Hurricane Irene has the skyscrapers of New York firmly in her sights and the Mayor Michael Bloomberg's taking no chances.  The time to leave is right now.

In an unprecedented move the mass transit system's being shut down completely.

A mandatory evacuation ordered for low lying parts of the city - a quarter of a million people are being told to head for higher ground.

"We’ve never done a mandatory evacuation before and we wouldn’t be doing it now if we didn’t think it had the potential to be very serious. The best outcome would be if the storm veers off to the east and doesn’t hit or doesn’t hit us hard.  But we can’t depend on mother nature being so kind and we have to prepare for the worst and hope for the best."

The monster storm comes bearing high winds, a lot of rain and very high tide in New York harbour.

The biggest problem in Manhattan is thought to be the risk of flooding ... as the storm surge from Hurricane Irene forces water down the Hudson and East rivers.  It wouldn't have to be very deep to flood the subway and numerous other tunnels and basements.

Power may be out and there may be some building damage.

That would mean New York might not return to normal business in time for Monday's return to work happened when the subway flooded four years ago.

High winds, flying debris and flooding are also likely to damage middle class homes in the city's bedroom communities like nearby Long Island, Staten island and it's close neighbour the state of New Jersey.

These areas could suffer badly when the bad weather hits.

People are being urged to leave their homes early and not wait 'til the last minute.  

Frank Durand, Shelter Manager told me:

"We want them to go to friends and family first, this is a measure of last resort but we're ready."

New York doesn't have a lot of experience of hurricanes.  People here watch them ravage other parts of the country ... but right now there's a sense among hard-bitten New Yorkers that this one's for real.