Météo France has upgraded its storm warning for the Alpes-Maritimes to a red alert - while the Alpes-Maritimes préfecture has ordered all schools and town halls to remain closed on Friday.
Torrential rainfall is forecast between 4am and 10am on Friday. The department is also on an orange alert for strong winds and high waves.
The forecaster has warned of "a particularly intense rain storm episode" overnight.
Locally, up to 25cm of rain could fall within a few hours - in an echo of the deadly floods that hit the Riviera in October 2015.
The City of Nice has opened a crisis centre overnight to monitor the worsening weather conditions.
It has already ordered the closure of all parks, gardens, cemeteries and coastal paths until further notice.
All schools and creches in the Alpes-Maritimes will be closed on Friday. Boarding school pupils are being sent home by bus.
The Alpes Maritimes préfecture is asking people to work from home on Friday wherever possible.
Lignes d'Azur bus services around Nice are not running on Friday morning - but trams are unaffected. In Grasse, public transport operator Sillages has cancelled all of its morning bus services as a precaution.
Train services will continue to run along the coast, with possible delays, but trains into the Roya valley and the Train des Pignes will not run. The Var river is on a flood alert.
In Monaco, schools and creches have also been closed and outdoor activites cancelled. This means the opening of the Monaco funfair on Port Hercule will be postponed until Saturday.
What's happening tonight is a so-called Mediterranean episode - a weather phenomenon in which hot and humid air from the Mediterranean generates violent storms, most commonly in the autumn, that can see a large amount of rain fall in just a few hours.
The storm could see waves up to four metres high and winds gusting up to 110 kilometres per hour.
In the event of violent storms, the official advice is to avoid driving and stay at home, stay away from coastal paths, avoid going into cellars or underground car parks and stay informed with your local mairie.
The City of Nice has told shops and restaurants to close their terraces, secure their outdoor furniture and close any shelters and awnings.
The weather is due to calm down again on Saturday.
The Full English Breakfast show returns on Riviera Radio at 7am with all the latest local news.
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