Storm Forecast
Valid: Tue 30 Jul 2024 06:00 to Wed 31 Jul 2024 06:00 UTC
Issued: Mon 29 Jul 2024 14:25
Forecaster: TUSCHY

A level 1 was issued across parts of N/E France into Belgium/Luxembourg for heavy rainfall. Isolated large hail and a few severe downburst events are also forecast.

SYNOPSIS

Extensive blocking has been established over the European mainland with its axis anchored over the far W Mediterranean/France into SW Germany/NW Italy. Hot and partially humid conditions accompany this strong subtropical anticyclone.

This blocking ridge gets framed by numerous upper troughs. One is the rather impressive but gradually filling trough over far W Russia, which extends down towards the Red Sea. It starts to interact with a more progressive feature SE of Iceland, which crosses the Faroe Island NE but remains just shy S of the Artic circle until 06Z. A resulting transient zonally aligned trough will bring unsettled conditions to the Nordics.

Over the far NE Atlantic another upper trough has no intention to become more progressive as part of a weak/stationary wave train between 30-50 degree N. Its main effort will be to back the blocking ridge over CNTRL Europe and help to advect moisture from the ITCZ and the subtropics into SW and W Europe.

All eyes then turn to another and rather intense (for this time of year) upper trough, which crosses S Greenland during the forecast and apporaches the Labrador Basin/the Reykjanes Ridge during the day as this feature could play some role in the build-up of another heat wave for CNTRL Europe later on.

Frontal wise no major boundary will impact today's thunderstorm probabilities. However a structuring warm front over N-France could become the main player regarding CI.


DISCUSSION

... France to Belgium/Luxembourg into far W Germany ...

A complex scenario for thunderstorm development is forecast in the highlighted area.

Moisture quality of this airmass will be exceptionally good at all levels. A combined channel of mid-level moisture advection from the subtropics but also from the ITCZ during the past few days caused a plume of TPWs of 40-50 mm, which now advects from the Bay of Biscay NE into W/NW and later on also N France and far S-UK. This means PWAT anomalies in the 2-4 sigma range, which is rather unusual.

Along the surface, a diffuse low-pressure area evolves from the Landes Plateau (far SE Bay of Biscay) into CNTRL France (GEFS) with IFS-ENS pushing this channel into N-France. An extensive member spread and blurred MSLP gradient keep this development rather uncertain.

Anyhow, falling pressure pushes a weak warm front gradually N before stalling somewhere over N France. The BL airmass quality along/S of the warm front features impressive moisture values with dewpoint readings in the upper tens to mid twenties! The lower troposphere remains very warm with 850 hPa readings in the upper tens, so cap holds strong until the afternoon.

From then on, conditions improve steadily for CI with peak T2m readings in the lower to mid thirties, dewpoints around 20C and an approaching low-/mid tropospheric wave from the W, which slides E along the warm front.

This moisture combined with a N/NE-ward fanning Spanish/N-African EML plume push MLCAPE in the 2500-4000 J/kg range with EFI CAPE in the SOT 2(and higher) range!

CI probably starts with surface based activity along the best moisture axis. T2m readings gradually exceed the convective temperature and the region with highest CI probabilities most likely extends from the Pays de la Loire region to the Ile-de-France (Nantes to Paris) during mid/late afternoon with some models also indicating CI down to the Auvergne-Rhone-Alpes region. This activity (well timed) should take profit of an approaching low-amplitude wave from the W and further upscale growth into a cluster is forecast as this activity moves into Paris during the evening/overnight hours. These storms will probably be (near) surface based and can take profit of all the DLS (around 10 m/s) before CIN increases rapidly beyond sunset.

Intense pulsating storms bring large hail and with this amount of CAPE even a very large hail event won't be ruled out. In addition, a few severe downburst events and excessive rain are forecast with a few exceptionally high rainfall rates possible.
It remains unclear, if convection grows upscale into a large cluster (more likely) or evolves into a more progressive line but the main focus should shift into NE/E France betimes.

Another focus for CI becomes the sea breeze area along the N coast of France, where mesoscale lift could induce thunderstorms, which then spread E. If convection evolves in this area, weaker CAPE values should limit the severe risk with isolated hail and excessive rain the main issue.

During the night any cluster shifts E and could affect a broad area from Belgium into SW Germany, dependant on where the cold pools push convection, if a MCV evolves from any cluster and also how strong the mid-level wave modifies the mid/upper height geometry. Despite heavy rain this activity should become more and more sub-severe.

Beside the mentioned activity, erratic/pulsating new development is possible along outflow boundaries or along the orography with ID2-EPS showing signals down to the Massif Central, which depends on the strength of the low-amplitude wave (still ongoing NWP discrepancies).
For now the AROME solution will be used as a runaway with an extensive MCS down into CNTRL France. This model is the only one which tracks an outflow boundary from the SE Bay into CNTRL France during the morning and forenoon hours.

We issued very broad lightning and level areas due to higher uncertainties than usual. In addition, if this mid-level wave verifies to be weaker than currently anticipated, CI could be rather short-lived and spotty. This for sure becomes a heavily weighted nowcast event!

... Other lightning areas ...

Numerous lightning areas with sub-severe thunderstorm activity were added, surrounding the blocking ridge (somewhat like a ring-of-fire version in small).