Storm Forecast
Valid: Sun 21 Oct 2007 06:00 to Mon 22 Oct 2007 06:00 UTC
Issued: Sat 20 Oct 2007 19:52
Forecaster: TUSCHY

SYNOPSIS

Investigating the current European weather maps reveals a very dynamic pattern resulting in a completely disturbed streamline pattern over Europe. To get an understanding about the overall situation we have to describe the streamline pattern upstream a little bit.
During the past few days, very warm air east of North America streamed northwards with temperatures at 500hPa approaching -5°C!
This air now gets incorporated into a developing trough SW of Greenland, spreading rapidly towards the east. As a consequence, pressure begins to rise over the northern Atlantic Ocean, causing a weakening trend of a positively tilted upper-level trough west of Ireland. This in turn temporarily weakens the WAA, supporting a strong ridge over NW Europe, which resulted in the excessively high temperatures well to the north during the past 2 days.
Nevertheless...downstream of this NW-European high pressure area, a pool of very cold mid-level air spreads straight to the south and is accompanied by intense wind fields due to strong geopotential height gradients over southern Europe.

Warm and stable conditions continue over parts of eastern Europe.

DISCUSSION

... Confidence is growing that a high-end severe weather event will be imminent for parts of SE Europe. Please be up-to-date to the latest statements of your national weather services ! ...

... Greece, Crete, western Turkey and the Aegean Sea...

Yesterday, both the 12Z sounding reports and WV loops showed an intense 120kt upper-level jet over southern Germany, translating rapidly towards the south. The jet is forecast to strengthen and will surround the base of an intense upper-level trough placed over Italy during the forecast period, tilting the trough in a strongly negatively inclined position. In addition an intense subtropical jet should merge with the polar jet somewhere south of Greece, resulting in a strongly sheared environment.
As upper-level trough approaches from the west, pressure at lower levels will fall and GFS showed consistent signals during the past few runs that a well developed low pressure channel evolves south of Greece, shifting rapidly towards the northeast during the evening and night hours.
Forcing at mid-/upper-levels will be quite strong as well developed trough axis shifts eastwards ( adopting the aforementioned negative tilted position ).

SSTs of 21-23°C and a rapidly cooling lower troposphere result in a broad area of 500 - 1000 J/kg MLCAPE release, which is not excessively high, but in combination with those kinematic parameters it is more than enough. Mixing ratios of 10 g/kg and more and mid-level lapse rates of about 7K/km also confirm that this amount of instability release looks reasonable.

Thunderstorms should start to develop around 14-16Z south of Greece well ahead of the approaching cold front and thunderstorms are forecast to rapidly increase in coverage.
Another focus for enhanced convective activity will be the cold front itself, racing eastwards and reaching western Crete at about 03Z.

Thunderstorms will also significantly increase over Greece and the Aegean Sea at 22-00Z onwards as upper-level trough approaches from the west / southwest.

Right now there are still some model-to-model and run-to-run uncertainties especially regarding the strength of the surface channel ( resulting in shifts of shear maxima and shear variations over one place from one run to the next one of more than 20m/s ! ).
It looks like that there are 2 areas of an enhanced severe weather risk:

Crete and SW Turkey:

Shear in this area is impressive with 0-3km shear of 30-40m/s!, a belt of 10-15m/s LL shear, 0-1km SRH values of 200-300 J/kg and 600 - 800J/kg SRH at the lowest 3km!
Winds at 850hPa are forecast to reach 25-30m/s and about 35m/s at 700hPa and despite the fact that the atmosphere ahead of the cold front stays quite warm, moderately steepened lapse rates and increasing 0-3km CAPE release hint on an increasing risk of downward mixing of those wind speeds to the surface. Dependant on the final storm mode, severe-damaging wind gusts will be the main threat especially in bowing storms and mesoscale convective systems. In addition, shear vector stands nearly orthognoal on any developing line of storms, increasing the risk of longer lived, discrete thunderstorms.
Strong moisture inflow, deep convergence and not too high LCLs also point to the risk of tornadoes....even strong tornadoes will be a distinct possibility.
The same for the cold front, although main risk should be a severe - damaging wind gust threat. As cold front advances eastwards, it becomes more parallel aligned to the background flow and slows down. This could help to decrease the severe weather threat a little bit during morning hours.

SE- and E- Greece and the Aegean Sea:

Another area of big concern will be southern / eastern Greece, where the ingredient based severe weather forecast method points to a considerably enhanced tornado and severe wind gust risk.
LCLs are very low and as trough axis approaches from the west the wind field at all levels increases ( between 00 and 06Z).
LL shear of 15-20m/s and 250-400 J/kg SRH1 values will be present . We see no reason why storms should not be surface based, as strong and very moist inflow from the southeast establish during the night hours.
In addition, GFS 12Z again increased the winds at 850hPa, which could point to a potential widespread severe wind gust risk.

We decided not to issue a level-3 for this time as it is still not clear which area experiences the worst conditions ( a lot depends on the strength of the low pressure channel ) but parameters point to a potential severe weather event matching our level-3 criterion. Updates, if necessary will be issued later-on.

As a side-note: This looks like to be the first intense rain event since the devestating wild fires during this summer. Missing vegetation and long-lasting dryness over Greece should support a possible high-end flash-flood event.

....Central Mediterranean...

A constantly deepening surface low is forecast to leave the Tyrrhenian Sea during the evening hours, entering the western Ionian Sea during the latter part of the forecast period.
The severe wind gust / tornado threat will be augmented over Sicily as winds at 850hPa stay at 25m/s in addition to strong LL shear ( 0-1km shear at or above 15m/s and SRH1 values of up to 200J/kg ).

Otherwise LL shear will be enhanced over a broad area and as a result, tornadic storms can be expected over southern / central Italy, coastal areas of the eastern Aegean Sea and over northern Tunisia, mainly along the coastal areas, where LL shear gets maximized.

...Eastern Romania and extreme northern Bulgaria...

Although best shear and instability will be displaced, environment is still be conducive for an isolated tornado event. We therefore issued a marginal level-1, which could be downgraded in later updates.