Storm Forecast
Valid: Fri 03 Oct 2008 06:00 to Sat 04 Oct 2008 06:00 UTC
Issued: Thu 02 Oct 2008 19:06
Forecaster: TUSCHY

SYNOPSIS

Latest Hovmoeller diagram of the 500 hPa geopotential height gradient (GHG) measurements of the NE Atlantic region indicates a final break-down of the nearly 15 days lasting positive GHG anomaly and outlooks for the upcoming days show a broad pool of negative values over most parts of Europe which is in line with latest NAO outlook, showing another peak of positive anomaly.The NE Atlantic and Europe will be in a transition mode during the next few days as a more zonal flow establishes. This will already occur during the forecast period, as ridging (50°-60°N/30°-40°W) over the far NE Atlantic flattens at least temporarily, as a rapidly stengthening large-scale vortex over NE Canada will start to establish a compact WAA regime over the N-Atlantic. This eventually enhances geopotential height gradients over UK/Ireland as polar air upstream of the European low ( analyzed with 982hPa over S Norway/Sweden at 12Z) advects southwards. Strong 60m/s upper streak evolves and pulls developing upper trough far to the south, over France at 06Z and W-CNTRL Mediterranean 24h later. Lowering geopotential heights just downstream of the trough axis support unsettled but warm conditions for SE Europe while warm and tranquil conditions prevail over most parts of Spain/Portugal.


***Additional informations on post-tropical storm Laura ***

Post-tropical storm Laura, at roughly 52°N/45°W, now encounters sub-11°C SSTs, while moving to the north, just S of Greenland. 06Z surface analysis (OPC) already has a complete transition into a frontal low pressure system and latest WV image hints on the inclusion of drier high tropospheric/low stratospheric air. Last QuikSCAT scan had uncontaminated 35-40kt along its SE quadrant, which is in line with the last advisory of the NHC, keeping it a 40kt system (discussion number 10). Ex-Laura should reach the extreme southern tip of Greenland during the morning hours of Friday, before turning to the east while increasing its forward speed.Further track of this system will be hard to follow as highly diffluent pattern overspreads the region south of Greenland, resulting in a broad area of pressure fall with numerous new low pressure developments. Latest indications are that not Laura itself but a new developing low will reach Ireland during SAT morning while the circulation of Laura decays somewhere SE of Greenland but a modified but still high energetic pool of tropical airmass will be available for further cyclogenesis over NW Europe.



DISCUSSION

... France/Bay of Biscay ...

There a various foci for isolated to scattered thunderstorm chances. To keep track of all those regions, the whole paragraph was splitted into numerous time-frames:

** Extreme SW-France/SE Bay of Biscay 06Z - 18Z**

Timing of thermal trough is very good as coldest air (-10 to -12°C at 700hPa and -28 to -30°C at 500hPa ) overspreads the area during daytime heating. Vigorous convection should be already in place over the SE Bay of Biscay during the morning hours where SSTs remain between 17-20°C, so enough instability release for deep convection as EL temperatures drop to below -30°C. Strongest activity will be until 12Z as cold front finally clears the Bay. Question remains where to draw thunderstorm area/when electrification of convection starts, so only the SE corner was highlighted ( not the eastern parts as fetch for turbulent mixing of LL will be too short). As trough axis at mid-levels/cold front at LL swings onshore, diurnal heating,very cold mid-levels and a fetch of better moisture along the S/SE-ward moving front should support thunderstorms mainly in the highlighted area. Surface front will clear extreme SW-France around sunset and both, losing frontal structure and very meager BL moisture should lower thunderstorm chances. Overall, shear is weak but stronger thunderstorms could produce isolated large hail in addition to an isolated waterspout risk over the SE Bay of Biscay.

**ESE France and extreme SW-Germany 15Z-18Z**

The main focus for a few isolated thunderstorms arises during peak heating, as weakening surface cold front approaches from the NW. As front reaches the Black Forest area, orographic enhanced lift could support a few stronger updrafts and weak LL convergence and marginal parcel layer depth superpose somewhat. Not much activity expected but enough for a small lightning area.

... Belgium, the Netherlands and extreme NE France ...

