Storm Forecast
Valid: Mon 10 Oct 2011 06:00 to Tue 11 Oct 2011 06:00 UTC
Issued: Sun 09 Oct 2011 17:45
Forecaster: SCHLENCZEK

A level 2 was issued for most parts of W Turkey, the Bosporus region and the coastal areas of the W Black Sea for widespread excessive precipitation, tornadoes, large hail and severe wind gusts.

A level 1 surrounding the level 2 was issued for the same types of severe weather, but with a lesser density of reports expected.

A level 1 was issued for the Ionian Sea, S Greece and parts of the Aegean Sea mainly for waterspouts.

A level 1 was issued for parts of the Netherlands, Germany, Denmark and the Baltics mainly for severe wind gusts and tornadoes.

SYNOPSIS

An upper trough over the Adriatic Sea undergoes a cut-off process during Monday. Its center of upper level cold air is initially located over the central Mediterranean and will shift eastwards during the forecast period. Most portions of Turkey and also the Black Sea / Bosporus region are located directly ahead of this upper low in a region of strong DCVA, provided by a cyclonically curved 60 m/s southwesterly upper jet streak. This will create a strongly forced environment with favourable kinematic conditions for severe thunderstorms. Moderate to high values of MLCAPE (up to 2 kJ/kg) are expected ahead of the low.

In the wake of the low, vertical shear is much weaker but some hundred J/kg of CAPE are still enough for thunderstorm development.

Another region of unstable air is found at the western coast of Norway in the vicinity of a very cold upper low which moves eastwards towards the coastline.

A strong 70 m/s westerly upper level jet is found over the North Sea, associated with a shortwave trough affecting the northern parts of the Netherlands, Germany, Denmark, Poland and the Baltic States. In the vicinity of the cold front, some low-end instability is simulated by GFS, WRF-ARW and WRF-NMM. Strong QG forcing and a good kinematic environment may allow the formation of narrow convective rainbands with a threat of severe wind gusts and tornadoes.

DISCUSSION

...Norway...

In a weakly sheared environment with a few hundred J/kg of CAPE, some scattered showers and thunderstorms may develop during the day. Severe weather is not expected to occur.

...N Netherlands, N Germany, Denmark, S Sweden, N Poland, Baltic States...

A rather tricky weather pattern is expected to unfold over N Germany and other countries along the coastal areas of the S Baltic Sea as a strong shortwave trough races across this area. The degree of embedded convection in the cold front is quite uncertain but if convection develops, very strong LL shear, high values of SRH3 and strong deep layer shear may favour broken line segments and some embedded low-topped supercells with intense wind gusts and a distinct tornado threat. The WRF-ARW model with 4 km resolution shows such a scenario with strong linearly shaped convection but as the affected region is close to the northern boundary of the model domain, one should be very careful when interpreting the output. A level 1 is issued for this possible threat of severe wind gusts and tornadoes. This event should start in the late afternoon / evening hours over the Netherlands and will move across the area until Tuesday morning.

...E Mediterranean, Aegean / Ionian Sea, Greece...

In the wake of the upper low, some 500 - 1000 J/kg of MLCAPE are forecast and scattered thunderstorms are expected to develop. Overall shear is rather low but some storms may produce a short-lived tornado / waterspout due to high LL CAPE and low LCL heights.

...W parts of Turkey and Bosporus region...

Moderate to high instability overlapping with some 20 - 30 m/s of deep layer shear, high 0-3 km SRH and rather low CIN and low LCL heights are found at the southwestern and western parts of Turkey during Monday and also on early Tuesday. As strong QG forcing is in place, thunderstorm development should continue throughout the day. Especially in the mountaneous regions some kilometers further inland, this continuous regeneration of storms will lead to excessive precipitation in many places with flash floods being the main threat. If storms stay more isolated, they are very likely to become supercells and may produce large hail, severe wind gusts, and also tornadoes. The tornado threat should be augmented along the coastal regions where high LL moisture and strong LL shear overlap with sufficient instability. Especially in the evening hours, when LL moisture generally tends to increase, some strong tornadoes (F2 intensity and higher) may occur. The overall situation is an almost classic high-end level 2 setup for the eastern Mediterranean region.
Please note that flooding may be more intense than one would expect solely from the 24 hour precipitation sums as there already was a heavy rainfall event on Sunday affecting the same region of Turkey. The regions further east, which are not displayed in the forecast map, will also be affected by a major excessive rainfall threat.

Towards the Bosporus, a small area with augmented instability is located near the left exit region of the upper jet streak and intense thunderstorms are likely to occur. Very strong LL shear ahead of the SFC low and therefore high values of SRH3 suggest a significant tornado threat for this region. Some large hail and severe wind gusts may occur as well as there is a high probability for any storm to become supercellular. Excessive rainfall is also not ruled out but the main threat should be tornadoes and large hail / severe wind gusts.