Storm Forecast
Valid: Fri 06 Sep 2013 06:00 to Sat 07 Sep 2013 06:00 UTC
Issued: Thu 05 Sep 2013 20:06
Forecaster: TUSCHY

A level 1 was issued for far NE France, parts of Belgium and the Netherlands mainly for large hail, strong to severe wind gusts and an isolated tornado event.

A level 1 was issued for W-UK and NE Ireland mainly for an isolated tornado event. The northern part of the level 1 will see excessive rain.

A level 1 was issued for NE Spain for the evening/overnight hours mainly for an isolated large hail event.

A level 1 was issued for Sicily and S-Italy mainly for large hail and excessive rain. An isolated tornado event is possible.

SYNOPSIS

A high amplitude omega-like upper air pattern has established over parts of Europe, as a strong mid/upper level ridge remains stationary over CNTRL Europe. Two upper lows flank that ridge to the east and west, with the western one being the most dynamic feature. This low moves south and affects all of Ireland, Scotland and UK with attendant weak impulses also influencing areas like France and Benelux. The eastern low over the Ukraine remains stationary and it plays a minor role for today's thunderstorm probabilities.

A surface depression is forecast to evolve over the eastern English Channel and Benelux with a gentle northward motion expected during the forecast. Attendant fronts play a major role regarding thunderstorm coverage.

DISCUSSION

... NE France, Belgium, the Netherlands and the S-North Sea ...

Low-tropospheric streamline forecasts show a broad convergent flow regime over the area of interest, created by increasing cyclonic (SW-erly) flow in response to the approaching upper low / the evolving surface depression and the outflow from the NE-ward departing surface high over the Baltic Sea. A leisurely SE-ward moving cold front becomes more active/progressive during the development of the surface low and moves in from the NW during the forecast. Models were persistent in showing a prefrontal convergence zone to affect the area of interest well ahead of the actual cold front. This pattern supports a deepening BL moisture layer ahead of those fronts with increasing BL dewpoints during the forecast.
At upper levels, a mid-level impulse/IPV anomaly crosses France from SW to NE and affects Benelux during peak time heating. This impulse however weakens due to another, much stronger one digging south to its west, so there is some uncertainty, how strong this feature will finally be. Hence NWP solutions are diverse especially with thunderstorm coverage and also with placement to some degree.

A plume of steeper mid-level lapse rates overspreads the area and moves atop the aforementioned moist BL. Some diabatic heating is possible and forecast soundings show a rapid destabilization trend of the prefrontal air mass. GFS remains consistent with 1-2 kJ/kg MLCAPE, WRF showing 1-1.5 kJ/kg and EZ now seems to agree on .6 to 1 kJ/kg.

Shear will peak during the passage of the weak impulse, which has a perfect timing as it crosses Benelux during peak heating. 20 m/s DLS along and ahead of the front and 15 m/s 0-3 km shear along the front itself create enough shear for organized DMC.

Most models agree well in CI over far NE France around noon, as forcing, increasing CAPE and approaching front/prefrontal convergence zones convene. 25-30 kt SW-erly storm motion vectors indicate rapid NE-ward motion of that convection towards W-Belgium and the Netherlands, where environmental conditions remain supportive for organized DMC. Models diverge a bit, if convection remains attached to the convergence zone, riding more to the north and hence offshore, or if the convection becomes detached and races to the NE and hence remains more onshore. Obviously the latter scenario would be of greater concern, as a few organized multicell/isolated supercell events are possible. Latest 12Z (5th Sept.) NWP guidance had a consistent shift of the main activity to the east, so confidence in the second scenario increased. It remains questionable, if discrete convection verifies, given the small angle of storm motion vector to the eastward sliding boundaries. Current expectation is to see numerous showers / thunderstorms with only a few of them becoming longer lived/severe. Final strength of the impulse, exact timing of the boundary and degree of prefrontal heating determine the final risk. Large hail and strong to isolated severe wind gusts will be probable, especially in case 15-20 m/s 3 km bulk shear verifies, which could support a few bowing line segments with an enhanced severe wind gust risk. As the surface low deepens to the NW, ageostrophic deflection of the BL wind also enhances LL shear. As LCLs drop to below 1 km over parts of W/C Netherlands and W-Belgium during the late afternoon hours, an isolated tornado event can occur.
The activity grows upscale into a thunderstorm cluster, which moves offshore and affects parts of the North Sea during the overnight hours within a N-ward expanding MUCAPE tongue.

A level area was issued, where confidence in organized DMC is the highest. Further east (e.g. NW Germany), weak convergence signals and diffuse mid-level forcing kept the confidence below a level area for now. In case a longer lived storm is able to evolve, large hail and strong to severe wind gusts would be the primary hazard.

... Ireland, Scotland and UK ...

The major upper low drops south and moves over SSTs, which increase from 14°C to 18°C from N to S. Therefore the main convective activity is expected over offshore and coastal areas of Ireland and W-UK, where LL CAPE increases during the forecast. Some signals of enhanced coastal convergence and low LCLs increase the chance for a few funnel and waterspout events.
Mixed signals are present for the Irish Sea during the evening and overnight hours. Wrap-around moisture enters the scene and moves beneath the center of the cold-core low. An increase in CAPE is forecast. However, strengthening frontogenesis might produce a shield of more stratiform rain with embedded convection, affecting parts of the Irish Sea, N-UK and NE Ireland. Also, increasing 15-20 m/s LL winds are more in line with an increasing risk for LL rotation and therefore mesocyclonic tornado activity. Current thinking is that a healthy deformation band will set up north of the surface depression with increasing CAPE supporting enhanced and embedded convection. Heavy to excessive rain will be the main hazard, but an isolated tornado risk offshore/along the coast of the Irish Sea can't be ruled out as well.

I kept the offshore 50-% lightning area close to the maximum of SST and SST anomaly.

...Sicily and S-Italy ...

A moist marine layer with BL dewpoints in the upper tens and lower twenties remains in place and with cool mid-levels atop, MLCAPE of more than 1000 J/kg is forecast. A slowly departing upper trough provides enough forcing for scattered thunderstorm development. Shear remains weak to moderate with 10-15 m/s DLS, but yesterday's active clusters showed that CAPE probably partyl offsets weaker shear. Hence, expect numerous strong to severe thunderstorms with isolated large hail and a few strong downbursts. A lone tornado event can't be ruled out with any mature supercell, as LCLs will be around 1 km and with some weak LLCAPE signals present. Heavy to excessive rain however will be the main risk.

... Spain and S/C-France ...

During the daytime hours, isolated to scattered thunderstorms are forecast over the Pyrenées and S/C-France. Despite moderate DLS on the order of 15 m/s, limited CAPE keeps severe probabilities low. Marginal hail and gusty winds can be expected. A temporarily organized cluster may emerge of the Pyrenees, affecting far S-France with heavy rain for a few hours. It can't be ruled out that this cluster might survive well into the night, as it intersects a northward running CAPE tonuge from the NW Mediterranean. We expanded the lightning area far east to include that possibility.

During the night, an approaching upper trough and a strengthening subtropical jet gradually increase thunderstorm probabilities over Spain and S-Portugal. For most of the Iberian Peninsula, modest MUCAPE and 15 m/s DLS support a few elevated thunderstorms with a marginal hail risk. Towards the coasts however CAPE strengthens and probably becomes near surface based, which also enhances the severe thunderstorm risk. The main hazard will be large hail ... especially over NE Spain.