Thunderstorms, heavy rainfall and flash flood risk forecast across northern Italy, Slovenia and Croatia, ending late-summer heat

Italy’s Department of Civil Protection forecasts widespread precipitation, mainly in the form of showers and thunderstorms, across eastern Piedmont, Lombardy, northeastern Italy, central-western Liguria, northwestern Tuscany and central-western Emilia-Romagna. Rainfall accumulations are forecast to be generally moderate and locally high.

Scattered to widespread showers and thunderstorms are also forecast across the remainder of northern Italy and Tuscany, as well as Umbria and western Marche, with generally light to moderate accumulations.

The bulletin forecasts a locally marked drop in temperatures across northern Italy and locally strong southerly winds over Liguria, Tuscany and northern Lazio. The Ligurian Sea is forecast to become very rough offshore.

ARSO has thunderstorm warnings in effect across Slovenia on September 9, with the threat developing progressively from west to east during the day.

In southwestern Slovenia, the warning is valid from 09:00 to 23:00 local time. Thunderstorms may produce locally intense rainfall, gusty winds and lightning.

The warning begins in northwestern Slovenia at 13:00, central Slovenia at 15:00 and southeastern Slovenia at 16:00, with all four regional warnings continuing until 23:00.

ARSO warns that intense local rainfall could rapidly raise torrential watercourses and produce surface-water flooding. People are advised to avoid exposed locations, including areas beneath trees and power lines, during thunderstorms.

The Croatian Meteorological and Hydrological Service (DHMZ) has issued Yellow thunderstorm warnings for the Zagreb, Karlovac, Gospić and Rijeka regions. Yellow denotes potentially dangerous weather.

Zagreb’s warning is valid from 15:00–23:59 CEST (13:00–21:59 UTC). DHMZ forecasts locally severe thundershowers, wind gusts above 65 km/h (40 mph) and a lightning probability above 50%.

Karlovac is under a Yellow warning from 17:00–23:59 CEST (15:00–21:59 UTC), with locally severe thundershowers possible and a lightning probability above 50%.

Gospić’s warning is valid from 18:00–23:59 CEST (16:00–21:59 UTC). DHMZ forecasts locally severe thundershowers and a lightning probability above 60%.

The Rijeka region is under a Yellow warning from 15:00–23:59 CEST (13:00–21:59 UTC). DHMZ forecasts locally severe thundershowers, possible hail, wind gusts above 65 km/h (40 mph) and a lightning probability above 70%. Interruptions to outdoor activities are possible.

Severe Weather Europe attributed the developing storms to an upper-level trough advancing across central Europe and a secondary low forming over the northern Mediterranean. The associated frontal system is forecast to move from the Alpine region toward the northern Adriatic and western Balkans.

The specialist forecast indicated convective available potential energy (CAPE) exceeding 3 000–4 000 J/kg in parts of the broader affected region. Potential hazards identified in the forecast include large to giant hail, damaging winds, tornadoes, and localized two-day rainfall totals of 150–200 mm (5.9–7.9 inches).

The system will also mark a sharp break from the recent late-summer heat across parts of the region. Italy’s Civil Protection forecasts a locally marked temperature drop across northern Italy, while the advancing trough and frontal system are expected to bring substantially cooler air into the Alpine region, northern Adriatic, and western Balkans behind the storms.

References:

¹ National Meteorological Vigilance Bulletin of September 7, 2026 — Department of Civil Protection — September 7, 2026

² Thunderstorm warning for southwestern Slovenia — Slovenian Environment Agency— issued September 8, 2026

³ A major pattern flip arrives: Severe weather outbreak and sharply colder air to end Europe’s late-summer heat — Severe Weather Europe — September 8, 2026