Storm Forecast

Storm Forecast
Valid: Tue 18 Jun 2024 06:00 to Wed 19 Jun 2024 06:00 UTC
Issued: Mon 17 Jun 2024 12:19
Forecaster: TUSCHY

A level 2 was issued a broad belt from extreme N Spain to France to Luxembourg/Belgium to Germany into W Poland mainly for large to very large hail, severe to damaging gusts, heavy rain and a regionally enhanced tornado threat.

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

A level 1 was issued over NW Russia mainly for heavy rain, hail and a few tornadoes.


As energy dives S along the E side of a blocking ridge at 30 degree W into a deepening trough just W of Portugal, a gradual cut-off process occurs. Framed by positive 500 hPa height anomalies, this cut-off gradually slides into Portugal and a potent SW-erly flow regime into CNTRL Mediterranean backs an healthy subtropical ridge over the Mediterranean (with v wind anomalies at 250 hPa on both sides of the trough exceeding +/- 3 sigma).

Using PV data, the cut-off process won't get finished until 06Z (19th), but a weakeness in this long-wave structure evolves over CNTRL Europe as a more progressive and lower-amplitude wave starts to shift E over Germany.

Along the surface, an extensive and more or less W-E aligned warm front runs from NW France into N-Germany and starts to wave with a diffuse LL depression being forecast to emerge along this boundary. IFS-ENS member show a rather tightly clustered member cloud with a substantial zonal spread due to onging issues where it might evolve. It probably becomes noticeably detached from the environmental channel of lower MSLP somewhere over NE Germany into NW Poland, where NWP guidance in general agrees in a closed LL depression. Intensity spread is marginal with a deepening from 1013-1015 hPa to 1010 hPa.

The warm front features a steep N-S slope over Belenux and Germany with LL frontogenesis enhanced over N-CNTRL parts behind the departing/structuring depression, as weak CAA begins next to the convectively induced tightening of the LL thermal gradient. Mid-level frontogenesis spreads NE over far N Germany into the S Baltic Sea. A broad belt of unstable mid-levels affects that boundary, which is important regarding CI probabilities and heavy rain issues well N of the surface boundary.

Low geopotential heights also cover most of Scandinavia with a daytime driven thunderstorm risk in a weakly capped environment, whereas a progressive low-ampltiude wave enters NW Russia during the day with another round of thunderstorms forecast.


... France into Belgium and Luxembourg ...

The first round of organized convection starts right away from 06Z onwards and lasts well into the evening hours with a focus over NE France into S-Benelux.

An opening LL vortex over the Bay of Biscay and far W France pushes the warm front into N-CNTRL France during the forenoon hours. Peripheral along that boundary, BL mixing ratios around 12 g/kg beneath neutral/slightly unstable mid-level lapse rates create 400-800 J/kg MUCAPE, before diabatic heating pushes this into the 1 kJ/kg SBCAPE magnitude over NE France/Luxembourg around noon.
Effective shear increases as cap erodes with DLS up to 25 m/s and 15-20 m/s of 0-3 km shear into Luxembourg around noon.

NWP guidance shows WAA rain over N France along the sloped boundary with a long-lived tail-end supercell/bowing segment racing E/ENE. ID2-EPS is rather tightly clustered and persistence of this signal in past few runs increases confidence of a long-tracked supercell or bow echo to cross NE France into Luxembourg until the early afternoon. Large hail risk dominantes with an increasing severe wind gust and tornado risk betimes as activity becomes more surface based. LCLs around 500-600 m and lots of LL streamwise vorticity in the inflow region indicate a growing tornado risk between Reims and Nancy into Luxembourg. This risk exits E thereafter. Of note: Some models bring this convection more into Belgium with similar hazards, so final path still somewhat uncertain.

