Storm Forecast
Valid: Tue 20 Sep 2016 06:00 to Wed 21 Sep 2016 06:00 UTC
Issued: Tue 20 Sep 2016 06:50
Forecaster: BEYER

A LVL 2 was issued for Crete and Southern Turkey mainly for severe wind gusts, hail as well as tornados and to a lesser extent for excessive precipitation

A LVL 1 was issued for the surrounding area extending from the southern Aegean Sea to the Crimean Peninsula mainly for the same reasons as in the LVL2 area.

A LVL1 was issued for parts of Italy (Venetian coastline) mainly for excessive precipitation.

SYNOPSIS

A strongly amplified and omega-like upper level flow is present over most parts of Europe which is supposed to be a quite persistent general weather pattern.

One trough is situated over the near Northeastern Atlantic Ocean influencing Greenland, Iceland and with its forward flank parts of the British Isles

Another long wave trough can be found over far Eastern Europe and Western Russia. This trough is extending further south and combines with the upper level low that was active over the central Mediterranean last weekend. The axis of this complex trough is travelling eastward from the Eastern Balkan Peninsula in direction to the Aegean Sea and Turkey as well as the Black Sea.

In between the two long wave troughs a highly amplified ridge is present over the northern half of Central Europe as well as over large parts of Northern Europe bringing stable weather conditions.

Finally a short wave trough can be found over Southern Europa that starts from the Iberian Peninsula and travels eastward over the Western Mediterranean. It is deamplifying throughout time and reaches the central portions of the Mediterranean at the end of the forecast period.

DISCUSSION

...Crete and Southern Turkey...

Humid and warm airmasses are present downstream of the cold front, that travels eastward during the day and will likely cross most parts of the affected area until afternoon. Thus the risk for severe weather is highest in the first half of day until the early afternoon, whereas the regions further downstream (not on the map) will be affected later on.

Quite a high mixing ratio with values locally up to 13 g/kg are present. They can combine with steep lapse rates so that CAPE values of around 1500 J/kg, locally 2000 J/kg, are present. Having a vivid mid- and upper-level Jet maximum (aligned SW to NE) quite strong vertical shear values are present. DLS (0-6km) has values of 20 to 25 m/s, and also 0-3 km shear values can be found between 15 to 20 m/s.
Finally it should be added that surface stream lines indicate a nice low level convergence along the cold-front that is followed by a region of positive pressure tendency.

Taken all this together organized storms are possible that can easily become supercells bringing large to locally very large hail as well as severe wind gusts. The latter is also supported by Corfidi vectors showing >25 m/s of forward propagation.

In addition, LLS (0-1 km) does also show enhanced values (>10 m/s). Together with SRH3 values of up to 250 m2/s2 (SRH1: up to 200 m2/s2) and low LCL heights (<1000 m) there is a risk for tornadoes that may come along with developing supercells.

Finally, since ppw values are quite high (values locally up to 40 mm) a local excessive precipitation event along the Turkish coastline cannot be ruled out.

...Western Mediterranean and far Northwestern Black Sea...

In the named regions weak upper level winds (due to the respective trough axis) can combine with low LCL heights. In addition AGL CAPE (0-2km) shows locally more than 100 J/kg. Taken all together there is an enhanced threat for waterspouts. This is well pronounced over the Black Sea. Concerning the Western Mediterranean, the threat increases in the late afternoon and during the night time and is travelling eastward with the trough axis.

...Venetian coastline...

Instability and convergent low level streamlines coming from the Adriatic Sea give an enhanced threat for storms that may bring excessive precipitation due to rather high ppw values. This risk is most pronounced during the night hours when the diffluent forward flank of the upper level trough offers the needed lift mechanism.

In addition Corfidi vectors indicate a slow storm motion that also supports this threat.