Greece's National Meteorological Service (EMY) has issued an orange weather warning for a sharp deterioration in conditions from early Tuesday through Wednesday morning, forecasting heavy rain, frequent lightning and localized hail across the northwestern Aegean.
The system will bring the heaviest rain and thunderstorms to Thessaly — particularly coastal Magnesia — from the early morning hours, spreading to the Sporades islands and Halkidiki through the afternoon, and to central and northern Evia by midday.
The storms are expected to persist into early Wednesday over coastal Magnesia before easing.
A cold front sweeping down from northern Europe will drive temperatures down 6 to 8 degrees Celsius by Wednesday and Thursday, as colder air collides with warmer conditions still lingering over southern Europe.
Forecasters describe the shift as a two-stage event: the first system arrives overnight and persists through Wednesday, followed by a brief improvement Thursday and Friday — though temperatures won't return to recent highs — before a second, less predictable low-pressure system moves in from central Europe over the weekend.
Monday will begin with summer-like warmth before unstable clouds build over central and northern mainland Greece and inland Peloponnese by midday, bringing afternoon showers and thunderstorms to mountainous areas.
Conditions will intensify overnight as the main disturbance moves in from the northern Balkans, with Halkidiki, the Sporades, Central Macedonia, Thrace and Thessaly facing the greatest risk.
Northerly winds will also strengthen, reaching 5 to locally 6 Beaufort across the central and southern Aegean, including the Cyclades, Karpathos Sea and Crete, with the western and northern Aegean seeing the sharpest overnight increase.