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Storm Forecast Valid: Sat 29 Jun 2024 06:00 to Sun 30 Jun 2024 06:00 UTC

28-06-2024 20:19 | Estofex | m.b.t. 29-06-2024 t/m 30-06-2024

Storm Forecast
Valid: Sat 29 Jun 2024 06:00 to Sun 30 Jun 2024 06:00 UTC
Issued: Fri 28 Jun 2024 20:19
Forecaster: TUSCHY

A level 3 was issued across parts of E-France to far NW Switzerland into SW Germany for damaging gusts, very large hail and excessive rain.

A level 2 surrounds the level 3 and covers the similar hazards but with a somewhat lowered confidence level. A regionally augmented tornado risk is present with a strong event possible.

A level 3 was issued for NW Italy into far S-Switzerland mainly for giant/destructive hail, excessive rain, damaging gusts and we also explicitly add a strong tornado risk for this area.

A level 2 surrounds the level 3 and covers the similar hazards but with a somewhat lowered confidence level.

A level 2 was issued for SW France mainly for excessive rain.

A level 1 surrounds all level areas with all kind of hazards but lower probabilities.

A level 1 was issued from Finland/W Russia into Romania for a few large hail/heavy rain and isolated gust reports in its southern part, turning more into a severe wind gust and isolated tornado risk towards its northern part.

A level 1 was issued for parts of the Caucasus mainly for hail and severe gusts.

A level 1 was issued for parts of S Turkey mainly for large hail and heavy rain.

SYNOPSIS

Modest phase shift occurs with the dominant wave pattern over Europe as a stout subtropical ridge over the CNTRL Mediterranean connects to the Scandinavian anticyclone and causes a major ridge, which runs from Algeria to W Russia.
In the meantime, an extensive trough evolves between negative height anomalies over the Norwegian Sea and Portugal.
A long lived cut-off over the Iberian Peninsula gets caught in-between those waves and lifts NE while filling. It will cross E-France as an open low-amplitude wave during the overnight hours (into Sunday), where interaction with a very volatile airmass is forecast (EFI CAPE/shear SOT >+2 with similar signals in the integrated vapor transport field). Of interest is the rapid fracturing of this wave in the IPV field as it enters Germany during the night with at least three IPV maxima forecast. This broadens the background forcing and affects a vast area from NW Italy to E France to Germany.

The EZ member plot for MSLP still indicates a substantial spread in any surface response but tries to emphasize the development of a low pressure channel from CNTRL-France into Belgium/W-Germany with a more pronounced vortex just to the E (shifting from S to N-CNTRL Germany). NWP guidance may have some issues with the massive convective blow-up in its guidance, but dynamics also support a growing LL vortex into N-Germany later in the forecast period.

The deep digging cut-off over SW Europe with its well placed intensifying 60-70 kt mid-level speed maximum over NW Algeria induced another round of dust entry into its growing WCB (yesterday and verified in latest RGB remote sensing data), which advects towards CNTRL Europe. Dust load values start impressive in the 2000 mgr/m2 range but weaken/shift east during the afternoon hours. NWP guidance shows a broad WCB cirrus shield with breaks, whereas models with dust input provide a rather thick cirrus blanket over France into the Alps during peak heating with some shortage of the short wave radiation input. However even this solution offers some breaks in the clouds and with more than 70-80% short wave radiation still forecast beyond noon, we expect adequate input for CI - maybe delayed a bit.

Once again, the effect of the dust proves difficult to judge and the nowcast trends from the previous night into the forenoon hours of any dust induced cloudiness has to be added to the nowcast procedure! Hence this outlook has rather substantial lingering uncertainties but we keep an aggressive forecast strategy for now due to the impressive thermodynamics/kinematics and a weakening dust concentration betimes.

DISCUSSION

***A regional outbreak of severe thunderstorms with very large to destructive hail, damaging wind gusts and excessive rain is forecast, including a regionally enhanced strong tornado risk***

... E-France into Germany and NW Switzerland ...

A weakening but still potent low-amplitude wave draws near from the SW during the day. It gets accompanied by a 80kt upper jet with a diffluent and divergent exit region overspreading a very unstable warm sector, although this jet weakens before entering Germany. This wave induces constantly lowering pressure over most of France into W-Germany during the day, which remains rather diffuse and results in a SW-NE aligned broad low pressure channel probably with an embedded surface circulation over E-France. This low merges with another structuring low over SE Germany, which shifts N during the night and both merge to one dominant LL depression over N/NE Germany towards the end of this forecast period.
Embedded in this low pressure channel is a wavy front, which runs from CNTRL France to CNTRL Germany and turns into a structuring and northward moving warm front.

