The storm, named Ianos, hit Lefkada Island on Friday morning, according to the Hellenic National Meteorological Service, and is expected to impact mainland Greece and the Peloponnese peninsula later.
Ianos was traveling with sustained winds of 100 kph (62 mph) just before landfall, making it the equivalent of a strong tropical storm in the Atlantic.
So far there have been no reports of injuries or deaths, but the medicane has caused property damage and some Greek islands have been hit by flooding and power cuts, local media reported.
It is now expected to track southeast, bringing high winds and heavy rainfall. Widespread totals of 150-250mm of rain are possible over the next 48 hours, with some local areas seeing up to 500 mm.
Nikos Miliotis, the Citizen Protection Deputy Governor for the Ionian islands, told Greek media that Kefalonia, Zante and Ithaca are the three islands being hit hard by Ianos right now.
Miliotis said there were power cuts across Ithaca, that trees have fallen and some sailboats have sunk. There are also power cuts in Zante.
The Hellenic National Meteorological Service has issued a weather warning, saying strong winds and storms will impact the Ionian islands, the Peloponnese and Central Greece on Friday. In Athens, storms are expected to strengthen Friday evening into Saturday.
A medicane has features similar to hurricanes and typhoons. Medicanes can form over cooler waters and usually move from west to east, whereas hurricanes move from east to west.
Ianos could end up being one of the strongest medicanes on record.
On satellite imagery the storm is taking on an appearance that resembles a hurricane you'd expect to find in the Caribbean.
Winds will also be a concern, especially if the systems maintains strength, while slowing down, as some of the models suggest. Some models are forecasting the storm to have sustained winds of at least 125 kph (77 mph) with gusts of 180 kph (112 mph). Strong winds for a longer duration will result in more widespread hazards and damage.
The Greek government has issued urged citizens in the affected areas to stay indoors, avoid basements in case of flooding and to secure objects that could fly away.
Climate change to make medicanes worse
According to a , only one or two medicanes occur per year. These powerful storms usually happen during the months of September and October, when sea surface temperatures in the Mediterranean are still quite warm, although they can occur at any time of year.