You can't prepare for the best case, you have to prepare for the worst case, New York Mayor Michael Bloomberg tweeted Saturday morning, as NYC faces its first ever evacuation order due to the upcoming threat of Hurricane Irene.

All New Yorkers in the low-lying Zone A coastal areas and the Rockaways are required to evacuate by 5 p.m. Saturday as Irene hits the city with hurricane conditions likely including destructive wind, heavy flooding rain, and storm surge with coastal flooding, as Irene is expected to bring the worst conditions later Saturday night and Sunday.

The Category 1 hurricane is likely to pack winds of 55 to 70 miles an hour, Bloomberg said at a briefing Saturday morning at 9:30 a.m.

Now, we've heard that lots of people are leaving, and that's great, and we're joined here by some people who can help us get the message out, said the mayor.

If the storm were to head directly east and get away from us, nothing could make us happier. But you can't prepare for the best case, you have to prepare for the worst case.

Con Edison may preemptively shut off their power lines in the low-lying areas in the city before the storm surges, possibly leaving residents and business without power until Monday. Because if salt water gets on these cables when they are being used, the damage is substantial. There's a lot less damage to cables if salt water gets on them when they don't carry any current, the mayor explained.

Our engineers have to first see how high the water is coming in. Once we figure out about what the flooding levels are at, then we can make a decision on all low-lying areas, said Alfonso Quiroz, a spokesman for Con-Ed . So, it's certainly isn't going to be just in Lower Manhattan.

ConEd will make the final decision in the coming hours, said Bloomberg.

The most important thing is to make sure their facilities aren't damaged, which would take a long time to repair. If salt water gets into the underground cables and those cables are carrying electricity, there is a real chance of damage to those lines, said Bloomberg. There is a lot less chance of damage to the lines if those power cables are not carrying power.

NYCHA buildings will shut down elevator service in all buildings in the evacuation area. The Staten Island ferry will be closed beginning at 10 p.m. Saturday or earlier.

Friday, city officials elected to shut down New York City's sprawling transit system, which encompasses both the subway and an array of commuter rail lines that serve Long Island, Westchester County and Connecticut, starting on Saturday until further notice.

If winds reach 60 miles per hour for a while, bridges including the George Washington Bridge, the Robert F. Kennedy Triboro Bridge, the Throgs Neck Bridge and the Whitestone Bridge will be closed, according to Governor Andrew Cuomo.

According to an Associated Press report, around a thousand National Guard soldiers and airmen are being dispatched Saturday from across New York to Long Island and the New York City areas to help out with evacuations, cleanup and other duties.

The City has increased the police protection throughout the evacuation zone, and readied two teams of firefighters in each of five boroughs to help evacuate homebound elderly and nursing home patients.

So far, around 7,000 patients and residents have been evacuated.

The NYPD is also sending vehicles with sound equipment throughout the area to broadcast the message that this is a dangerous hurricane that is approaching and residents must evacuate to safety. They will be doing that all afternoon into the evening, but keep in mind, if you want to evacuate later on, you're going to be on your own. You'll have to walk, or you're going to have to find some way to use a car or a taxi, added the mayor.

He further asked New Yorkers to stay off streets and sidewalks to prevent injury, and stay away from the windows for those living on or above the tenth floor of a high rise.