Storm Forecast
Valid: Fri 21 Nov 2008 06:00 to Sat 22 Nov 2008 06:00 UTC
Issued: Thu 20 Nov 2008 23:56
Forecaster: TUSCHY

...SYNOPSIS...

Long anticipated arctic outbreak is underway as very cold air spreads southwards between an intense 1040hPa high over the NE Atlantic and an extensive cyclonic vortex over N-Europe. Degree of extreme mid-level cooling and moderate BL moisture result in slightly unstable conditions over most parts of N/CNTRL Europe. Downstream of this vortex, cool conditions prevail in a more hostile environment for deep convection while warm and stable conditions continue over far SW Europe.
A very large level-1 was issued, but parts of this level area will see different kind of severe weather risks during different time frames. Therefore the level was split in the discussion.

... Germany and Austria ...

Main story is the cold front passage during the daytime hours. A slowly organizing short-wave races SE wards over N-CNTRL Germany, reaching E-Germany during the midday hours. Attendant surface cold front crosses W-Germany around 09Z and approaches the Alps only a few hours later. Main inhibiting factor will be quite meager BL moisture but some recovery is forecast just ahead of the cold front passage. Placed in the left exit region of impressive 80-90 m/s upper streak and accompanied by an intense UVV field, a forced line of intense showers/thunderstorms could evolve although latest high resolution models don't support that. Enough persistence in latest model runs regarding aforementiond parameters are present to stick with a forced line of storms and an augmented severe wind gust risk as LL wind field is quite blustery ( winds at 850hPa at or above 25m/s also in the well mixed postfrontal airmass). Forecast soundings support the risk for deep convection with such intense forcing. LCLs are very low and speed / directional shear augmented ( LL 15m/s and SRH above 300m^2/s^2), so beside intense wind gusts, an isolated tornado can't be ruled out.
The tornado risk should increase after the cold front passage over parts of CNTRL / SE-Germany after 12Z, as still some low-end BL moisture and rapidly cooling mid-levels result in neutral/slightly unstable profilers. Speed and directional shear remain strong so severe wind gusts and an isolated tornado are possible beside white-out conditions in intense showers. As electrified convection could pop up everywhere, we included the whole region in a thunderstorm area.

It is even more difficult to define highest thunderstorm chances for the rest of Germany. Conditions are supportive for isolated thunderstorms all day long over NW Germany and also increase during the night over N/NE Germany. See lines below.

For Austria, cold front passage becomes somewhat diffuse somewhere between 12Z and 18Z. As decribed above, forcing is very strong and line of intense showers/thunderstorms could affect NE Austria with strong to severe wind gusts and marginal hail. Thereafter, mid-level temperatures drop to below -40°C at 500hPa, so shower/thunderstorm activity will keep going although current thinking is that activity will be mainly embedded in more stratiform precipitation areas. Decided to issue only a small thunderstorm area with the level area somewhat larger to reflect severe wind gust threat during the not necessarily electrified cold front passage.


... Coastal areas of the Netherlands and NW Germany until 00Z...

Prolonged period of onshore flow persist throughout the forecast as modified artic air comes ashore. Latest OSI-SAF product had SSTs of 8-10°C well offshore and a belt of 10-12°C just off the coastal areas with even higher values over the English Channel. Temperatures at 850hPa gradually decrease from -5°C during the early morning to near -10°C at night, which should set the stage for abundant LL CAPE release but despite long path and mixing, airmass remains very dry which is also reflected in forecast soundings and attendant LCL forecasts of 1-2km above sea level. Despite the impossibility to forecast a short spin up, missing boundaries and aforementioned conditions limit risk of waterspouts for most of the day. Placement of upper jet max and attendant rapid increase of EL temperatures to the east were heavily weighted for the shape of the thunderstorm area. Coastal convergence above belt of higher SSTs and very low temperatures at 500hPa ( at -40°C) keep chances for electrified convection high enough for a thunderstorm area. Already marginal BL moisture gets mixed out quickly onshore, but constantly steepening mid-level lapse rates to well above 8K/km could offset that effect so thunderstorms could move far inland in an environment with high speed/directional shear and hence a level-1 was issued due to an isolated tornado/marginal hail and strong to severe wind gust risk. Shallow convection will occur over Belgium and the Netherlands and it is interesting to see that GFS still sticks to a low-end tornado risk. Shear would be abundant but very low EL heights and minimal instability preclude any area for now. Thunderstorm chances will increase during the night over the Netherlands as cold mid-levels spread westwards with the same risk as decribed above.


... NW coast of Germany and the extreme E/SE North Sea between 00Z-06Z ...

As first upper wave exit the area during the late morning hours, strongly meridional component of boundary layer flow is progged, resulting in decreasing shear at lower column of troposphere and an overall increase/organisation of shower streets during the day and night hours. In addition, wrap around moisture from Norway/Sweden advects SW-wards during the night and a strengthening convergence zone sets-up over the area of interest at midnight and onwards. Another weak perturbation should exit the area until 00Z and conditions seem favorable for a strengthening band of deep convection with a long fetch all the way to the west of Norway. Thermodynamic support will be favorable with very steep lapse rates through LL/mid-levels, combined with very moist air in the dendritic growth zone. Deep and potential electrified convection cells should affect NW Germany during the time with marginal hail / strong to severe wind gusts and an isolated tornado threat. This risk could extend well onshore and hence the level area was drawn far inland.

... Slovenia, Hungary and Croatia ...

During the past few days,signs of moisture advection ahead of the cold front passage became more robust during each run. Surface cold front will slow down during the passage of the Alps while cold mid-/upper levels overspread the area during the midday/afternoon hours. Good BL moisture beneath steepening mid-level lapse rates will be adequate for a concentrated area with nearly uncapped 400-700J/kg MLCAPE. Surface cold front exits S-Austria during the early afternoon hours and races southwards, so thunderstorm activity will increase rapidly during the afternoon hours. Impressive shear values are present ( 0-6km bulk shear 30-35m/s, LL shear 15-18m/s and enhanced directional shear) so thunderstorms will gain organisation very fast. A concentrated swath of severe to damaging wind gusts, large hail and a few tornadoes are forecast and a level-2 was issued. Instability diminishes during the evening and early night hours, but intense forcing should keep line of thunderstorms organized well into the night ( 18-21Z) with a diminishing severe wind gust/tornado and hail risk all the way to western Romania. Hence the level area was expanded well to the east.

N-/E- Adriatic Sea was also included into the level-1 area as DLS values in excess of 30m/s overspread southward racing cold front and severe wind gusts are forecast. As cold airmass at upper levels sinks southwards, instability along the coastal areas of the E-Adriatic and Ionian Sea are on the increase and as LL shear is very strong, a level-1 becomes necessary to reflect the isolated tornado risk.