Storm Forecast
Valid: Fri 17 Jan 2025 06:00 to Sat 18 Jan 2025 06:00 UTC
Issued: Thu 16 Jan 2025 20:16
Forecaster: TUSCHY

Numerous level 2 areas were issued for excessive rain.

A broad level 1 surrounds the level 2 areas mainly for heavy rain. In addition, a few tornadoes are possible with a strong event not ruled out.

SYNOPSIS

There is not much to say regarding most of the European weather as tranquil/stable conditions persist beneath an extensive and long-lived anticyclone, which is placed over CNTRL Europe into Scandinavia. A ridge extends S from this anticyclone and covers most of SE Europe while connecting with a subtropical ridge over Libya.

The main story becomes an anomalous strong cyclonic vortex over Algeria, which captured a NE ward moving tropical disturbance (emerging from 15N near Senegal/Mauritania) during the past 36h. A resulting spin-up/vertically deepening vortex is forecast to exit NE Algeria before moving towards N Tunisia during the day. Some modifications of the LL vortex can occur due to the frictional effects of the variable orography (coast/land) and we expect the vortex to move back towards extreme NE Algeria during the overnight hours. In the meantime, a pronounced IPV maximum spreads E just south of the vortex, advecting towards Sicily during the day. Cross sections through this broad/deep cyclone reveal numerous weak LL PV anomalies, with at least one probably driven by enhanced surface fluxes as it moves offshore for some time (during its approach to the NE Algeria coast during the night).

DISCUSSION

... Sicily into far S Italy and E Sardinia, NE Algeria ...

The placement of a constantly deepening/structuring vortex (at least until the overnight hours of the 18th January) over far N Tunisia and later over NE Algeria brings a well structured occlusion over Sicily and far S Italy during the day before expanding N towards Sardinia during the afternoon onwards.

Until the afternoon hours:
Advected layered PWs show anomalous moist influx from the deep tropics for the mid/upper levels, wich at least partially feeds into the occluding front/closing warm sector over the area of interest. Beside this moisture transport, we also see effective surface fluxes from the +2K SST anomalies of the S-CNTRL Mediterranean. Forecast soundings within this occlusion point to deep/moist and slightly unstable conditions. Orographic support certainly assists in thundery episodes already during this time frame next to convectively enhanced (but non-thundery) rainfall, which probably dominates (due to overall warm mid-levels). Extreme rainfall amounts with rapdily evolving flash flood issues become likely during the day, mostly confined along the orography.

During the afternoon onwards, a substantial drying occurs with the approaching IPV max from the SW in ongoing intense background lift, so a substantial increase of thunderstorm activity occurs not only over Sicily and far S Italy but also further N towards far SE Sardinia during the afternoon onwards. The main concern with this activity is training activity along the coast/orography in a 30-40 kt southerly LL flow regime. This band of enhanced convection shifts E and weakens during the night. Combined with the rainfall amounts before, this could cause a few serious flash flood issues along but also away from the orography. We expanded the level 2 deep into E Sicily with some westward adjustment of the NWP guidance in the latest cycle.

The same is valid for SE Sardinia with serious flash flooding possible in a similar scenario.

During the day, onshore moving convection towards the level 2 areas poses also a risk for tornadoes with a strong event not ruled out, given ample LL CAPE, strong shear and effective/strong streamwise vorticity in forecast hodographs. Right now the highest tornado risk seems to evolve over far SE into E Sicily and Calabria. An isolated hail event is possible, too as mid-level lape rates steepen during the day.

Another area of concern for excessive rainfall amounts arises over NE Algeria along the W fringe of the vortex, where a well structured back-bent front points normal into the orography. A few serious flash flood events are forecast.