Storm Forecast
Valid: Tue 15 Oct 2013 06:00 to Wed 16 Oct 2013 06:00 UTC
Issued: Mon 14 Oct 2013 19:37
Forecaster: TUSCHY

A level 1 was issued for parts of Albania, Republic of Macedonia and N/NW Greece mainly for an isolated large hail and excessive rainfall event.

SYNOPSIS

At 20Z, WV imagery and phase diagrams reveal a symmetric and deep cold core low, centered over the SE UK. Pronounced banding structure is diminishing with temporal loss of latent heat support of the 12-14 °C [SST] warm North Sea. Latest loop keeps this feature more or less stationary over SE UK, but an eastward drift is anticipated within a short time, as a potent E-Atlantic trough approaches from the west. This feature is accompanied by a strong warm conveyor belt with influx of subtropical air (MIMIC-TPWs aoa 40 mm) and this air mass is forecast to become absorbed by the UK low. Therefore a constant eastward motion of that warm and humid air mass towards C-Europe is anticipated.
Very dry and cold air invades most of NE Europe with convection confined to the offshore/coastal areas. Low topped convection should remain weakly electrified.

DISCUSSION

... S-Balkan States, Greece to Romania ...

20Z surface synops show dewpoints in the mid-tens over the area of interest. Only weak impulses are embedded in a slightly diffluent mid/upper flow regime. Persistent SW-erly flow in the lower troposphere advects a moist air mass from the Adriatic/Ionian Sea to the NE, so BL dewpoints should hold steady in the mid-tens during the forecast. 500 hPa temperatures below -15 °C, aforementioned BL moisture and temporal diabatic heating should support a broad swath with 200-600 J/kg MLCAPE....increasing to more than 1000 J/kg towards the N-Ionian Sea. Expect daytime driven showers/thunderstorms from the Republic of Macedonia to Romania. Limited CAPE should keep the severe risk very low. A few stronger storms could produce marginal hail and strong wind gusts.

Albania and Greece see better CAPE/shear overlap, but stronger capping and weak forcing induce uncertainties regarding the degree of CI. Expect isolated to scattered thunderstorms with locally heavy rain/large hail. A marginal level 1 was issued mainly due to good shear profiles and access to rich BL moisture along the coast (heavy rain).

Thunderstorm probabilities also increase over the N-Adriatic Sea during the end of the forecast as cold air overspreads 19-20 °C SSTs. An isolated waterspout risk is possible.

... Benelux, Germany ...

The clash of the cold-core low with the eastward spreading subtropical air mass causes a tightening gradient flow along the southern fringe of that vortex. A 500 hPa jet streak intensifies to aoa 35 m/s within an eastward spreading warm conveyor belt , which brings heavy rain to S-Germany. The left exit region of that jet overspreads C/E Germany. Combined with DCVA ahead of the approaching vortex, enough forcing is available for scattered CI within a weakly capped air mass and atop a moist marine BL. Eastward surging drier mid/upper air affects the occlusion, which seems to transform into a split front as 850 hPa theta-e holds steady in the lower 30s (°C) over C/E Germany until 18Z . In addition, wrap-around moisture moves beneath the cold-core low from the W/NW. 500 hPa temperatures drop to -25°C or less, so low to moderate SBCAPE build-up is anticipated. Marginal hail and gusty winds will be the main hazard. Repeatedly onshore moving storms with slow storm motions could pose a heavy rainfall risk for Belgium/the Netherlands, but effective PWs below 20 mm should keep the flash flood risk too marginal for a level area. The activity weakens after sunset but isolated storms could persist until 21Z...mainly over E-Germany, SW Poland and the NW Czech Republic.