Storm Forecast
Valid: Sat 28 May 2011 06:00 to Sun 29 May 2011 06:00 UTC
Issued: Fri 27 May 2011 19:40
Forecaster: SCHLENCZEK
A level one threat was issued for parts of Iberia, N Morocco and NW Algeria mainly for large hail and severe wind gusts.
A level one threat was issued for a narrow area from Hungary towards S Belarus mainly for severe wind gusts and large hail.
SYNOPSIS
An upper shortwave trough, initially located over Scotland, moves across the North Sea towards S Scandinavia. Scattered showers and thunderstorms are likely to occur near the upper cold core in an environment with weak vertical shear.
Moderate instability is forecast for most parts of Iberia in a warm and moist airmass ahead of an upper cut-off low. Some decent WAA and diurnal heating may lead to approx. 1500 J/kg MLCAPE over the western parts of Iberia which partly overlaps with enhanced 0-6 km deep layer shear.
A tongue of unstable air attached to a weakening upper trough over east central Europe will extend northeastwards across NE Hungary towards Russia. Even though convective activity will be mostly diurnally driven, some potential for large hail / severe wind gusts will exist.
Overall QPF signals are far below the thresholds for widespread excessive precipitation.
DISCUSSION
...Iberia and N Maghreb States...
Deep layer shear increases from about 10 m/s in the northern half of Iberia to more than 20 m/s towards Morocco and N Algeria as an upper jet stream is located near the southern tip of the upper cut-off low. In contrast to the horizontal gradient in vertical shear, instability will likely reach its maximum over central Portugal where 0-6 km shear is rather low. GFS shows a slightly veering wind profile up to approx. 500 hPa which leads to some 100 - 150 mē/sē SRH3. If convection starts to develop, the evolution of isolated well-organised multicells and a few supercells will be likely. The main threat is expected to be large hail and some isolated severe wind gusts as LCL / LFC heights are quite high (around 2 km) and steep mid level lapse rates indicate a dry EML which is favourable for evaporative cooling. Flash floods and tornadoes are unlikely in such a situation, but they are not completely ruled out. The peak activity of severe thunderstorms is expected in the late afternoon / early evening.
...Hungary, W Romania, Slovak Rep, Serbia / Montenegro, W Ukraine, Belarus...
Near the surface cold front of the trough which crossed central Europe on Friday, CAPE in order of some hundred up to 1000 J/kg is forecast in an environment with 10 - 15 m/s 0-3 km shear. The tropospheric wind maximum in this region is near 700 hPa which should be favourable for some linear multicell segments. If the linear shape is the dominant convective mode, isolated severe wind gusts will be the main threat. Apart from wind gusts, some storms should be capable of producing marginally severe hail as well.
...Greece, S Adriatic, Aegean Sea, Balkan region, Turkey...
Some hundred J/kg CAPE are forecast over a broad area in the vicinity of a weak upper cut-off low. Vertical shear is below 10 m/s in most places and organised severe thunderstorms are unlikely to occur. Nevertheless, one can't rule out an isolated waterspout over the Aegean / E Mediterranean as LCL heights are below 1 km and low level lapse rates are close to a dry adiabat. Slow storm motion and high effective PWAT may allow an isolated heavy rain / flash flood event over Greece / W Turkey but overall threat is below the level 1 threshold.
