Storm Forecast
Valid: Mon 27 Aug 2018 06:00 to Tue 28 Aug 2018 06:00 UTC
Issued: Sun 26 Aug 2018 23:32
Forecaster: BEYER

A level 2 was issued across parts of the east coast of Greece mainly for extreme rainfall and to a lesser extent for waterspouts

A level 1 was issued for large parts of Greece, FYROM, as well as parts of Albania, Serbia, Montenegro, and Bulgaria mainly for excessive precipitation and to a lesser extent for waterspouts as well as locally large hail and severe wind gusts.

A level 1 was issued for the coastal areas along the Skagerrak, Kattegat, and part of the Baltic Sea for waterspouts.

SYNOPSIS

An upper-level low is moving from the Adriatic Sea to the Aegean Sea during the forecast period. During this time this upper level low crosses Greece and will be gradually weakening. The upper-level low is accompanied by an upper-level jet maximum on its southern flank.

A second upper level low can be found west of the Iberian Peninsula and is only slowly moving east-northeastward, approaching its northwestern coast.

More to the north two long wave troughs can be found embedded in the frontal zone. One of them is moving from the North Sea to the Baltic Sea during the forecast period whereas the other is still west of Iceland. At the end of the period, the British Isles are getting on its forward flank.

A ridge is influencing most parts of Southwestern Europa and Western Europe. Another well-amplified ridge can be found over Western Russia.

DISCUSSION

... Parts of the Balkan States ...

The region is on the forward flank of the slowly eastward moving upper level low and thus in an upper-level diffluent flow and on the left exit region of the upper-level jet. These are good indications for robust lift mechanisms.

In addition, unstable airmasses are active which is represented by steep mid-level lapse rates. Overlapping with good moisture values that will lead to several hundred up to 1500 J/kg of MLCAPE. Since LL moisture significantly decreases inland, MLCAPE is highest along the coasts. On the other hand Lapse Rates are higher inland which counters the lack of moisture.

Taking everything together and taking into consideration the good lift ingredient, widespread CI can be expected. However, DLS is mostly weak with values only locally reaching 10 m/s. Thus convection is mostly unorganized and of pulsating nature. To the south, with the help of the jet, shear values are getting better but then storm coverage decreases.

In the affected region winds in 500 hPa are rather weak with 5 to 15 kn due to the passing center of the low. Therefore storm motion is forecasted to be rather weak and as a consequence, the main hazard of storms is the occurrence of excessive precipitation.

More to the south storms should move faster. In the transition zone in between, there is a risk for locally large hail and severe wind gusts.

Finally, there is a risk that waterspouts can be observed. A slow upper-level flow, a convergent low-level flow, and slow storm motion are good hints for spouts. This is possible especially over the Western Aegean Sea and the Eastern Ionian Sea.

... Eastern coast of Greece ...

This area was highlighted with a LVL2 for the night to Tuesday. LAMs and different model ensembles show an impressive consistency that this area will see high amounts of convective precipitation.

Having a look at the ingredients, that looks quite plausible. With northeasterly LL winds continuously advection of LL moisture should feed backbuilding storms. This is also supported by a maximum of moisture flux convergence.

Having these robust hints we decided to issue a LVL2 for the highlighted area.

... Coastal areas of Skagerrak, Kattegat and Baltic Sea ...

The upper-level trough is moving over this area. Cold mid-levels and a warm water surface lead to steep mid-level lapse rates. Although LL moisture is only modest, a few hundred J/kg of CAPE can build up. Whereas most of the cold air convection is not severe, the potential of waterspouts is enhanced.

Ingredients for waterspouts are quite good. The trough shows a distinct low-level center that is moving over the highlighted area. Thus mid and upper-level flow is rather weak and the same is true for the storm motion. In addition, LCLs are forecasted to be only between 400 and 600 m. Having also the surface low in that region, LL convergence zones can be found.

Taking all these hints together, the potential for the occurrence of waterspouts is clearly enhanced. The waterspout risk is moving eastward during the forecast period.