Storm Forecast
Valid: Fri 26 Mar 2010 06:00 to Sat 27 Mar 2010 06:00 UTC
Issued: Thu 25 Mar 2010 20:22
Forecaster: TUSCHY

A level 1 was issued for parts of Germany and Denmark mainly for large hail and an isolated tornado (Germany) and locally excessive rainfall (Germany and Denmark).

A level 1 was issued for SE-France and N-Italy mainly for large hail, severe wind gusts and an isolated tornado.

A level 1 was issued for SE-/E-France and parts of Switzerland mainly for excessive rainfall.

A level 1 was issued for W-France mainly for severe wind gusts and an isolated tornado threat.

A level 1 was issued for N-Portugal and NW-Spain mainly for severe wind gusts.

SYNOPSIS

An omega-like pattern is anchored over Europe with two major troughs (first one is placed over the Bay of Biscay, drifting northeastwards during the forecast and a second one over west Russia with a marginal eastward movement) and a stout low/mid-level ridge in-between. Zooming into the western trough, a complex nature of various waves with different amplitude circle that feature, causing a tricky mid-/high-level streamline pattern.

i) First short-wave exits the S/E North Sea (06Z-12Z) to the north while weakening over Norway (15Z onwards)

ii) A second one over SW-France (06Z) with an ENE-ward motion, affecting E-France (12Z) and SW-Germany (18Z)

iii) The most dynamic wave, moving over S-France and N-Italy to the east during the outlook period

iv) The upper trough axis, entering the W-English Channel/N-Bay of Biscay area during the evening hours with slow eastward movement.

At lower levels, a cold front/convergence zone ranges from SE-France to SW-Germany during the morning hours, which already reveals impressive LL convergence signals. This slow moving cold front intensifies thereafter due to numerous reasons:

i) augmented convergence due to the rough orography over Switzerland (SE-France/Switzerland)

ii)the dynamic / kinematic reasoning due to the NE-ward traveling short waves (SE-France/SW Germany)

iii)falling surface pressure over SE Germany due to diabatic heating but also due to adiabatic warming as strong foehn winds continue

This surface low lifts out to the north beneath a favorable jet coupling and good high-level divergence.
It is a bit hard to determine the structure of this cold front, but many signs hint on an ana-type cold front. This type of front is characterized by a broad stratiform postfrontal rain shield and a narrow zone with enhanced convection along its leading egde. In addition, due to continued geostrophic adjustement, as front strengthens, the thermal circulation ought to play an increasingly important role for potential initiation over parts of Germany later-on.


DISCUSSION

The model bias is insignificant with NOGAPS, GME, GEM, EZMWF and GFS all print the sub-990hPa depression SW of UK and a developing 1005hPa surface depression over SW-Germany beneath impressive upper divergence. Minor discrepancies in strength and placement of this depression are visible, but nothing too significant. This similarity is also present over NE-Germany during the night hours with the northward moving depression. Major discrepancies exist in the BL air mass quality with EZMWF running below the GFS output, so CAPE fields still reveal differences. For now, we stick with a medium between EZMWF and GFS, as surface dewpoint reality is found somewhere in between both outputs. Run to run consistency of the model outputs is good.

...Germany ...

Anafront-type cold front over SW/W-Germany (morning hours) drifts slowly towards the east. Constantly falling prefrontal surface pressure and N-ward drifting depression over E/NE-Germany thereafter result in a gradually strengthening front, bisecting Germany from north to south. Around noon, the front accelerates eastwards over S-Germany as the surface depression lifts to the north. As mid/upper jet breaks apart into numerous smaller streaks, favorable upper divergence remains in place, so in respect of forcing/lift, no negative reasoning is seen.
Prefrontal air mass over SE-Germany is warm but relatively dry as adiabatically warmed foehn winds cause a decrease in LL mixing ratios. This also places the CIN bullseye over east/southeast Germany with marginal SBCAPE forecast. As cold front pushes eastwards during the afternoon/evening hours, a rapidly enlarging isallobaric wind vector points to a windy frontal passage but with absent deep/persistent convection and probably subsevere gusts, no level will be issued. It has to be noted, that WRF came up with a prefrontal convergence zone (outrunning the front itself), which may help to increase dewpoints somewhat ahead of the approaching cold front and this solution even supports initiation over S-Germany, as mid-level lapse rates remain steep. We excluded extreme SE-Germany due to ongoing foehn effects.

