Storm Forecast
Valid: Fri 04 Oct 2013 06:00 to Sat 05 Oct 2013 06:00 UTC
Issued: Thu 03 Oct 2013 16:33
Forecaster: TUSCHY

A level 2 was issued for NE Spain mainly for large to isolated very large hail, strong to severe wind gusts, excessive rain and an isolated tornado event.

A level 2 was issued for the S-coast of France mainly for excessive rain, an isolated tornado, marginal to isolated large hail and severe wind gusts.

A level 1 was was issued for E/NE Spain, parts of W-Italy and the Tyrrhenian Sea (inlcuding adjacent areas) mainly for large hail, excessive rain, strong to severe wind gusts and an isolated tornado event.
The same for the level 1 over parts of France.


SYNOPSIS

Steering pattern is evocative of an omega pattern but its western fringe is already in progress of weakening as the W-European trough tries to close off during the end of the forecast. This feature however taps into a moist/unstable air mass over SW Europe and sparks a quite robust severe thunderstorm event. A strong cold-core low over SE Europe draws a very dry continental air mass all the way to the E-Mediterranean and in combination with a southbound building surface high, no deep/organized or long-lived updrafts are expected. The ridge in-between, which extends from Italy all the way to the Baltic Sea is about to break down with a constant 500 hPa height fall expected.

DISCUSSION

... Spain, S and CNTL France and parts of the NW/W Mediterranean ...

The dynamics are not really strong despite a marginal speed-up and strengthening of the 500 hPa trough over the Bay of Biscay. Looks like an healthy vorticity lobe extends from the Bay of Biscay to Spain with a strong impulse circling its base. Hence, the main trough axis shifts from a positive to a negative tilt, which increases dynamics somewhat through the forecast. In combination with weaker short waves along its downstream fringe, enough lift is expected to sustain long-lived DMC. The approaching trough and northward arching subtropical ridge create a brisk 25-30 m/s mid-level SW-erly speed maximum, which extends from E-Spain all the way to SE France. DLS hovers aoa 20 m/s with simlar values in the lowest 3 km AGL. Latest synop and sounding data show a seasonably moist BL with dewpoints in the lower 20s and mixed-layer mixing ratios in excess of 15 g/kg. Forward trajectories confirm an undisturbed inflow from the W-Mediterranean to the north towards S-France. Some drier air tries to mix northward from N-Africa to S-France, but should not play a serious role right now for the French convection.

Overall set-up resembles long-lived MCS events from the past, which evolve over far NE Spain and move towards S/SE France during the forecast. CI will probably occur over E/NE Spain over the rough orography, as an eastward moving cold front collides with moist inflow from the W-Mediterranean (fueled by falling surface pressure over C-Spain). CI already occurs well before noon. Discrete multicells/supercells with large hail, excessive rain and strong to severe wind gusts are possible. However, rapid upscale growth into storm clusters is expected, as this activity translates to the E/NE, entering an increasingly unstable air mass over far NE Spain and offshore areas. Training does not yet seem to be a serious problem, but flash flooding could be already a regional problem with clustering storms. CI also occurs over SW France, where multicells/isolated supercells are forecast, given 800 J/kg MLCAPE and 20 m/s DLS. Large hail and strong wind gusts will be the main risk with an isolated tornado event also possible, as LCLs drop to below 800 m and some LL shear exists. A cluster of storms should emerge out of this activity while moving to the NE, affecting most of C-France until 20 Z. Then activity should turn more elevated in nature with a decreasing severe risk (mainly heavy rain). Numerical CAPE outputs remain diverse as convection along the coast of SW France could limit moisture return. We therefore stuck with the less aggressive EZ model. In case higher CAPE verifies, an upgrade may be considered.

Thereafter, during the afternoon and evening hours, the ENE-ward moving Spanish cluster(s) should take profit of a 15 m/s S-erly LLJ, a coupled upper jet structure and increasing cyclonic curvature of the mid/upper level flow to sustain a huge cluster over far SW France, which gradually shifts to the east. Latest models still focus on an area between Montpellier and Marseille, where backbuilding with excessive and flash flood producing rainfall amounts could be a distinct possibility. Tail-end storms certainly reveal supercell structures with a large hail, tornado and severe wind gust risk. We expanded the level 1 far inland to cover the rainfall risk, induced by that convection. We also expanded the level 2 far to the east to cover that long-lived MCS feature as it should become a bit more progressive. Ongoing LLJ should keep this feature alive for most of the night.

Beyond midnight, organized DMC also evolves from Corsica/Sardinia to Italy. Multicells and isolated supercells with large hail, severe wind gusts, an isolated tornado event and excessive rain are all possible. Not yet sure how serious a training risk could be along the W-coast of Italy with mixed signals in models, but effective PWs in excess of 40 mm show a high chance for flash flood producing storms. Highest probabilities exist over far NW Italy, where an upgrade was performed.

It remains a bit uncertain right now how far south storms evolve over the W-Mediterranean ... e.g. towards the Balearic Islands. This area will be affected by the aforementioned northward mixing of drier African air. Also, meager forcing and strong capping don't support widespread CI of DMC, but I don't want to rule out at least isolated convection, approaching the Islands from the W during the evening hours. Also, with expected cluster over S-France, MCS-induced subsidence could limit CI to its south. A low confidence level 1 was expanded all the way to the Balearic Islands for now. In case a longer-lived storm manages to develop, all kind of severe could occur with a favorable shear/CAPE overlap.

... Benelux and far NW Germany ...

NE-ward spreading moisture but weak mid-level lapse rates should create 300 to 700 J/kg SBCAPE during the afternoon hours. Shear is on an increase with 15-20 m/s DLS present around peak heating. This area however resides between two weak UVV maxima and hence CI will be a big concern. GFS and EZ show sporadic initiation and CAPE/shear overlap could temporarily support a strong storm with marginal hail and gusty winds. Around sunset, CAPE rapidly vanishes, but elevated clusters of the French activity should move in from the SW. Locally heavy rain will be the main hazard.