Noon onwards into the evening, far N France (N of Paris-Luxembourg) into Belgium becomes the focus for scattered CI along the structuring LL depression. Enhanced frontogenesis might induce a rapid upscale growth into a broken line of (semi) discrete supercells. Although currently dismissed in many models, a few prefrontal discrete supercells are also possible. Main uncertainy remains when/where LL depression evolves with ongoing spread in guidance. Any more discrete storm poses a risk of large hail with strong to severe gusts forecast. In general there exists a tornado risk with ID2 03Z showing a substantial risk with 20 m/s helical inflow into low-based convection. The activity before/around noon will also dictate that risk, which exits into Germany until midnight. The heavy rainfall risk increases in this area with gradually clustering convection and some training issues.

Finally, another structuring depression evolves over SW France around sunset which then shifts N/NE into CNTRL France until 06Z. This depression once again assists in a tightened moisture gradient, which evolves from SW France into Paris with BL mixing ratios in excess of 12 g/kg. EML atop pushes MLCAPE in excess of 1000 J/kg.
CI will be the eastbound moving cold front over Spain during the daytime hours with organized convection moving rapidly towards and over the far SE Bay of Biscay into SW France. This activity resides beneath abundant upper divergence along the left exit of a 120 kt upper-level jet/80 kt mid-level jet. Hence upscale growing convection into a cluster is possible with (semi) discrete activity along its S end. Final path remains a bit uncertain but all of SW France will be in an augmented risk for severe. If clustering convection occurs, upscale growth into an MCV event is possible, which shifts into W-CNTRL France during the night (increasing shear along its fringes). Large to isolated very large hail, severe wind gusts and heavy rain are all forecast. Lowering LCLs during that time frame (into the night) also enhance the tornado risk.

Further NE (CNTRL France into Luxembourg), evening CI is also forecast but with diffuse background forcing, coverage remains a bit uncertain. Improving LL convergence however should support isolated to scattered storms, which ride NE along the moisture flux zone and can easily grow into long-lived (semi) discrete supercells. DLS remains in excess of 20 m/s, so growing nocturnal BL dictates the final magnitude of the severe risk. Right now, convection should be accompanied by all hazards until midnight, including a strong tornado risk, large hail and severe gusts. Multiple events for the same area may cause an enhanced excessive rainfall and flash flooding risk (probably shifted a bit more to the N of the sfc boundary).
Beyond midnight, the risk for more elevated supercell activity continues until the end of the forecast while shifting into far W-CNTRL Germany. Nowcast dictates the degree of how elevated storms will be. Any surface based activity will see all kind of hazards.

Some WAA thunderstorms occur over far W France beyond midnight with isolated hail/rain issues.

... Germany into W Poland...

Initially anticyclonic mid/upper flow pattern turns more cyclonic during the day and combined with the structuring LL depression, conditions improve rapidly for CI beyond noon.

Especially the shear parameter can't get much better with EFI(CAPE/shear) backing that (SOT up to 1+). MUCAPE improves from 500-1000 J/kg over N-CNTRL Germany to aoa 1.5 kJ/kg over CNTRL parts and even higher (capped and probably not activated) to more than 2 kJ/kg south of the Danube.

The main question will be, how discrete the activity will stay. Weak background forcing is more supportive than current storm motion/boundary orientation. With still mixed NWP signals we try to focus on the following time-frames/regions:

... W-Germany...

will see a decent risk for supercells moving in from France/Luxembourg beyond noon. Some models try to weaken this activity but it is more probable that it just keeps going E/NE. All depends on where the LL depression organizes but current idea is to see more discrete activity from Rhineland Palatinate into Hessen, whereas convection tends to become semi-discrete more to the W (where frontogenesis increases). Anyhow, this activity will see favorable conditions for broad/organized updrafts with 20 m/s+ inflow into lots of streamwise vorticity ingesting updrafts. Right moving cells also move just S of its precipitation cores, so long-lived activity is forecast. Hazards include large hail, severe gusts and heavy to excessive rain (especially along the backside of the depression). The tornado risk is enhanced and with some available forecast soundings/hodographs from CAMs we would not want to rule out a long-tracked and intense tornado event especially where STPs resides in the +1 to +2 region (W-CNTRL to CNTRL Germany). During the night the activity weakens.

