Storm Forecast
Valid: Wed 09 Oct 2024 06:00 to Thu 10 Oct 2024 06:00 UTC
Issued: Tue 08 Oct 2024 21:18
Forecaster: TUSCHY

A level 2 was issued across far S Bay of Biscay into NW Spain and far SW France mainly for severe gusts.

A level 1 surrounds the level 2 for similar hazards including an isolated tornado risk and heavy rain.
That level 1 extends into CNTRL/E-France/W Germany mainly for a severe gust risk. A tornado is not ruled out.
That level 1 also extends into SE France mainly for severe gusts and a few tornadoes.
That level 1 also extends into far NW Italy mainly for heavy rain and a severe gust risk.

A level 1 was issued for the SE coast of the Adriatic Sea mainly for heavy rain.

SYNOPSIS

A deep trough is centered from the Norwegian Sea to the Bay of Biscay with numerous low-amplitude waves rounding its base. This trough gets framed by positive mid-level height anomalies, so not much net motion to the E is forecast for this wave.

Ex-KIRK enters the trough's base from the W but in a more destructive way, e.g. with only a weak upper PV anomaly/short-wave approaching from the W. Hence despite a quite favorable coupled upper jet pattern with increasing divergence, no real connection of the diabatically driven LL PV anomaly with the weak upper PV anomaly is forecast. As Ex-KIRK moves ashore over France, BL friction probably offsets upper venting and results in either a steady state pressure minimum or a gradual filling trend as Ex-KIRK moves towards Germany. This is now well reflected in ensemble data, which decreased more intense/polward displaced members significantly compared to yesterday. Still a healthy warm-seclusion with a graudally eroding asymmetric/shallow warm core reflects Ex-KIRK's center for most of the day with a rather strong isallobaric gradient (> +/- 10hPa/3h) along its S fringe.

A potent/progressive trough exits the Adriatic Sea E during the start of this forecast and leaves behind a rather zonal flow regime. With deep frontal intrusion, best BL moisture remains displaced south of strongest mid-level flow, which keeps the severe risk on the lower side for most of the Mediterranean.

DISCUSSION

... CNTRL Mediterranean ...

A few lingering and still training thunderstorms occur along the SE coast of the Adriatic Sea in response to the departing upper trough. With rapidly warming mid-levels from the W, this convection diminishes beyond noon with lowered CAPE. A few more isolated storms are still possible all day long with spotty heavy rain issues. A marginal level 1 was issued for the forenoon activity and covers the localized heavy rain threat.

... France into N-Spain and W-Germany/far NW Italy ...

The main focus for some organized convection resides along the S fringe of Ex-KIRK, where some dry slotting occurs (emerging from the mentioned rather weak upper PV anomaly, which approaches Ex-KIRK from the W).

The main focus for DMC activity resides along the eastward moving cold front, which affects the far S Bay of Biscay area and NW-Spain beyond noon and far SW France into the evening hours.
The latest idea is that a growing convective line occurs in an impressive background kinematic setup with shear at all levels exceeding 20 m/s. The main concern will be extreme gusts at elevated/mountainous areas, as gusts probably exceed 40 m/s. For lower elevations, any convective segment will be accompanied by severe gusts (in excess of 25-30 m/s). This convective line outruns better MUCAPE betimes but severe gust could occur all the way to the Toulouse area. Combined with LCLs below 500 m, a few tornadoes are also possible with a strong event not ruled out.
A non-thunder level 1 area was expanded far S into CNTRL Spain, as any convective element could support severe gusts.

The picture becomes rather nebulous further downstream towards CNTRL France and W Germany. A weakening dry slot with a still rather healthy warm seclusion features a potent warm front (NE France into NW Germany) and an ill-defined eastbound racing cold front. Some moisture advects from the NW Mediterranean into the warm sector, but very warm mid-levels should keep thunderstorm probablities confined to SE France. Towards CNTRL/E France into W Germany, meager MUCAPE could still support broken lines of deeper (but probably mostly non-electrified) convection. BL lapse rates steepen next to the cold front with still 20 m/s 0-1km shear present, so this activity could still add some enhanced gust potential to the background flow and even a tornado event cannot be ruled out.

We want to emphasize that a somewhat augmented nocturnal tornado risk exists along the departements Franche-Comte, Rhone-Alpes and Paca (SE France). Better prefrontal moisture inflow ahead of the cold front assists in a belt of healthy LL-CAPE with local peaks in excess of 300 J/kg. CAPE barely hits the -10C and hence a few thunderstorms are possible next to broken line segments with enhanced but non thundery convection. Limited hodograph curvature is forecats with intense LL shear, which points to an augmented severe gust risk. However there is also a risk of a nocturnal tornado event with any deviating/tail-end cell. We kept this also in a level 1 but with augmented lightning probabilities.

Thunderstorm probabilities increase also over NW Italy during the end of the forecast with heavy rain and severe gusts possible on a local scale.

... Other lightning areas ...

More regions with more or less augmented thunderstorm activity are forecast but these thunderstorms should stay sub-severe.