Storm Forecast
Valid: Sat 14 Apr 2012 06:00 to Sun 15 Apr 2012 06:00 UTC
Issued: Sat 14 Apr 2012 02:34
Forecaster: TUSCHY

A level 2 was issued for W/NW Turkey mainly for an augmented tornado risk and large hail/severe wind gusts.

A level 1 was issued for W-Turkey, east of the level 2, mainly for an isolated tornado, large hail and severe wind gust event.

A level 1 was issued for SW-Turkey mainly for excessiv rainfall.

A level 1 was issued for Greece, Bulgaria and parts of Romania mainly for large hail and an isolated tornado event.

A level 1 was issued for the coast of NW-Algeria mainly for an augmented tornado risk and large hail/severe wind gusts.

A level 1 was issued for parts of Italy and the E-coast of the Adriatic Sea mainly for locally heavy rainfall amounts and an isolated waterspout along the coasts.

SYNOPSIS

A deep trough remains situated over Europe with reinforcement along its western fringe, as another shot of cold air approaches from the north. A cyclonic vortex over SE-Europe, already well structured at low/mid-levels continues to deepen (vertically) with scattered to widespread showers and thunderstorms anticipated over most parts of S/SE-Europe. Another disturbance over Spain enters the SW-Mediterranean after sunset and sparks numerous showers and thunderstorms. The rest of Europe remains covered by a cool/dry air mass.

DISCUSSION

... Albania, Macedonia, Greece, Bulgaria and Romania ...

A strong warm front, placed over Bulgaria/Romania with an appreciable tilt to the NE, continues to lift to the N/NE. Strength of this frontal boundary is reflected well by isolated nighttime thunderstorm activity over CNTRL Romania. Postfrontal warm sector is featured by deepening BL moisture with dewpoints increasing to 10°C or more over Bulgaria and Romania. At the same time, WV imagery show dry slot moving in from the SW, which overspreads the warm sector. As a result, atmospheric stratification becomes increasingly unstable, reflected by isolated to scattered convective clusters with cooling cloud tops after 00Z.

Placed beneath the dry slot, good daytime heating is forecast, which pushes surface temperature rapidly beyond the convective initiation temperature. Hence, scattered to widespread thunderstorms are forecast as CIN erodes. Mid-level lapse rates are not excessively steep, but ongoing BL moisture advection and a slight cool-down at mid-levels allow modest MLCAPE build-up of roughly 500 J/kg, probably maximized over S-Romania/N-Bulgaria. CAPE magnitude increases towards Greece where higher dewpoints are present.

Sharp shear gradient is present with enhanced DLS next to the mid/upper jet over E-Romania, Bulgaria and Greece. Well organized mutlicells with large hail and strong wind gusts are forecast. LL shear is also enhanced, so an isolated tornado event is possible. The level 1 was expanded westwards, where strong pulsating storms may also pose an isolated large hail risk. The overall thunderstorm activity diminishes after sunset with a gradually cooling BL. Nevertheless, near neutral/slightly unstable stratification of the atmosphere assists in sporadic thunderstorm activity well into the night. Otherwise, daytime convection clusters betimes with locally heavy rainfall.

... Turkey ...

For Turkey, two foci for severe hazard exist:

The first one is situated over W/NW Turkey, where moist onshore flow pushed dewpoints well in excess of 10°C during the past 24 hours. Similar mid-level conditions are forecast compared to areas further north with moderate mid-level lapse rates but increasingly moistening low-levels. Widespread modest MLCAPE of roughly 500 J/kg is forecast. A 25-30 m/s mid-level streak sprawls into the highlighted area, which keeps 0-6km DLS at or above 20 m/s. Forecast soundings of W-Turkey show WAA regime with strongly veering flow and modest CAPE. The main concern is probably messy/widespread initiation owing to prolonged period of diffluent flow aloft and modest CAPE/weak capping. LL SRH relaxes somewhat during the day as wind field in lowest 2 km backs in response to the approaching cyclonic vortex, but rough topography may locally increase directional shear significantly, given strong background flow. Also, LL CAPE forecast is marginal at best. However, when weighting all those facts, we can't exclude tornadic activity mainly along the coasts and over NW-Turkey, so a level 2 was added. Next to that, large hail and severe wind gusts are also expected. Well organized storms are also forecast east of the level 2, but probably with less dense coverage. Similar hazards are forecast. Thunderstorms will last well into the night well onshore and probably all night long along the coasts. As shear decreases betimes, the severe risk should rapidly decline until midnight.

The second area of concern will be SW-Turkey after 18 Z. This area will be placed beneath a coupled high-level jet configuration with strong and deep tropospheric lift. BL airmass just offshore remains moist with 0-1 km average mixing ratio of more than 10 g/kg. Would like to see stronger LL flow for forced influx of moist air, but forecast wind field still supports a nighttime MCS, probably backbuilding with an overall gradual movement to the east. Model solutions diverge significantly with precipitation amounts and latest thinking is that strongest activity may occur just outside of our forecast area. Hence a high-end level 1 for excessive rainfall was issued for that area.

... SW-Mediterranean ...

Another strong impulse enters the Mediterranean area after sunset. Again, offshore flow features low dewpoints, reflected by latest readings over Spain with surface dewpoints in the lower single digits. However, rapid destabilization offshore is forecast, as cold mid-levels cause a rapid steepening trend of mid-level lapse rates in accordance to moistening LLs. We think that the coast of N-Algeria may again see the main activity during the evening hours onwards. The coastal areas see a favorable overlap of 200-500 J/kg MLCAPE and rapidly increasing SRH onshore, so any onshore moving storm may rotate with an attendant tornado, large hail and severe wind gust risk. We decided to stick with an high-end level 1 due to the very confined overlap of supportive parameters. Thunderstorms spread rapidly eastwards until 06Z, probably affecting N-Tunisia during the end of the forecast.

... CNTRL Italy ...

We remain concerned regarding locally heavy rainfall next to the well structure cyclonic vortex, which crosses the region slowly during the forecast. This time, mainly the eastward facing coasts were highlighted with a level 1. High res. models show locally strong convergence lines atop the W-Adriatic Sea with enhanced LL CAPE, so would not be surprised to see an isolated waterspout event with that activity. The risk gradually relaxes during the night as the vortex fills/weakens and becomes more diffuse.

Heavy rain, but probably less convective, also affects parts of the E-Adriatic Sea. A low-end level 1 covers that risk.

... The rest of the thunderstorm areas ...

Either shear or CAPE or both remain too marginal for organized convection. Again a vast area was covered with a 15-% thunderstorm region due to low-end chances. Reasons for daytime initiation probably can be found on the mesoscale, which can't be forecast that far out and over such a large area. Overall activity rapidly diminishes after sunset.