Valid: Sun 27 Apr 2014 06:00 to Mon 28 Apr 2014 06:00 UTC
Issued: Sun 27 Apr 2014 06:33
Forecaster: VAN DER VELDE
Levels 1 and 2 were issued for parts of Netherlands, Germany, Czech Republic, Austria, and northern Balkan mainly for excessive convective precipitation.
Levels 1 and 2 were issued for parts of Turkey and Aegean Sea mainly for large hail.
A level 1 was issued for eastern Poland mainly for a chance of excessive convective precipitation and large hail.
A level 1 was issued for Sardinia mainly for large hail.
Low pressure systems stretch from Ireland to Turkey with major low pressure centers in the English Channel, Italy, and Aegean/Turkey,with associated upper shortwave troughs ensuring upward motion. Thermal low pressure is present over eastern Germany and Poland. A main feature is the stalled cold front between Netherlands and Austria. Associated linear convergence falls on the warm side due to the thermal trough versus cloudy cool airmass side and is the trigger for convective developments.
...Netherlands, Germany, Czech Republic, Austria, northern Balkan...
There is only a few hundred J/kg CAPE, more in the east where lapse rates are steepest. Mixing ratio around 8 g/kg. Weak flow and mean wind vectors oriented along the convergence line should lead to slow-moving and training multicell systems which can locally deliver tens of millimeters of rain, as mesoscale models predict. The airmass is very weakly capped leading to early development, but the inflow of warm air into the convergence zone may keep it from turning into a soup of convective remains.
Enhanced deep layer shear (10-15 m/s) in the afternoon especially in the north of the area and better upper level CAPE support a slight large hail threat. Stationary convergence lines can also trigger some slow-moving cells with some chance of excessive local rainfall.
...SW Turkey and surroundings...
A strong upper level trough is accompanied by a strong upper- and mid level jet. The region of interest is in the left exit region. This results into strong lift due to upper level divergence. Mixing ratios of 10 to 12 g/kg in combination with large mid level lapse rates lead to MLCAPE or MUCAPE values of 800 to 1200 J/kg. High shear values (0-6km: 20 to 25 m/s and 0-3km: 15 to 20 m/s) should lead to organized thunderstorms that have the risk for large hail. Thunderstorms will likely organize along a line of enhanced convergence. This line will travel from Greece to southern Turkey during the day, leaving the forecast area in the evening hours. Hodographs suggest a potential for more than gusts resulting from Corfidi vectors around 25 m/s. LCLs are low in the frontal zone which would benefit tornadoes, but low level shear appears lacking (<10 m/s 0-1 km).
A much drier environment with slight CAPE and LCL around 2000m with steep lapse rates above and deep layer shear of around 20 m/s suggests a good potential for large hail. However this area does not see the strong lift from the shortwave trough and initiation might be isolated of orographic nature.
The cold front passing between 06Z-12Z is associated with some CAPE and moderate deep layer shear and SREH. Intense multicell or perhaps a supercell can form and release large hail. Low-level shear is not so strong which limits tornadoes.
A very slight tornado chance exists as vertical shear increases towards the western part oft he upper level low (15 to 25 m/s DLS and over 10 m/s LLS) in the presence of CAPE.