30-07-2024 23:03 | Estofex | m.b.t. 31-07-2024 t/m 01-08-2024
Storm Forecast
Valid: Wed 31 Jul 2024 06:00 to Thu 01 Aug 2024 06:00 UTC
Issued: Tue 30 Jul 2024 23:03
Forecaster: TUSCHY
A level 2 was issued for N/E France into SW Germany, Switzerland and W Austria mainly for large to isolated very large hail (western part) and excessive rain.
A broad level 1 encompasses the level 2 area mainly for heavy to excessive rain. In addition a few large hail events are forecast next to regional swaths of severe gusts (SW Germany)
SYNOPSIS
The persistent and blocking subtropical anticyclone still resides over SW into CNTRL Europe with the highest 500 hPa height anomalies affecting SE Spain to the Balearic Islands and NW Africa.
In the meantime, ridging persists over CNTRL Europe but weakens during the forecast period as numerous weak impulses traverse this ridge east.
As we move into August this ridge gets framed by an extensive trough over Russia and a vertically stacked but very imperssive (for this time of year) upper low over the Irminger Basin (500 hPa height anomaly of -4 sigma and MSLP anomaly in the -6 sigma range). Finally the stationary wave train with a deep trough just W of the Iberian Peninsula persists and continues to back the subtropical ridge over SW Europe at least for this forecast period.
A wavy frontal boundary affects Sweden but has no impact on today's DMC activity, which is mainly driven from the BL, orography and from the mentioned passing low-amplitude waves.
NWP guidance still has substantial discrepancies regarding the structure of the first wave, which should be analyzed over CNTRL/E France around noon before reaching Switzerland/SW Germany as a flattened/broadened wave during the evening. Cross sections through this wave indicate weak thermal advection and lift becomes rather nebulous as this wave breaks up into smaller impulses (which could be an artefact due to ongoing NWP convection). Combining all that we should see numerous eastward travelling weak impulses with variable Q vector signals and not much change in the thermal stratification.
Another wave crosses France eastward during the overnight hours.
DISCUSSION
... France to Germany, Belgium, Luxembourg, Switzerland into N Italy ...
The plume of very moist air with TPWs in the 40-50 mm range (TPW anomalies of 150-200% to the climatology) and high BL moisture (Tds in the lower twenties with BL mixing ratios in the 14-16 g/kg range) spread E into Belgium/Luxembourg and W/SW Germany/Switzerland.
Not much convective modification occurred, so expect still rather healthy mid-level lapse rates of the Spanish/North African EML plume to overspread that BL moisture. The result will be MUCAPE values in the 1500-3000 J/kg range with higher peaks likely (e.g. yesterday's 12Z sounding from Trappes with 3500 J/kg MUCAPE). DLS remains weak (around 10 m/s), so convection should be pulsating.
Regarding timing and regional coverage of DMC activity there are still a lot of uncertainties left (e.g. from the previous night).
However some more coherent signals exist that a slowly E/NE ward moving pulsating cluster affects Belgium/Luxembourg during the morning as it rides along a diffuse/wavy warm front boundary. This convection probably builds an MCV, which reveals its structure during the convective minimum of the diurnal cycle. If this MCV verifies, this vortex could be the main focus for new development around noon/into the afternoon hours along its S/SE quadrant with convection pushing into W Germany later on, before weakening as it outruns the CAPE plume. Heavy rain will be the main hazard.
Otherwise the region just downstream of the mid-level wave and S of the MCV cloudiness will see a rapid uptick in CI during the afternoon, which would bring E-France / SW Germany into the focus. Intense pulsating convection grows upcale into numerous eastward moving clusters. Also, the orography of the Swiss Alps acts as focus for scattered/widespread CI, which could grow upscale into an extensive cluster. Ensemble data/simluated IR/WV data show a realistic chance for a cluster covering all of E-Switzerland/ W-Austria and SW-Germany during the evening hours into the overnight hours.
A manageable BL moisture depth over SW Germany/Switzerland is prone to diurnal mixing with 2m dewpoints dropping into the mid tens, yielding deep/well mixed low-tropospheric conditions (inverted-V profiles). Therefore there will be a risk for severe downbursts as thunderstorms move in from the W or develop on site. Severe gusts are also possible as upscale growing convection develops extensive cold pools. This scenario is covered in the ID2-EPS with a broad swath of Bft 8/9 gusts over SW into S-Germany and weaker signals for Bft 10/11 gusts.
Initiating convection also poses a large hail risk as top-down weighted CAPE profiles offset weak shear and even a very large hail event (in excess of 5 cm) is possible, especially in E France (Grand Est to Bourgogne-Franche-Comté region).
The main story however becomes the rain as very high rainfall rates are forecast with DMC activity. Flash flooding on the local scale is possible and as DMC interacts with the orography even a significant flash flood event is not ruled out with more than 40% probabilities for more than 40 l/qm/h along the cluster's path (W-Alps).
In addition, expected amount of latent heat release should induce another MCV feature somewhere over S/CNTRL Germany. Along its periphery, training convection becomes an issue as convective elements slow down due to modified SR winds. Latest data indicates the highest risk over CNTRL Germany but this remains uncertain. Nonetheless we expanded the level 1 well into E Germany to cover this option.
The cluster should weaken during the night as it outruns the instability axis but we could see an impact as far E as far SW Czechia/CNTRL Austria.
We upgraded parts of yesterdays SWODY 2 to a level 2 as confidence increased in a developing corridor with enhanced severe including a few extreme events (rain/hail).
During the night, the attention turns to the next wave and if CI occurs, an untouched airmass with MUCAPE in the 2000-3000 J/kg range interacts with regionally enhanced DLS along the wave's periphery (ID2 up to 15-20 m/s DLS). This would support another extensive cluster to impact most of N/NE France during the night. Large hail and flash flood producing excessive rain will be the main hazard next to strong/ isolated severe gusts. NWP guidance however diverges substantially with this activity.
Despite ongoing uncertainties in NWP guidance, we think that enough BL recovery (behind the nocturnal cluster and forenoon activity) exists for a high CAPE setup as the wave approaches. ID2 also constantly showed nice signals for active DMC initiation. The same for the ensemble data but also for models, which had the best handling with last nights convection over France. Hence a level 2 was added also for this area.
As a side-note: MCV driven shear can increase on a regional scale along its periphery, so we cannot rule out a few better organized storms next to the MCV track. Also, any developing cluster should produce an impressive amount of lightning with forecast CAPE/moisture distribution.
Onweer/blikseminslag | Verwachtingen/waarschuwingen | Wateroverlast/(zware) neerslag
