Severe thunderstorms expected in parts of France, Switzerland and North Italy



  • Valid: Sat 06 Jul 2019 06:00 to Sun 07 Jul 2019 06:00 UTC
    Issued: Thu 04 Jul 2019 21:33
    Forecaster: TUSCHY

    A level 2 was issued across E-Spain mainly for large to very large hail and severe wind gusts.

    A level 1 surrounds the level 2 for similar hazards with lower probabilities.

    A level 2 was issued for parts of France, Switzerland and N-Italy mainly for large to very large hail, severe to damaging wind gusts, excessive rain and over NE-Italy and W-Slovenia also for an augmented tornado threat.

    A level 1 surrounds the level 2 for similar hazards with lower probabilities.

    A level 1 was issued for S-Sweden mainly for isolated large hail.

    A level 1 was issued for parts of the E-Ukraine mainly for large hail.

    A level 1 was issued for the E-Turkey mainly for large hail and severe wind gusts.

    A level 1 was issued for N-Spain mainly for large hail and severe wind gusts.

    photo:ESTOFEX;desc: Extended Forecast valid Sat 06 Jul 2019 06:00 - Sun 07 Jul 2019 06:00 UTC.;

    SYNOPSIS

    An extensive zonally aligned channel with low geopotential heights covers Scandinavia. Embedded cold-core vortices plow east and maintain unsettled/cool conditions over N-Europe.

    A quasi-stationary upper trough just west of the Iberian Peninsula backs a healthy subtropical ridge over the CNTRL Mediterranean, where hot and dry conditions prevail.

    Attached to the frontal zone, numerous synoptic-scale fronts cross CNTRL-/E-CNTRL Europe with repeated surges of modified polar air moving south. A very moist and warm/hot air mass resides over the Mediterranean and surroundings. In-between an extensive thermal/moisture gradient evolves from France to the Alps to Hungary, where organized convection is forecast.

    DISCUSSION

    Please note that only level areas were issued. Attendant lightning areas will be added in the Day-1 outlook.

    France, Switzerland, S-Germany, Austria, N-Italy to Slovenia, Croatia and Hungary

    Certainly, the main focus for severe convections resides in this extensive swath over CNTRL-Europe.

    Most robust low tropospheric moisture is forecast over N-Italy with days without any frontal passage and just temporal/regional convective overturning. Moisture over SW France improves substantially in response to a weakening cyclonic vortex just W of Portugal and resulting veering winds over the S-Bay of Biscay, which push a marine air mass onshore. Mixed signals about timing/strength of moisture recovery with an adequate moisture depth against diurnal mixing exist from S/SE France to Switzerland, S-Germany and parts of Austria. Most of the guidance, however, brings the 10 g/kg isohome into these areas.
    A pronounced EML over the W-Mediterranean turns NE towards the Alps with gradually lowering values and atop the moist air mass.
    Hence widespread 2000 J/kg MLCAPE evolve over W/SW and S-France with up to 3000 J/kg over N-Italy. Otherwise, MLCAPE resides in the 800-1500 J/kg range.

    The proximity to the deep frontal zone pushes the 6km bulk shear in the 15-20 m/s range and hence a mixed mode of organized multicells and supercells is forecast. Most forecast hodographs offer rather straight signatures with somewhat better mid-level turning/veering over SW-France and augmented 0-3 km SRH from NE Italy to Slovenia.

    A smooth mid-level flow regime turns a bit more wavy/disturbed at lower levels with numerous eastward moving small-scale waves. The strongest disturbance seems to approach CNTRL-Europe during the evening and overnight hours from the W and is accompanied by a S-ward sagging cold front.