Initiation becomes more likely in a 12-15Z time frame over central Germany and northwards, where roughly 500 J/kg MLCAPE (a medium of EZMWF and GFS) and the western verge of the CIN area overlap. DLS remains modest with 15m/s, but locally augmented 0-3km directional shear and EL model output below -40°C indicate a fair chance for more organized storms (multicells) with a large hail threat. A risk for isolated tornadoes is confined to a narrow stripe along the cold front over north to south-central and eastern Germany, where best LL CAPE, some LL veering and strongest updraft evolution overlap. We decided to stick with a level 1 due to the upper streamline pattern with abundant divergence, which ought to foster rapid storm development into one large or numerous smaller clusters (probably trailing stratiform), which keeps discrete storm mode more limited. BL stabilization causes a rapid decline of unstable surface parcels and therefore, the main risk will be strong wind gusts and isolated large hail after sunset.

A stripe of intense rainfall evolves along the postfrontal side of the cold front over central/north-central Germany, due to the intense divergence/convergence at upper and lower-levels respectively but also due to the developing depression and increasing frontogenesis.

... SE France, Switzerland, Italy, S-Austria and Slovenia ...

All ingredients are present for a band with heavy rainfall during the 06-12Z time frame over SE-France and over S-Switzerland until roughly 15Z-18Z. No frontal penetration occurred over the W-Mediterranean during the past days, so moist air is advected northwards to the area of concern. We went with a level 1 for that risk, as some MUCAPE is present as far north as E-France during the morning, so despite overall stratiform nature of this event, an isolated embedded storm is possible, justifying the issuance of a level.
A conditional severe thunderstorm risk is also present over S-France (morning hours) and N-Italy thereafter as surface cold front gradually moves eastwards. The front stalls over W-Italy due to a gradually consolidating surface depression to its east and remains aligned parallel to the southern Alps until the night hours. DLS is very strong with 30-40 m/s and roughly 10-25 m/s 0-3 km shear, increasing from west to east but the limiting factor for a more robust risk will be marginal thermodynamic environment with SBCAPE below 500 J/kg. The thunderstorm coverage may be more isolated, but any stronger updraft will be able to acquire rapid organisation with a large hail/severe wind gust threat. The LL shear ramps up, as the surface depression evolves with an isolated tornado risk possible mainly over NE-Italy due to enhanced LL speed/directional shear. Thunderstorms spread eastwards during the night but as CAPE diminishes betimes, the severe risk vanishes. Influx of moist Mediterranean air and orographic/ isentropic lift support a developing stratiform rain shield over Austria, moving eastwards during the night. No level was issued for that risk at expected rain amount will stay sub-severe.

... France, Ireland and parts of UK ...

In the postfrontal air mass, daytime driven showers and thunderstorms occur over central France and westwards, as diabatic heating, a well mixed BL and steep mid-level lapse rates result in 200-500 J/kg SBCAPE. Shear is weak at best, so the main risk will be marginal to isolated large hail due to a low WBZ level. The activity wanes after sunset. The same will be the case over parts of Ireland and SW-UK.

The next convective flare-up occurs over NW-France at midnight onwards, as the upper trough axis draws near from the west. This feature is of special interest, as the occluded depression beneath the upper trough results in an increase of the BL moisture in a strongly sheared environment (0-1km speed shear 15-20m/s and direcitonal shear 150-250 m^2/s^2). Despite a severe wind gust risk with 850hPa winds increasing to 25m/s, the tornado risk will not be negligible. A level 1 mainly for severe wind gusts was issued as the final tornado risk has to be evaluated in upcoming model runs, but at least an isolated tornado event can't be ruled out.

... N-Portugal and N-Spain ...

Moist onshore flow continues during the forecast beneath cold mid-levels. Deep convection will be confined to the level 1 region, where severe wind gusts are forecast. The overall thunderstorm risk diminishes during the late afternoon hours as the region resides beneath the anticyclonic shear side of the high-level jet.