Storm Forecast
Valid: Fri 31 Jan 2014 06:00 to Sat 01 Feb 2014 06:00 UTC
Issued: Thu 30 Jan 2014 19:46
Forecaster: TUSCHY

A level 2 was issued for parts of N/CNTRL and S-Italy mainly for excessive rain.

A level 1 surrounds those level 2 areas mainly for isolated large hail, severe wind gusts or an isolated tornado event.

A level 1 was issued for N-France and S-UK mainly for severe wind gusts and an isolated tornado event.

A level 1 was issued for SE France mainly for heavy rain and an isolated tornado event.

SYNOPSIS

Mid/upper streamline pattern features a disturbed pattern with ongoing meandering flow over Europe. Main topic will be a major and rapidly deepening cyclone to the west of Ireland with significant mid-layer height and temperature rises over SW Europe (e.g. Portugal and Spain). The evolution of the height field forces quasi-stationary trough over S-CNTRL Europe to build further to the south towards Algeria as 40 m/s mid-layer jet streak builds south along its western flank. Further east over SE/E Europe, upper WAA atop ongoing LL CAA pattern creates a very effective blocking pattern with a 1050 hPa plus surface high center over SW Russia.

At the surface a quasi-stationary surface trough extends from Tunisia to CNTRL Italy with numerous weak low pressure areas. A complex pattern of various fronts affects the CNTL Mediterranean.
Beside that feature, the rapidly developing depression west of Ireland becomes the most dominant player for NW Europe especially during the overnight hours. Latest (19Z, 30th Jan) modified IR loops already show this impressive looking depression over the N-Atlantic, just parked beneath a 80 plus m/s upper jet and moving east along an intense baroclinic zone (where mT and cA converge). This dynamically driven intensification occurs until this outlook period starts. Thereafter however (so just in my forecast period), a rather impressive plume of unstable (occluded) air wraps around the center while upper jet dynamics exit to the E. Phase diagrams once again show an healthy warm-core structure ... deep and stable with surface pressure likely leveling off just below 940 hPa. This should induce a slowdown of the depression and a stable motion to the SE with only slow increase of the depression's central pressure being forecast during the rest of the outlook. Past model runs already came in later with the approach of the depression to Ireland and this trend should continue a bit ... at least until the warm anomaly of the core finally forms and models catch up with that scenario. With this trend in mind, we doubt that strongest winds will arrive in Ireland until 06 Z, as warm-seclusion draws near. Nevertheless, forecast soundings from the potential sting-jet region show 50 m/s winds at 850 hPa, which itself lowers to less than 1000 m! Well mixed air mass should assist in BL gusts in excess of 40 m/s, but again ... strongest winds remain just west of Ireland until 06Z. With little to no convective activity expected, no level area will be issued for now. 15-% lightning area covers spotty activity during the day and well ahead of that jet.

DISCUSSION

... N-France and UK ...

Aforementioned depression approaches the area in a well occluded stage with ongoing separation of eastbound racing surface fronts and incoming high-level tropospheric/lower stratospheric air. So not much concern exists with fronts, which cross the area during the daytime hours from W to E. During the night hours, a strong impulse at 500 hPa circles the sub-940hPa depression and approaches W-UK until 00Z from the W/SW. This feature takes profit of access to the tropopause fold with a pool of drier high-level air approaching and probably crossing the cold front over UK. Further south, over N-France, forcing will be weaker and mid-levels quite warm, but better influx of modified subtropical air also increases concerns in weak MUCAPE build-up along the cold front.
No surprise, shear will be intense, so forcing and CAPE will be the parameters which dictate how organized (in respect of convection) the cold front might be.

Current thoughts are that the southern part of the front probably outruns dynamics but remains in a favorable shear environment. Organized narrow cold-frontal rain band structure or even an LEWP structure crosses N-France from W to E at 00Z onwards. Depending on how structured this front will be, strong to severe wind gusts and an isolated tornado event can occur. With departing forcing, a break-up of that front is expected betimes with ongoing but more isolated severe risk in the eastern part of the level area. Of interest is also rapid weakening of LL directional shear along the front over N-CNTRL France during the end of the forecast as frontolysis begins.

S-UK has better forcing and meager BL moisture could be offset (at least partially) by dry slot/front interaction. Despite missing support of models, the concern of a LEWP-ish line exists, which races east. Intense shear at LL once again indicates an isolated tornado and severe wind gust risk. Also width of the warm sector (if not already occluded) dictates the potential severe risk and if some prefrontal backing of the LL wind field can occur.

... Italy ...

With quasi-stationary features at all levels, a conveyor belt of warm/humid air becomes established with no serious net motion to the east expected. This belt covers all of Italy and points to the S-Alps. Most of the embedded fronts will be aligned nearly parallel to the background flow or will be squeezed along the Alps and also don't show any impressive looking motion vectors.

Forecast soundings show a deep unidirectional flow regime from the Ionian Sea to NE Italy with 20-25 m/s winds just above the BL. Effective PW remains high with 15-25 mm and cool mid-layers atop warm SSTs also offering modest SBCAPE of 400-800 J/kg over the Adriatic and W-Ionian Sea. An ominous agreement between models exist in placing a 100-250 mm/24 h QPF bullseye over N/NE Italy, where prolonged period of upslope flow is expected. Unstable air mass with northward fanning MUCAPE plume should support embedded convection far inland and probably all the way to S-Austria. An isolated severe risk could exist along the N-coast of the Adriatic Sea due to rapidly increasing LL shear, so an isolated tornado event is possible. Otherwise, excessive rainfall will be the main hazard ...all the way to SW-Austria. Uncertainties arose if this would be a level-2 situation as falling snow levels could keep runoff on a more modest side, but areas below the snow levels will still see excessive and likely flash flood producing amounts of rain, so we decided to stick with a level 2 area. Dangerous and probably life-threatening winter conditions exist in the S-Alps, so please keep up-to-date with your local weather office!

Another area of concern in respect of excessive rain runs from the SW Tyrrhenian Sea to C-Italy and also from Sicily to the northeast. Uncertainties remain high regarding placement and strength of surface depressions and the structure of any surface front, but the set-up is prone to slow moving or training convection in this general area. Beneath those training MCS events, rainfall amounts in excess of 100 mm in a short amount of time (e.g. in a couple of hours) are possible and serious flash flooding is a distinct possibility. Certainly not all areas will see level 2 conditions, as mesoscale heavily influences the outcome of any MCS event.
Next to the rainfall risk, shear/CAPE overlap also supports organized convection. Any more discrete activity (e.g. tail-end storms) could become well organized with large hail, severe wind gusts and an isolated tornado event ... a strong event can't be ruled out!

... SE France until 12 Z ...

Ongoing cluster of training showers and thunderstorms from the night probably continues until noon before weakening. Heavy rain and an isolated tornado event are forecast.