Storm Forecast
Valid: Fri 24 Aug 2012 06:00 to Sat 25 Aug 2012 06:00 UTC
Issued: Thu 23 Aug 2012 23:04
Forecaster: TUSCHY

A level 1 was issued for SE Ireland mainly for heavy rain.

A level 1 was issued for parts of W/SW UK and NW France mainly for a few tornado events, marginal hail and strong wind gusts.

A level 1 was issued for E-France, parts of Germany, the Czech Republic and Poland mainly for an isolated tornado risk, large hail (S-Germany may even see an isolated very large hail event), strong to severe wind gusts and heavy rain.

A level 1 was issued for parts of Slovakia and the Alps mainly for large hail, strong to isolated severe wind gusts and heavy rain.

SYNOPSIS

Brisk SW-erly flow pattern continues over most of W/C-Europe. This assists in the set-up of an active baroclinic boundary, which runs from S/C France to Poland. Numerous disturbances graze that boundary and cause scattered to widespread thunderstorm initiation. Hot and dry conditions prevail over the Mediterranean.
A strong depression enters the forecast area from the west with unsettled conditions affecting Ireland and UK.


DISCUSSION

... Germany ...

At 21 Z, a quasi-stationary boundary from SW- to E-Germany separates a moist (15°C surface dewpoints and more) air mass to its south from significantly drier air to its north. A sharp vort max is about to enter E-France with increased pressure falls noted in latest surface data (21 Z). It marks the start of a gradually consolidating depression, which moves from E-France to the NE. This scenario results in the stationary front to transform into a northward moving warm front, approaching C-Germany during the start of the forecast.

First area of interest will be a weakly defined warm sector (magnitude depends on how strong the depression evolves), which covers E-Germany during most of the daytime hours. Mid-level lapse rates temporarily steepen until noon, before weakening thereafter. As a vort max passes, subsidence strengthens from the west during the afternoon hours onwards. Hence, any DMC occurrence will probably be transient with peak activity around noon. That's the time, when the warm sector features best LL moisture and model pool indicates roughly 500-800 J/kg MLCAPE (EZ as an outlier on the lower end side). Deep and low level shear are supportive for storm organization. Main uncertainty will be the type of storm mode with most models showing a prolonged period of clustering storms, which could limit discrete storm mode. At least a few isolated/more discrete thunderstorms are possible with strong to severe wind gusts, heavy rain and an isolated tornado the main risk. Around sunset onwards, the severe risk wanes, as cold front (transformed warm front) moves through from W to E.

Further to the south, a leisurely northward moving warm front becomes again quasi-stationary during the day from SW-Germany to N-Bavaria and transforms back to a SE-ward moving cold front during the night. This active frontal boundary and weak mid-level impulses result in repeatedly initiation all day long. Straight hodographs with 20-25 m/s 0-3 and 0-6 km shear are forecast with 500-800 J/kg MLCAPE present, so well organized multicells/isolated supercells are expected, which move rapidly to the NE. Given background wind fields, storms may grow upscale into bowing segments with concentrated swaths of severe wind gusts. Beside that, marginal to isolated large hail occurs with strongest storms. Also, any storm, which rides along the warm front ingests higher helical flow and an isolated tornado event can't be ruled out.
To the SE (e.g. SE Bavaria), strong SW-erly flow results in subsidence and mid-level warming with 850 hPa readings in excess of 18 °C. This type of pattern with plenty of BL moisture assists in mainly capped conditions during most of the daytime hours. An Alpine EML fans out to the NE which causes a local CAPE max of 1 kJ/kg and more just north of the Alps within 25 m/s DLS. Current thinking is that isolated storms move off the N-Alps towards the ENE/NE. Those storms may be able to acquire supercellular structures with large to very large hail and strong wind gusts. It has to be noted that mesoscale models keep this activity very limited during most of the forecast period.

A broad level 1 covers all of the aforementioned regions. An upgrade may be considered for areas along the warm front (SW-Germany to N-Bavaria) due to the wind gust risk.

... E-France ...

A cold front squeezes along the W-fringe of the Alpine crest and assists in a 15 m/s LLJ, advecting rich moisture from the Mediterranean (favorable and unmodified trajectories) towards E-France. Prolonged period of lift and orography support CI all day long. Despite 25 m/s DLS and intense LL shear, confidence in the severe risk is modest due to limited diabatic heating and probably a large shield of stratiform rain with embedded thunderstorms. Anyhow, isolated storms could easily organize and then would be able to produce all kind of severe, including large hail, severe wind gusts and an isolated tornado event. The main risk will be excessive rain. Thunderstorm probabilities decrease after sunset with CAPE decreasing to zero. An upgrade may be needed, if storms remain more discrete than currently anticipated.

... Alps ...

Scattered thunderstorms evolve during the day within a strongly sheared and moderate CAPE set-up. Well organized multicells with large hail, severe wind gusts and heavy rain are forecast. The severe risk decreases after sunset.

We added SE-Austria to the level 1 as rich BL moisture and modest mid-level lapse rates result in more than 1 kJ/kg MLCAPE. Any storm, which moves off the mountains into this environment will become severe with large hail and strong wind gusts. Confidence in that scenario is low however and therefore only a 15-% lightning area was issued.

... The Czech Republic, S-Poland and Slovakia ...

A broad warm sector features rich BL moisture and moderate lapse rates, supportive of 500-1000 J/kg in the highlighted area and up to 1.5 kJ/kg over S/SE Poland. 20 m/s deep layer speed shear and enhanced LL shear allow thunderstorms to gain some good organization. LL shear will be maximized along the warm front itself (e.g. W/C and N Poland), where any tornado risk will be maximized. Otherwise, large hail, strong to severe wind gusts and heavy rain will accompany strongest storms. During the late afternoon/evening, storms grow upscale into numerous thunderstorm clusters, which affect parts of Belarus and the W-Ukraine with heavy rain and marginal hail.

... W/SW-UK, parts of Ireland and parts of NW France ...

A strong 995 hPa depression moves from Ireland to W-UK. Attendant well structured cold front enters the forecast area from the west during the day with a slow eastward motion forecast. A pronounced dry slot overtakes the cold front and assists in an active thunderstorm period mainly within the highlighted 50% lightning area. Those storms evolve within an environment with strong directional shear (e.g. 15 m/s LL shear and 200 J/kg SRH-1) and LCLs well below 1000 m. A few tornado events are possible next to strong wind gusts and marginal hail. The severe risk decreases after sunset although some surface based convection may persist over N-France. Strong shear still supports an isolated better organized thunderstorm event with strong wind gusts and an isolated tornado event. Hence the level 1 was expanded all the way to NE France.

An occlusion wraps around the depression's center with an augmented probability for training thunderstorm activity mainly over S-Ireland. Heavy rain will be the main hazard as modified subtropical air wraps around the center of the depression.