Storm Forecast
Valid: Sat 19 Oct 2013 06:00 to Sun 20 Oct 2013 06:00 UTC
Issued: Fri 18 Oct 2013 16:17
Forecaster: TUSCHY

A level 1 was issued for parts of C France mainly for isolated large hail, excessive rain and an isolated tornado event.

A level 1 was issued for a small part of S France mainly for isolated large hail and an isolated tornado event.

SYNOPSIS

A flat and transient omega-like pattern has established over Europe. A weakening upper trough over SE Europe remains in place with only a low-end motion to the east. Numerous vortices over the far E-Atlantic steer a warm/humid air mass far to the NE while a stout branch of the polar vortex over N-Norway/Sweden advects a cold airmass to the south. A deep baroclinic zone (e.g. in excess of 20 K difference at 500 hPa between Germany/N-Norway) is the result with an evolving 40 m/s mid-level streak over the Baltic Sea.
This kind of pattern results in a bisection of Europe from the North Sea to Greece. Areas to the east will feel the influx of cool/cold and dry continental air from the N whereas areas to the west see a gradual recovery of moisture, both due to advection of a modified subtropical air mass and a moistening W-Mediterranean air mass. Hence thunderstorm probs. over E/NE Europe remain confined to offshore and coastal areas with isolated/weakly electrified storms. Scattered and stronger storms however occur over far W Europe.

DISCUSSION

... Ireland and United Kingdom ...

Ingredients for thunderstorms will be in place with some iffy signals however. Behind a ENE-ward moving front (already transforming from a cold front into an occlusion from N to S) seasonable BL moisture remains in place. Mid/upper jets also reveal a good placement for some lift in addition to a weak vort.max. crossing the area of interest from SW to NE between 12-18Z. However, forecast soundings show a meager depth of the BL moisture with constant drying above. Therefore onshore turbulent mixing next to some temporal heating lower confidence in adequate CAPE build-up well inland. Coastal areas (SW-ward facing) will see best moisture and at least 300 to 700 J/kg SBCAPE. Some veering in the lowest 2 km AGL is present along the coastal areas with 50-100 J/kg 0-3 km CAPE, so an isolated spout/short-lived tornado is possible. Effective PWs also approach 20 mm, which could lead to some heavy rain and 15-20 m/s 850 hpa flow should ensure gusty winds with stronger convection.

We expanded the lightning areas far offshore. Despite more hostile conditions for electrified storms with this type of air mass, approaching cooler mid-level air should increase the CAPE disperal in the mid-level graupel layer a bit. Hence confidence in more activity compared to yesterday has increased.

We also splayed the eastern part of the 15-% lightning area all the way to Denmark to cover a few elevated thunderstorm events beyond 12Z. This also includes parts of N France . This activity will be non-severe.

... S/C-France ...

Odds increase to see an heavy and convectively enhanced rainfall event between Marseille and Montpellier .

A very moist air mass is in place over the far W-Mediterranean with no serious frontal intrusion forecast. Persistent SW-erly mid/upper flow and falling surface pressure to the W/NW should assist in a constant onshore flow and hence better onshore moisture over the area of interest. However, position and strength of the mid/upper jets are not yet favorable during most of this forecast period but improve after midnight. The same with the LLJ, which showes some strengthening during the overnight hours. Hence, expect mainly onshore convection during daytime hours. We issued a 50-% lightning area and a level 1 as persistent upslope flow towards the Massif Central could induce a cluster with isolated large hail (20 m/s DLS). We also don't want to exclude an isolated tornado event with LCLs below 1 km and enhanced LL shear. Interaction of mid/upper dynamics and aforementioned LLJ could also support a temporal back-building with excessive rain the main risk. Therefore the level 1 was expanded far inland.

Beyond 00Z, environmental conditions improve for a developing coastal cluster with back-building tendencies. With 20-25 m/s DLS and increasing SRH, any coastal storm should become organized and probably supercellular. Large hail and an isolated tornado will be the main hazards. Current thoughts with late development of that cluster preclude an excessive rainfall - level issuance for now. In case of earlier CI, excessive rain will be added to the current level 1 wording.