Storm Forecast
Valid: Wed 15 May 2024 06:00 to Thu 16 May 2024 06:00 UTC
Issued: Tue 14 May 2024 20:32
Forecaster: TUSCHY

A level 2 was issued across parts of N-Italy (S Alps) mainly for excessive rain.

A level 2 was issued for parts of NE Italy (Emilia Romagna into SW Veneto) mainly for severe wind gusts, excessive rain, large hail and a tornado threat. No level 2 markers were added due to the small nature of the level areas.

A level 1 surrounds both level 2 areas with excessive rain
the main threat but also for an isolated tornado/hail risk.

A level 1 was issued from SE France into W/SW Germany mainly for heavy to isolated excessive rain, a few hail and strong wind gust events but also for an isolated tornado event.


SYNOPSIS

NAO- regime continues with stout Scandinavian blocking in place during this forecast. Blocking assists in an extensive split-flow regime over Europe with one jet diverted to N-Scandinavia and the other branch plowing S towards CNTRL Europe. As a result from this configuration, two major cut-offs meander S of the anticyclone - one exits the Bay of Biscay to the SE with the second one targeting the Black Sea with a gradual drift to the E. Between both vortices, weak ridging affects parts of S-CNTRL Europe.

Frontal-wise the the main player for this forecast will be a wavey and N-S aligned frontal boundary, which runs from the UK to W-Switzerland and further S into the CNTRL Mediterranean region. A deep and uncontaminated northward push of a moist marine airmass from the Mediterranean well into Benelux/Scotland is forecast and to some extent reflected in latest EFI forecasts (e.g. water vapor flux). SST anomalies over the Mediterranean run 1-2.5 K above climatology for this time of year and will be an additional moisture source next to the gradually cut off advection from the subtropical Atlantic.

Both cut-offs have surface imprints with the western one the more aggressive one (around 990 hPa but filling). This vortex is about to move into the Bay of Biscay and it assists in this frontal boundary next to a broad high pressure area over E/NE Europe, as both features cause converging conditions along this extensive quasi-stationary boundary over CNTRL Europe.


DISCUSSION

... N-Italy ...

The first area of interest will be the N Piedmont, maybe extending into the Vall D'Aosta and the adjacent Swiss regions. An extensive cluster with embedded thunderstorm activity from the previous night is kept alive beneath abundant upper divergence / left exit of an 100 kt upper jet max and with unimpeded BL inflow from the SE (Adriatic Sea / BL mixing ratios in the lower tens g/kg). Already 100 l/qm and more were forecast until the start of our forecast period and NPW guidance has clear signals for another 100 to 250 l/qm in this region due to the static nature of the front to the W, which funnels the moisture right into the orography. There could be a temporal lull in activity during the afternoon, before QPFs ramp up again for the overnight hours. A flash-flood driven level 2 was issued. Tail-end or more discrete storms could bring isolated hail and a tornado with some helicity-rich LL inflow and low LCLs.

During the day, also uplsope areas from the Lombardy to N-Friuli Venezia Giulia will see training convection with excessive rain. Again, NWP guidance indicates 24h rainfall amounts into the 100-200 l/qm range with most of the rainfall forecast in a shorter time-frame. Hence a second level 2 was issued and extended well E to cover this risk. Once again, tail-end storms pose an isolated tornado/low-end hail threat.

A third area of interest evolves over Emilia Romagna, as a rich BL marine layer sneaks beneath neutral/slightly unstable lapse rates atop, offering 800 to 1500 J/kg MUCAPE. CI awaits the noon/afternoon hours as another mid/upper wave rotates through this area of interest. Thunderstorms likely initiate along the N Apennines and then move NE, later E as they decouple from the background flow and become steered by internal (non) linear dynamics. Forecast soundings/hodographs show deep/unstable and moist inflow with lots of streamwise vorticity (including lots of LL streamwiseness). This kind of inflow magnitude fosters broad/long-lived supercells with all kind of hazards, including an augmented (and potential strong) tornado risk, severe wind gusts and large hail. The hail threat could be lowered a bit as no real top-down CAPE distribution is forecast, but mature supercells should still be able to produce large hail - probably in excess of 5 cm on an isolated scale. This activity weakens after sunset while decoupling from the BL with decreasing CAPE but mature storms could take the severe risk all the way to the and atop the N- Adriatic Sea.
Another level 2 area will cover this risk and the level areas were expanded well E to account for an ongoing severe risk atop the Sea and along the coastal areas (where CIN values remain low to absent).

... SE/E France, Switzerland into S/W Germany ...

Placed within the plume of moist/unstable air an active day with DMC activity is forecast. A diffluent and also divergent (Switzerland/SW Germany) upper flow regime assists in scattered to widespread CI. This activity gets framed by the mentioned wavey frontal boundary over E-France and a pronounced LL confluent flow regime/prefrontal convergence line, which runs from W-Bavaria to the NW. CI occurs along the convergence zones but also in-between as convective temperature will be reached.

Kinematic are modest with roughly 10 m/s DLS - a bit lower towards the Alps and higher towards W-Germany/Switzerland. Despite the limiting factor there is some concern for initiating storms to rotate for some time with lots of helical inflow into updrafts. So would not be surprised to see some hail e.g. over far S/SW Germany with initiating activity including an isolated tornado risk with rather low LCLs (SW Germany) and mentioned favorable inflow for deviating storms. However, weak mid to upper level storm relative flow should support rapid transition towards cold pool driven and upscale growing convection with heavy to locally excessive rain the main risk. There is a chance for a longer-lived MCS event from far E France/W-Switzerland to move into SW Germany during the evening but confidence in this is low with ongoing subtle model discrepancies of driving weak short-waves.
This risk supports a broad level 1 area with the main concern for a few flash flood events (e.g. along the orography).

In Switzerland, the S/E may see suppressed activity with thick cloud cover from the Italian storms but we did not downgrade this part as not much BL modification/insolation is needed for an isolated organized thunderstorm.

The level 1 was expanded well into SE France for a few organized thunderstorms with isolated hail and mainly heavy rain but limited diabatic heating and widespread CI should keep the severe risk in check.

... Balearic Island and the Dinaric Alps ...

Both areas could see a few strong to isolated severe thunderstorms, but the general risk seems to stay below a level 1 threshold mainly due to limited and partially capped CAPE.