The warm sector airmass features unseasonable moist BL conditions with BL mixing ratios in the 12-15 g/kg range with even higher peaks towards the Upper Rhine, translating to dewpoints in the upper tens to lower twenties. Some places along the northward surging warm front may see a Td increase in excess of 10K within 12-15h. A northward spreading EML with mid-level lapse rates in excess of 7K/km interacts with this warm sector airmass and yields afternoon MUCAPE values in the 2000-3000 J/kg range with local 4000-5000 J/kg peaks possible atop regions with maximed BL moisture (e.g. Upper Rhine). Strong WAA (partially enhanced by Alpine downslope component) pushes 850 hPa temperature in the 20-26 C range, which adds modest capping to most of the German warm sector throughout the day, whereas cap weakens towards CNTRL Germany and France. Also, a drying/mixing BL N of the Alps assist in capping issues.
DLS values during peak heating oscillate around 15-20 m/s with 0-3 km shear values hovering around 10-15 m/s. An increase of the 0-3 km shear vector is forecast from the evening onwards (as the wave approaches).

During the afternoon, a few thunderstorms may erupt along the warm front, as ongoing WAA interacts with the more complex orography and as a vorticity lobe/short-wave crosses the warm front east/northeastwards while CAMs show virtually now cap (if strong diabatic heating verifies!).
Point source forecast soundings indicate 3000 J/kg MUCAPE with 15-20 m/s DLS and enhanced SRH areas in a strong WAA regime along the boundary. A few longer-tracked supercells with very large hail, severe downdraft gusts and excessive rain are forecast in case they ride the boundary to the E (as forecast by the Bunkers motion). Somewhat lowered LCLs with pooled moisture adds a tornado risk to this activity with a strong event possible. This isolated risk extends from Belgium to CNTRL Germany into N Czechia. Although NWP guidance still diverges with this event (e.g. ID2 ensemble with only a few members showing CI), confidence in isolated CI is high enough for adding a level 2.

During the same time (early afternoon hours onwards), the focus then turns to SE into E France and W/NW-Switzerland. A broad belt of 15-20 m/s DLS and similar shear values in the lowest 3km overlap with abundant CAPE, so combined with the incoming forcing from the SW and diurnal heating, scattered severe thunderstorms erupt. Even with dusty cloudiness only temporal diabatic heating should be enough for CI as surface temperatures soar into the upper twenties. Amount of CAPE and incoming forcing should cause scattered to widespread CI.

Any maturing updraft becomes long-lived and has plenty of shear to work with. A mix of long-lived supercells and severe bow echoes is forecast with all kind of severe and extremely severe hazards, including very large to giant hail, excessive rain, severe/damaging swaths of severe wind gusts and a few tornadoes, including a strong one (especially in case of lowered LCLs by previous convection). Widespread CI may cause erratic storm interactions betimes with a fast tendency of growing clusters. Colliding convection may bring a risk of local life-threatening flash flooding issues!

As we move deeper into the evening hours, the focus then shifts from a more discrete storm mode to a clustered/semi-discrete one. A mixture of long-lived tail-end supercells, embedded bow echoes and a growing severe MCS should cause enough (extreme) reports for justifying a focused upgrade to a level 3 from NW-Switzerland into E-France and towards SW/CNTRL-Germany. If we get a severe MCS going while entering SW Germany, a long swath of damaging hail/severe gusts can occur. Highest confidence for such an event is a potential MCS tracking along the French/Swiss border into the Upper Rhine, where most extreme CAPE awaits convection.

Mature convection probably starts to outrun the surface based CAPE plume somewhere over CNTRL Germany, before riding along the sloped frontal zone towards NE Germany during the night. Although the severe risk decreases, hail and heavy rain remain an ongoing hazard. We would not be surprised to see a lingering severe risk into Saxony/NW Czechia as the MCS builds SE into the CAPE plume. All kind of hazards persist until the end of the forecast period with this activity including severe gusts in case we see enough BL modification for surface based activity.

We broadened the level 2 well into Bavaria to account for CI more E into the reservoir of rich CAPE. In case this scenario verifies, the risk turns to very large hail and severe downbursts with more elevated convection and thick MUCAPE profiles above the cap.

Another concern arises from CNTRL France into Belgium and NW-Germany from the afternoon into the overnight hours as the risk spreads NE.
Growing convection moves into the W fringe of any structuring low/ low pressure channel, where the storm relative component (e.g. northerly sfc component pushing Bunkers storm motion near zero) slows down the convection. In case we get one or two more dominant LL vortices, slow moving/training convection may bring major and (on a local scale) potential life threatening flash flood issues in a very moist background environment (PWs in the 40-45 mm range and abundant CAPE). Although extreme issues may occur more on a local scale, a rather broad level 2 was added to highlight this (partially nocturnal) risk, which also depends on the LL MSLP geometry. This risk continues over W-Germany beyond 06Z.

(alleen de voor NL en aangrenzende gebieden relevante tekstdelen overgenomen, klik hier voor de hele forecast, red.)

Hagel(-schade) | Onweer/blikseminslag | Tornado/hoosverschijnsel(en) | Verwachtingen/waarschuwingen | Wateroverlast/(zware) neerslag | Wind/storm (-schade)

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