Storm Forecast
Valid: Fri 23 Jan 2009 06:00 to Sat 24 Jan 2009 06:00 UTC
Issued: Thu 22 Jan 2009 20:36
Forecaster: TUSCHY

SYNOPSIS

The cyclonic vortex between Iceland and Scotland remains in place with numerous disturbances along its periphery, delivering unsettled and cold conditions to NW Europe. A flat upper trough is placed over SE Europe while cold/stable conditions prevail over NE Europe. A depression enters the forecast area during the evening hours from the west, growing into a major extratropical depression. A few more lines about this feature:

Discrepancies between past few model runs are still noticeable although all of them merged to the solution of a depression, crossing the S-Bay of Biscay and SW-France during the night hours. However, how low the central pressure will be remains somewhat obscure with GME/GFS still showing the most intense solutions (GFS12Z down to near 960hPa). Latest glance on IR-enhanced GOES-12 loop reveals a distinct baroclinic leaf structure between 40°-50°N/40°-50°W at 17Z. 12Z analysis of the Ocean prediction center already tracked this feature as an 1004hPa wave with dewpoints to its south up to 15°C, running roughly 10K lower to its north, which is in accord with GFS/GME. Also upper-levels with jet core now topping out at near 110m/s (250hPa) fits in the forecast philosophy of both models, so we decided to stick with them and keep the rest of the models in view. Connection to the slightly modified tropical airmass holds steady and favorable placement beneath the left exit of the slowly weakening upper jet should indeed result in strengthening and possible rapid cyclogenesis ( 24hPa/24h or more). This depression crosses the S-Bay of Biscay between 18Z-00Z, reaching W-France after midnight.

DISCUSSION

... W/SW coast of Turkey, NE Greece and S-Bulgaria [06Z-15Z]...

A constantly weakening depression, affecting a vast area over SE-Europe, slowly moves northeastwards. Ongoing convection / new thunderstorm development is likely over the Aegean Sea, especially along the coastal areas, where strong LL convergence and CAPE release become maximized. Rough terrain more onshore and an increasingly cold/stable boundary layer should keep thunderstorms more elevated, but persistent and strong influx of warm/moist air keeps surface based convection within the bounds of possibility along the coast and just offshore. A rapid increase of ageostrophic deflection onshore enhances LL shear and thunderstorms along the coast, rooting into the highly sheared layer could gain rotating updrafts in addition to 30-40m/s background shear until 09Z. Thereafter, shear relaxes bit by bit and so does the tongue of rich BL moisture, which lowers the severe risk after 12Z. A level-1 was introduced along the coast concerning the isolated tornado/severe wind gust risk and due to a large hail risk over NE-Greece and extreme SW/S-Bulgaria, where stronger directional shear and 400-800 SBCAPE overlap. Both, the severe thunderstorm risk but also the coverage of storms should decrease over the level-1 after 12Z, but thunderstorms will continue around Crete until the morning hours in a weakly sheared environment.


... Central/east France, W/SW-Germany ...

The focus for isolated thunderstorms arises along the eastward marching cold front although conditions appear only marginal supportive. Tongue of higher theta-e values ahead of this front dissolves noticeably while crossing E-France. However, rapid SE-ward advancing mid-level CAA in conjunction with a SE-ward advancing negative tilted upper trough axis offset detoriating BL conditions, so a tongue of marginal instability persists all the way to W/SW-Germany, 12Z. Some potential instability release looks reasonable as intrusion of dry low-stratosphere air, a weakening UVV field and cold front overlap over W-Germany/E-France around 9Z-12Z, so confidence in an isolated thunderstorm event along the cold front is high enough to include a general thunderstorm area. In addition to that, a level-1 was introduced to reflect an isolated severe wind gust risk due to winds at 850hPa of 20-25m/s. An isolated tornado event over E-France can't be excluded especially during the afternoon hours, as BL remains quite moist and SRH1 increases. The thunderstorm risk should ease after 12Z over Germany and hence no level/general thunderstorm area was drawn over central Germany despite the ongoing risk of isolated severe wind gusts. All in all, this is a low-end thunderstorm risk.

... NW/N-Spain and SW/central France [18Z-06Z and onwards] ...

The strong cold front will be the first focus for deep convection, which reaches NW-Spain around 18Z and continues its rapid eastward propagation thereafter. A concern is the fact that trajectories now pass over the Iberian Peninsula, so prefrontal warm sector loses its favorable connection to the high theta-e plume. Nevertheless, a strong cold front reaches SW/W-France at midnight as mid-level lapse rates steepen somewhat (6-7K/km). A forced line of showers/thunderstorms will likely accompany this cold front and this line will move well inland. Better BL moisture around Toulouse should ensure the line to keep its strength over SW-France, while a light weakening trend may occur further to the north. Instability release is on the low-end side, but intense forcing and rapidly strengthening shear point to a widespread severe-damaging wind gust/isolated large hail event. Thunderstorms along the cold front, rooting into the surface layer should be able to ingest SRH-1 values in excess of 400m^2/s^2. This combined with LCLs below 600m and some 0-3km CAPE release augments the tornado risk and a strong tornado is certainly possible, if persistent and surface based convection evolves. Thunderstorm activity will also increase over S/SE-France during the early morning hours, as cold front pushes eastwards. Severe wind gusts and isolated tornadoes are possible in this environment.

Of even bigger concern is the back-bent occlusion, which is forecast to cross the extreme SE Bay of Biscay and SW-France between 03Z and 06Z. It is still too far out to speculate about such more small-scale features, but this possible solution has to be kept in mind. A line of showers/thunderstorms could accompany the passage of this occlusion as mid-levels cool down rapidly. The main risk with this convection would be extreme wind gusts as winds at 850hPa reach 50m/s and forecast soundings support the mix down of such high values to the surface. Directional shear would be also conducive for tornadic activity. Later model runs have to be evaluated before sticking to this possibility. The level-2 was mainly issued as wind gusts along the cold front, associated with deep and/or electrified convection, will likely exceed 33m/s. The potential strong tornado risk, although still somewhat uncertain, also flows in this level-2.

The final area for enhanced thunderstorm chances will be just below the depression's center, which comes ashore somewhere around Nantes at 00Z onwards. The speed shear is much weaker, but directional shear remains high, so isolated tornadoes are possible with thunderstorms, evolving in this environment. Marginal hail and strong gusts are also forecast and hence the level-1 area was expanded well inland to reflect this risk.