Persistent and active convective period continues over the highlighted area as modified cool but moist polar airmass spreads southwards. A lull in activity should occur during the morning to afternoon hours, but as a well evolved back-bent occlusion approaches from the NNW after 15Z, thunderstorm chances increase again. LL lapse rate are not notably steep and strong wind gusts/marginal hail will be the main risk. Strong CAA on its backside finally support rapidly increasing 0-3km CAPE release, but as PWAT-values drop, confidence is low that more than very isolated storms will evolve offshore after 21Z. Only traces of instability onshore should limit updraft strength and hence thunderstorm chances.

... Baltic Sea ...

Cold atmosphere will be present during the forecast period and some low-end to moderate instability release offshore is expected. Isolated to scattered thunderstorms are forecast but weak shear should keep them below severe criterion.

As an additional information. A glance on numerous large inland lakes reveals SSTs of well above 13°C, with numerous lakes having readings of 15-17°C (e.g. Lake of Constance, Germany) and the southern part of Lake Garda (above 20°C). As mid-/low levels cool down and background flow relaxes, there will be a chance for waterspout reports over large and still warm inland lakes.

... W-CNTRL Mediterranean ...

A strong cold front crosses the western Mediterranean as the trough axis shifts eastwards. At Friday,06Z the cold front is forecast to just cross the Balearic Islands
from the NW although main upper impulse is still far to the west/northwest, so it may last until 10-12Z, before the surface front finally clears the area. Steep mid-level lapse rates, a moist BL and strong moisture convergence along the front result in a tongue of prefrontal 500-1000 J/kg MLCAPE and as winds at all levels increase during the morning hours (DLS 20-25m/s), thunderstorms along this front should rapidly gain organisation with a severe wind gust and tornado risk as also directional shear component is markedly enhanced. During the afternoon, evening and night hours, the front races eastwards and the risk for tornadoes/severe wind gusts and large hail continues along and ahead of this front. The past few runs of GFS pointed to a significantly enhanced severe thunderstorm risk over Sicily after 00Z, as weakly capped 1-2kJ/kg and constantly strengthening shear at all levels overlap. A level-2 was issued for this area as the risk for tornadoes - an isolated strong one possible- is augmented (low LCLs, very high LL CAPE release and increasing LL/DL shear). Southward extension of the level-2 is uncertain as dry and capped air could suppress storms, but only a small shift to the south of the strong upper impulse over the western Mediterranean could result in initiation south of Sicily in a very favorable environment. In addition,there was a slight southwards trend of the latest GFS QPF forecasts, so the level area was expanded well to the south.

Another round of organized thunderstorms evolves just offshore of NE Spain and eastwards at 15Z onwards as thermal trough overspreads the area. Rapidly steepening lapse rates over the warm Mediterranean result in 500-near 1000 J/kg MLCAPE with higher SBCAPE values possible. Again, abundant LL CAPE release, 0-6km bulk shear above 20m/s and enhanced 0-3km helicity values are present so there is a tornado and severe wind gust risk mainly over a region Balearic Islands and to the N/NE. The thunderstorm risk spreads eastwards during the night and early morning hours.

... NE Italy ...

Surface pressure falls south of the Alps and strong moisture advection will be underway over NE Italy already during the morning hours. Lapse rates at mid-levels stay steep and at least some low-end instability release is forecast. As a 35-40m/s mid-level streak crosses NE Italy from the SW during the midday/afternoon hours, DLS will increase to impressive 30-50m/s. EL temperatures hint on deep convection, which could take profit of those intense winds and a few organized storms can't be excluded. Any developing thunderstorm would be in a very favorable environment for storm rotation and an isolated large hail threat/strong to severe wind gusts risk is possible.

... SW/W-Turkey...

A northeastward moving EML affects the highlighted area during the forecast period. Steep mid-level lapse rates atop of a moist BL airmass should yield widespread 800-1000 J/kg MLCAPE. A slowly eastward progressing short-wave will induce enough lift fore widespread thunderstorm initiation in a moderately sheared environment (DLS 15-30m/s, increasing to the south), so thunderstorms will organize with multicells/supercells occurrence possible. Large hail, tornadoes and severe wind gusts are anticipated as LL shear/CAPE release is enhanced. A level-2 was issued as thunderstorm coverage will be high.