The focus then turns to the predawn hours over W-CNTRL Germany, as nocturnal activity from CNTRL France approaches. CAPE/shear space still supports a few supercells as SCP holds steady aoa +1. The main question will be, how stable BL conditions become as not much modification is needed for more surface based activity. For now we keep heavy rain and hail for the most likely hazard and this activity becomes the main focus for the following day (E-CNTRL Germany).

... CNTRL into E-Germany and W-Poland ...

could see numerous long-tracked supercells. CI probably occurs along the structuring warm front/moisture flux convergence zone and along the orography, which assists in more discrete CI over CNTRL/E Germany. The main issue becomes the CAPE magnitude with a rather well mixed BL airmass away from the frontal zone and rather warm mid-levels. Shear induced pressure perturbations certainly can increase final updraft speed (comparable to much more background CAPE) and there is not much in the way for entrainment. Hence this should still be enough for long-lived convection, as MUCAPE increases from Berlin to the Ore Mountains from 500 J/kg to in excess of 1000 J/kg.
Deviating cells ingest lots of streamwise vorticity with long/curved hodographs and large to isolated very large hail is forecast. In addition a better mixed BL airmass assists in fierce downdrafts with severe/dagaming gusts locally in excess of 120 km/h. With 0-3 km shear exceeding 20 m/s, bowing segments with swaths of (hail driven) damaging gusts are also well possible, probably maturing over E Germany into W Poland during the late afternoon into the evening hours. During the night, the bulk of activity shifts E into W-Poland with an overall decrease in intensity betimes.

... S-CNTRL Germany...

was added to the level 2 despite decreasing confidence in CI. This region resides more into the anticyclonic flow regime with less background support. Latest forecast soundings however show a virtually uncapped airmass with SBCAPE in excess of 2 kJ/kg and 20 m/s DLS during the afternoon hours. A weak IPV maximum traverses this area around noon and combined with upslope flow along the orography, a few cells may erupt, which gets supported by ID2 EPS data, showing a few long-tracked updraft helicity tracks. This area will see best top-down weighted CAPE distribution and very large hail in excess of 5 cm is possible. A well mixed BL and 0-3 km shear up to 15 m/s plus PWs up to 35mm could cause growing/intense cold pools which might result in one or two longer swaths of damaging hail/gusts outside of the current foci of deterministic runs. Hence the level 2 was expanded well S for now.

For all of those mentioned areas (France to Poland), a regionally level 3 upgrade was checked, but mainly uncertainties regarding final storm mode and/or coverage and/or CI kept this in an overall high-end level 2 event for now. Swaths of level 3 equal conditions however are possible (e.g. driven by the mesoscale).

... NW Spain and far N-Portugal ...

A level 1 was issued for this area mainly for a mixture of multicells/a few supercells with hail and strong gusts. Rich LL CAPE also adds an isolated tornado risk to this activity, which weakens during the night.

An isolated waterspout event is possible along the W coast of Portugal during the night as the LL vortex' core approaches.

... Scandinavia ...

Scattered to widespread CI occurs in a weakly capped airmass with slow moving thunderstorms, which tend to grow upscale into small clusters. Heavy rain, small hail and a funnel event or two are possible.

... W-Russia ...

An eastbound moving depression interacts with 500-1000 J/kg SBCAPE. DLS oscillates around 10 m/s and an uncapped environment assists in widespread CI. Will see numerous hail and heavy rainfall reports next to a few tornado events with regionally 0-3 km CAPE in excess of 200 J/kg. A level 2 upgrade was considered due to the amount of severe, but low confidence in any placement kept this a high-end level 1 event for now.

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