Storm Forecast
Valid: Mon 17 Sep 2018 06:00 to Tue 18 Sep 2018 06:00 UTC
Issued: Sun 16 Sep 2018 10:10
Forecaster: TUSCHY

A level 1 was issued across parts of Spain and the far W Mediterranean mainly for excessive rain.

A level 1 was issued across parts of Sardegna, Corse and N-CNTRL Italy mainly for isolated excessive rain and large hail.

A level 1 was issued for NE Algeria/NW Tunisia mainly for damaging wind gusts and excessive rain.

SYNOPSIS

Initial zonal flow pattern over N-Europe transforms into a more amplified one with the approach of a deep trough over the far N-Atlantic. Strongest height rises occur over W/NW Europe with onset of deep WAA, whereas not much height changes occur further east over S/CNTRL Europe. A broad ridge covers most of the W/CNTRL Mediterranean and CNTRL Europe but rather low thickness values support numerous waves cutting through that ridge or circle around its periphery (with most models even indicating the development of an upper trough over France during the night).
The long-lived upper trough over SW Europe weakens and pushes NE over the W Mediterranean during the forecast as a weakening mid-level wave.
A cut-off over far E-Europe drifts E and out of our forecast area, whereas a positive tilted upper trough continues to affect SE Europe.

Of interest is the extratropical transition of Ex-Helene just a few days after we handled a subtropical cyclone over the Black Sea - lots of warm-core activity lately.
Overall, Ex-Helene is covered well in most models and even deviating forward speed forecasts gradually merged in the latest model output. Phase diagrams support a rapid transformation from a warm-core into a cold-core feature and even the chance of a tenacious shallow warm core structure decreased in latest model data. Main reason for destructive interference with the westerlies is the missing phasing of Ex-Helene with an upper trough to its west. Hence no rapid baroclinic re-intensification is forecast. Therefore a weakening vortex is about to impact Ireland and Scotland during the night. Nevertheless, LL wind field is forecast to stay impressive next to the vortex' center with 850 hPa winds in excess of 30 m/s and long and looping hodographs. Despite drier air sneaking in from the west forecast soundings show stable conditions, so no convective threat is currently anticipated with the passage of Ex-Helene. Therefore no level was added despite the occurrence of strong to severe (offshore/coasts) wind gusts. Please refer to the UK Met Office and Met Eireann for further information.
The only thing of interest is the potential development of a precursor rainfall event as Ex-Helene interacts with the baroclinic zone to its N/NE, causing an extensive shield of heavy rain, affecting N-Ireland and Scotland during the forecast. Despite minimal CAPE (e.g. in IFS), not enough upright convection (let alone electrified convection) is forecast to add any lightning areas.


DISCUSSION

... W-Mediterranean and Spain ...

Not many changes are forecast compared to yesterday. A short-wave over the SE Bay of Biscay drifts to the E/NE with another wave over the Strait of Gibraltar doing the same towards the Balearic Island during the night. Both features add some background forcing to that setup. The main model difference remains the thermodynamic stratification of the lowest 1-2 km AGL with GFS showing the warmest bias (up to 2 K warmer compared to IFS f.ex.). This results in different CIN forecasts and hence different QPF maxima. Nevertheless most models agree in scattered CI west of the Balearic Islands and over parts of Spain.
Easterly/northeasterly BL flow continues to keep a plume with rich BL moisture just off the E coast of Spain. Onshore advection occurs but BL moisture mixes out during the day due to diabatic heating. Forecast soundings however support better LL moisture profiles compared to yesterday and hence augmented confidence in CI.

Weak mid-level lapse rates keep onshore MLCAPE in the 500-1000 J/kg range with higher offshore values forecast. DLS increases from less than 5 kn onshore to 30 kn offshore. Therefore the main risk will be slow moving/quasi-stationary thunderstorms over parts of S/E Spain with excessive rain and an attendant flash flood risk.
Offshore, two maxima exist with one occuring before noon with ongoing but gradually weakening thunderstorms from the previous night and another one during the overnight hours between the Balearic Islands and NE Spain. Excessive rain continues to be the main hazard.

... CNTRL Mediterranean ...

Despite weak ridging aloft, a passing mid-level wave affects the forecast area and adds some marginal synoptic-scale lift to today's setup. The main focus for CI exists along the mountains, where 500-1000 J/kg MLCAPE and weak shear overlap. Slow moving thunderstorms pose an heavy rainfall risk and bring some hail. A level 1 was added to parts of Corsica and Sardegna, where 25 kn deep layer flow and MLCAPE aoa 1kJ/kg coexist. Isolated large hail and excessive rain is a possible hazard.
Another small level 1 over N-CNTRL Italy was issued due to similar CAPE values and hazards.

... NE Algeria and NW Tunisia ...

Lowering thickness and weakening synoptic-scale subsidence could result in a few thunderstorms over the mountains of NE Algeria. Inverted-V soundings with better mid-level CAPE support a few severe downdrafts. Slow storm motion also points to locally excessive rain. A small level 1 was added.