Storm Forecast
Valid: Fri 14 Nov 2008 06:00 to Sat 15 Nov 2008 06:00 UTC
Issued: Thu 13 Nov 2008 17:29
Forecaster: TUSCHY

SYNOPSIS

Low geopotential heights over Europe remain as upper trough stalls over SW/CNTRL Europe, starting to close off. Surface pressure raises during the forecast over NW/CNTRL Europe while cold and stable conditions prevail over E-Europe. Main thunderstorm activity will be over the Mediterranean. Rapidly intensifying depression west of Norway shifts eastwards and so does the attendant strong wind field. Quality of airmass and weak convergence signals preclude any thunderstorm area for now with the highest risk of a few strikes just offshore of W-CNTRL Norway after 00Z.

DISCUSSION

... Parts of the Ionian Sea and S-Adriatic Sea ...

Positive tilted upper trough over the W-CNTRL Mediterranean keeps its tilt despite a gradual increase in jet dynamics along its SE quadrant. Intensification of the wind field at mid/upper-levels along the SE quadrant gradually works its way northeastwards, approaching the Ionian Sea after 00Z. As a consequence of the tilt, WAA in terms of a nice EML tongue builds northeastwards, causing lapse rates between 2-4km to increase constantly during the forecast. Latest MC SST image has a belt of 20°C SST along the coastal areas of W-Greece, increasing to well above 20°C towards the south/southwest. Thermal ridge at 850hPa holds steady until 18Z, before weakening quite rapidly, so area of strongest instability release should first concentrate on the mid-levels until 18Z and thereafter should also increase at lowest 3km. At the surface, broad cyclonic vortex, centered over Sardinia/Corsica during the midday hours, sliding southwards betimes, has no real fluctuation in strength, so cold front over Sicily is forecast to spread eastwards quite leisurely, becoming more diffuse somewhere over the central Ionian Sea during the night hours. Nevertheless, this front will be the focus for main initiation. Ahead of this boundary, maximized BL moisture advection beneath steepening lapse rates results in the built up of MLCAPE values in excess of 1-2kJ/kg and this tongue of maximized instability also builds northwards betimes,reaching the S Adriatic Sea during the night hours.

Speed shear will be the main issue as jet streaks not yet emerge out of the base of the upper trough with 0-6km bulk shear oscillating between 10 and 20m/s. However, degree of instability release should offset this negative effect and rapid thunderstorm organisation is expected. Veering with height is pronounced with SRH values up to 300 m^2/s^2 at lowest 3km. Combination of strong directional shear, good instability release at favorable hail growth zone and magnitude of CAPE all favor the risk of large hail and even isolated extreme events will be possible. Best instability stays offshore so this will be of special interest for the coastal areas of W-Greece. The highest tornado risk should evolve during the evening and night onwards, as LL cap weakens, LL lapse rates and in accord the 0-3km CAPE increase. Thunderstorms then also develop over the S/SE Ionian Sea and overall thunderstorm coverage will increase.

Will not go with a level-2 despite the significant hail risk as best environment for large hail could remain capped until the evening hours. However this will be a high-end level -1 due to the combination of an increasing tornado risk and large hail threat. Another uncertainty will be storm mode as slow moving surface front and numerous vort maxima, crossing this boundary during the day, should support a prolonged period of initiation and in combination with only modest DLS, fast storm clustering is not unlikely. An update may become necessary later-on.

... W-CNTRL Mediterranean ...

Continuous re-circulating of well mixed BL air in surface vortex beneath very cold mid-level air results in an extensive area of modest SBCAPE release. Shear remains weak, but LL CAPE release augmented. In addition, during the past 2 days, thunderstorms evolved along persistent and extensive convergence zones, which could serve as foci for enhanced waterspout risk if they set-up again.

After 00Z, surface pressure decreases north of the Algerian coast, also reflected in a maximized cyclonical curved pressure field. Influx of moist air from Sardinia/Corsica enhances instability with an augmented risk of showers/thunderstorms. There will be a tornado risk over a concentrated area along the NE-Coast of Algeria as friction results in an increasing ageostrophic wind component and augmented LL shear. Will not yet issue a level area as instability decreases rapidly inland and plume of most robust instability will come onshore just around 06Z onwards and hence out of the valid time frame.