Storm Forecast
Valid: Sat 21 Oct 2006 06:00 to Sun 22 Oct 2006 06:00 UTC
Issued: Fri 20 Oct 2006 17:59
Forecaster: TUSCHY

SYNOPSIS

Bipolar 500 hPa geopotential height distribution will continue during the next 24 hours ( refering to the 10/13-19/06 NCEP analysis, which had a -15 dam anomaly west of the Bay of Biscay and a 30 dam anomaly over Greenland )..... Smaller-scale low pressure areas will rotate around the pool of low geopotential heights west of Ireland.
The first one // 49°N 22°W at 20th October, 12Z // was rated as a 980hPa storm system, which should rapidly race towards the NE, reaching Ireland during the early morning hours ( already weakening ).
The other one // 43°N 42°W at 20th October, 12Z // should start to intensify while moving towards the east and will affect parts of SW Europe during the evening / night hours.
As a result, broad belt of SW-erly winds will cover the rest of Europe downstream of this broad E-Atlantic trough with embedded weak disturbances moving towards the ENE.
The extreme NE part of our forecast area will already see the first outbreak of cold air.

DISCUSSION

... Parts of United Kingdom and parts of France...

Aforementioned low pressure area will reach Ireland during the early morning hours... Downstream of this depression, a tongue of air with slightly higher moisture content will slide over S-UK from the SW and should leave United Kingdom during the later afternoon hours.... Right now it is somewhat difficult to determine the frontal boundary distribution at the surface, but it looks like that a strengthening convergence zone ( coupled with the axis of highest Theta-E ) will be the focus for a few storms to form..... Left exit region of approaching strong mid-/upper-level jet and good upper-level divergence indicate that enough lift should overspread the region from the SW to support at least isolated TSTM development.
Thermodynamic profilers become more favorable for convection due to some cooling at mid-levels and hence it looks reasonable for a broad area of low-end instability release over most parts of UK .
Overlapping shear and instability parameters look fine for some storms to get organized....A few tornadic storms will be possible due to low LCL's and combined with a broad area of enhanced LL shear mainly over the level-1 area ...furthermore, DLS of 15-20m/s should be enough for a few severe wind gust reports....Threat will vanish during the early evening hours, when instability should diminish and a field of strong NVA will overspread the area from the SW.

Nearly the same conditions can be found further towards the south along the English Channel and over N / CNTRL France, where broad area of enhanced LL shear and at least marginal instability release will overlap....
Again...I see the main support for storm development at the upper-levels and no clear frontal signal at lower levels (at most a very weak combination of an occluded front over UK transforming into a cold front over parts of N- France ).
A well mixed boundary layer and cooling mid-levels should be enough for some instability release... Did only include the N-part of France in the level-1 area due to stronger background flow and slightly stronger veering at lower levels, but if later soundings / satellite images indicate a more conducive environment for TSTMs further towards the south, a southward expansion may become necessary later on.

Combination of cold mid-levels and 16°C SSTs ( North Sea ) should support up to 500 J/kg instability release and kinematic parameters will be more than adequate for a few severe TSTMs with an isolated tornado / severe wind gust threat along the coastal areas of Belgium and the Netherlands

... Coastal areas of Portugal and S-Portugal / Spain ...

Weak instability will be present over extreme S-Spain and the Strait of Gibraltar and a few storms are forecast to form.... DLS should be enough for a few severe wind gust reports....
Expected storm coverage and pretty weak thermodynamic environment preclude a level-1.

Another area of interest will be SW / S Portugal...Exact strength of approaching depression still pretty uncertain ( GFS even opened it during the past few runs)....Advection of high dewpoints and still fine mid-level lapse rates should support some instability release well inland over S-Portugal during the night hours.
Low LCLs and good veering should support an enhanced tornado threat with any developing storm, but expected storm coverage will be low for any higher probabilities....Up to 20m/s DLS should also be conducive for a few severe wind gusts / marginal hail reports.
Extended the level-1 area further towards the north along the coastal areas of Portugal due to the possibility of a few severe storms to develop in an environment with 20-25m/s DLS and strongly enhanced LL shear.
Severe wind gusts / isolated tornado threat should rapidly ease further inland due to decreasing instability.

...CNTRL / E Mediterranean...

Slowly eastward moving upper-level low should support scattered TSTM development.... Low - moderate instability release will be present but shear should be too weak for any organized TSTM threat.... Despite the weak shear, any stronger pulsating TSTM could produce a few severe wind gusts / marginal hail .