongue of 5-6 g/kg mixed-layer mixing ratios beneath moderate mid-layer lapse rates of 7 to 7.5 K/km. 200-500 J/kg MLCAPE offshore and up to 800 J/kg onshore evolve within a strongly sheared 0-3 km environment (0-3 km shear 15-20 m/s, only slighty weaker 0-1 km shear and 150-200 m^2/s^2 SRH-1). Once again enhanced convection, e.g. lined up along embedded short-waves, crosses the level area from NW to SE with isolated large hail, strong to severe wind gusts and an augmented tornado risk. The N-Pyrenees see excessive rain due to a prolonged period of moist uplsope flow with aforementioned convection. The level 1 was expanded to N-CNTRL France, where forecast soundings show an overlap of modest LL shear (speed and directional shear) with low LCLs and enough mid-layer CAPE for strong updrafts. Isolated tornadoes are forecast as well.

Over NW-France and UK, steep mid-level lapse rates atop a well mixed marine air mass result in widespread 200-500 J/kg MLCAPE. Placed beneath the center of the vortex, shear will be weak. Low LCLs and the aforementioned stratification of the low-mid troposphere offer some LLCAPE and a few funnel/short-lived tornado events are possible. Slow storm motion also increases the risk for heavy rain/large amounts of sleet with strongest thunderstorms. This activity is daytime driven with a marked decrease after sunset.

W/CNTRL Mediterranean:
Deep CAA overspreads the warm waters of the W-Mediterranean, which causes 500-800 J/kg MLCAPE from the Balearic Islands to the east. Further north, warmer mid-levels keep magnitude of CAPE lower. Expect widespread initiation of pulsating storms or rather disorganized mutlicell clusters. Shear/CAPE separation lowers the overall severe risk, although waterspouts and isolated large hail are possible with strongest updrafts.
As the low-level vortex intensifies/deepens just off the SE coast of France, heavy rain with embedded thunderstorms affects the coastal area of SE France and NW Italy. The same for Corsica, where slow moving storms may produce heavy rainfall amounts. Level 1s for heavy rain were issued. Initiation over the Balearic Islands and to the SW remains uncertain and depends on strength of Spanish lee wave development. Also, long fetch over the rough topography of Spain lowers BL moisture with some time needed for the air mass to recover offshore.

Creative Commons License