    CI can be split into the following scenarios:

    a) Earliest CI seems to be a mixed mode of ongoing/developing elevated convection over CNTRL-/E-France, which moves into Switzerland. There, gradual rooting into the moist/unstable air mass is forecast with severe convection affecting Switzerland from noon onwards. Splitting cells and multicells occur with large hail and severe wind gust. A mixed/dry subcloud layer points to strong cold pools with potential line ups and concentrated swaths of severe. Constant recharging of the air mass points to repeated thunderstorm development during the day with ongoing severe. An upgrade to a level 2 was performed but may need to be broadened more in later outlooks.

    b) Active mountain convection over W-Austria and Switzerland is forecast also around noon onwards which is driven by diabatic heating and potential favourable timing of a passing short-wave. It remains unclear that far out how strong convection from a) affects CI of b) and in the end, we could see a mixture of both regimes, which build into growing and E-ward moving clusters. The main hazard will be heavy rain, large hail and a few severe wind gust events.

    c) Outflow boundaries from a)/b) finally serve as foci for CI over N-Italy or developing thunderstorms along the orography become mobile with maturing downdrafts and enter the lowlands from the N. Initiating storms are placed in an environment, which is favourable for long tracked supercells with large to very large hail, severe wind gusts and excessive rain. As convection approaches NE Italy/far W-Slovenia during the evening onwards, a tornado risk evolves due to backed LL winds. The level 2 was expanded far S to account for a growing and maturing MCS, which also affects parts of Croatia during the night.

    d) A bit of stronger capping and later timing of the short-wave may delay CI over SE Austria into N/E Slovenia a bit into the afternoon hours, but then multicells/transient supercells evolve with large hail, severe to damaging wind gusts and heavy rain. The level 2 was already expanded into those regions and may see further modifications in later outlooks.

    e)Beyond noon, a mixture of mountain convection (S-CNTRL France) and a prefrontal wind shift line over N-CNTRL France cause scattered to widespread CI. The latter activity probably starts more elevated but builds S into the unstable and weakly capped air mass while transforming into (nearly) surface based convection. CAPE/shear space supports organized multi cells/isolated supercells with all kind of severe. Growing and maturing clusters probably add concerns of swaths of concentrated severe/isolated extremely severe events. Hence an upgrade to a level 2.

    f) S-Germany remains more unclear with CAPE in the 500-1000 J/kg range but favourable DLS and a deeply mixed subcloud layer. Would expect mountain convection over SW Germany to move E in the form of organized multi cells/isolated supercells. Large hail and severe wind gusts will be the main hazard. During the night, weakening clusters from France/Switzerland may affect far S-Germany but with a lower severe risk due to diminishing CAPE. Hence only a level 1 for now.

    Far SW-France needs to be monitored closely for CI. Despite weak capping and mid-level ridging, not much modification is needed for a few severe thunderstorms with all kind of severe. That far out no level area was yet added but a level area may be considered in later outlooks.

    Both level 2 may merge in later outlooks. Confidence due to early CI and lots of clouds is too low for already merging both level areas.

    N/E-Spain

    A marine BL airmass beneath very steep mid-level lapse rates causes MLCAPE in excess of 2000 J/kg. DLS remains around 20 m/s. Despite a warm 850-700 hPa layer, anticipated diabatic heating and orographic support should help for isolated CI. Large to very large hail and severe wind gusts are forecast. The activity should weaken when moving offshore (E/NE) into a strongly capped air mass. Despite missing synoptic-scale forcing and isolated CI, favourable CAPE/shear space supports an upgrade to a level 2.

    N-Spain was added to a level 1 for organized mountain convection with large hail/severe wind gusts in a 15-20m/s DLS and 1000 J/kg setup.

    S-Sweden

    A small warm sector crosses the highlighted area during the daytime hours. Up to 500 J/kg SBCAPE and 20 m/s DLS overlap, so a few thunderstorms could turn severe on an isolated/temporal scale. Hail could become the main hazard with multicells and curved 3-6 km hodograph structures forecast.

    E-Ukraine

    SE-ward surging cold front/prefrontal wind shift interact with a NE-ward fanning plume of moist/unstable air (500-1000 J/kg MLCAPE). Strongest shear seems to be a bit displaced from the highest CAPE plume but still enough overlap exists for a few multicells with large hail.

    Far E-Turkey

    A few multicells bring large hail and severe wind gusts with locally heavy rain.



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