Storm Forecast
Valid: Sat 13 Oct 2012 06:00 to Sun 14 Oct 2012 06:00 UTC
Issued: Fri 12 Oct 2012 22:12
Forecaster: TUSCHY

A level 2 was issued for areas E of Tunisia, S Sicily and Malta mainly for excessive rain, large to very large hail, strong to severe wind gusts and an isolated tornado event.

A level 1 was issued for parts of the W and C Mediterranean mainly for heavy rain, isolated large hail and an isolated tornado event. A localized heavy rainfall event with training thunderstorm activity is possible.

A level 1 was issued for parts of N-France mainly for marginal to isolated large hail and strong wind gusts.

SYNOPSIS

The very complex synoptic pattern continues during the forecast period. The steering cyclonic vortex remains the oner over Norway/Sweden with a branch of this vortex affecting NW Europe. Another vortex evolves along the SW fringe and moves towards the Bay of Biscay during the night. The complex interaction of all those vortices results in widespread shower and thunderstorm development over most of NW-Europe. The zonal flow regime over the Mediterranean continues and assists in another round of active thunderstorm activity.

At the surface, numerous fronts affect the Mediterranean (described below). A gradually consolidating surface depression exits the English Channel during the forecast period and strengthens over the far SW North Sea. Otherwise weak surface pressure gradients prevail.

DISCUSSION

... W/C-Mediterranean ...

A complex frontal pattern continues over the area of interest. The cold front, which rushed southwards during the past 36h gradually dissolves over Sicily although strong convective activity over Sicily yesterday pushed a strong outflow boundary more to the south (ref. to lower BL moisture content over Sicily compared to Malta). Nevertheless, the main synoptic boundary starts to wash out, embedded in the prevailing westerly flow regime. Further north, e.g. Balearic Islands to Sardinia, a stalling cold front remains more intact regarding T/Td gradients and gradually transforms into a leisurely northward moving warm front during the forecast. This keeps a warm and moist air mass in place over most of the W/C Mediterranean.

For CI purpose, I would be glad to work with better forcing mechanism compared to yesterday, but again, only weak short waves either graze or cross the large area of interest all day long. A somewhat stronger impulse enters the W-Mediterranean during the afternoon hours with a constant motion to the SE (final motion depending on how strong this trough amplifies to the south). Like yesterday, the foci for CI will be multitudinous: decaying synoptic boundaries, strong outflow boundaries, the orography and aforementioned short waves. The 50-% lightning area will be a compromise between diverging QPF model signals and aforementioned foci for CI. What is more certain is the convective activity which continues all day long in the highlighted regions.

Instability increases from N to S with 500-1000 J/kg MLCAPE south of the Balearic Islands-Sardinia, increasing to 1000-locally 2000 J/kg MLCAPE over the Tyrrhenian and Ionian Sea and more than 3000 J/kg south of Sicily. This amount of CAPE assures deep and long-lived convection.

Longevity of DMC is also fostered by 15-20 m/s DLS, which is enough for well structured multicells and isolated supercells. Like yesterday, the mesoscale environment probably heavily influences the final storm mode with either large thunderstorm clusters ore more discrete cells expected.

The Balearic Islands and areas to the south/southeast see continued CI with slow moving storms at least until the early afternoon hours, before the approaching stronger mid-level impulse gradually causes a SE-ward shift of strongest DMC activity. Scattered to widespread convection is forecast with heavy rain and large hail the main risk. This activity also encloses the N parts of Algeria and Tunisia, where flash flooding is a distinct possibility with slow moving storms.

Over the Tyrrhenian Sea, S-Italy and the E-part of the Adriatic Sea, repeatedly initiation of thunderstorm clusters is forecast. Initiation is hard to pinpoint and bound to mesoscale features. Heavy rain, isolated large hail and an isolated tornado event accompany strongest activities. Thunderstorms keep going all day long. Similar to yesterday, moisture in the BL piles up along the E coast of the Adriatic and Ioanian Sea with modest westerlies atop. Any mesoscale boundary or the orography itself might be enough for temporarily training thunderstorm activity along the coasts with heavy to locally excessive. An isolated large hail and tornado event accompany strongest thunderstorm activity. Thunderstorms continue all day long.

Of main interest will be a corridor E-Tunisia-Malta/S-Sicily into the Ionian Sea, where impressive CAPE values exist. Latest MIMIC-TPW (12th October, 21Z) already indicates a rapidly deepening plume of very moist air east of the Gulf of Hammamet and Tunis with surface dewpoints above 20°C. Explosive thunderstorm development is possible all day long. A major cluster may evolve during the end of the forecast, but right now its main influence remains beyond this forecast period. Excessive rain, large to very large hail, strong to severe wind gusts and a tornado event or two are likely. A level 2 covers that area with favorable shear/impressive CAPE overlap.

Isolated (W-Black Sea) to scattered (Aegean Sea) thunderstorm development occurs during the forecast, but either weak shear or limited CAPE preclude organized convection. An isolated better organized multicell over the Aegean Sea and adjacent areas may produce heavy rain and large hail, but expected risk remains too diffuse for a level 1 right now.

... The English Channel, N-France and parts of Benelux ...

A marginal level 1highlights a strong impulse, which enters the W-English Channel during the start of the forecast with a constant shift towards the east. Attendant strong convergence zone/occlusion affects N-France during the daytime hours and reveals a fast motion to the east due to the strong impulse atop. With a slow down of that impulse over the S-North Sea during the evening hours onwards, a decoupling of strongest forcing, the occlusion and BL moisture/instability fields occurs, which should lower the thunderstorm and severe risk after sunset.

Current thoughts are that cold mid-levels atop onshore flowing maritime BL air mass create 500-800 J/kg SBCAPE, supportive of scattered thunderstorms over N-France and offshore areas. Isolated large hail and strong wind gusts accompany that activity. Betimes, thunderstorm activity also increases somewhat over NE France, Belgium and W-Germany with the eastward moving occlusion. A broken line of showers and isolated thunderstorms will pose a risk of strong to isolated severe wind gusts and marginal hail with a decrease in severe probabilities after sunset. Due to the occlusion, the low probability lightning area was expanded far to the E/SE.

Another strong impulse affects the Bay of Biscay during the overnight hours, which also increases the lightning probabilities over that area. Strong wind gusts and isolated marginal hail are forecast. Storms might also move ashore over W-France. With impressive upper divergence after 03Z, a cluster of storms may impact the W-coast of France with heavy rain although models diverge with that solution.