President Obama: 'This is going to be touch and go'

Debris and sand littering a road as Hurricane Irene came ashore near Atlantic Beach, North Carolina, 27 August 2011. Photograph: Reuters/Steve Nesius

6.37pm ET: The mayor of Philadelphia, Michael Nutter, has declared the city's first state of emergency in 25 years. He warns of extensive and long-lasting blackouts:

Due to severity of the storm, you may experience power outages of a week, 10 days or two weeks.

Meanwhile, New York City warns that ferry services will soon be ending. The Staten Island ferry already moved to an hourly service and will stop at 10pm, if not sooner. The subway system and other mass transit have been closed for some hours now.

The Washington Post's Capital Weather Gang (@capitalweather) says that DC can expect the worst of the storm-lashing between 8pm and 2am tonight.

6.23pm ET: Youjung Byun writes in with this photo as witness to the manic emergency supplies shopping that was going on earlier on Saturday in New York. Youjung captions his picture thus:

I took a photo of people stacking up foods and water from Key Food Supermarket on Grand Street in Williamsburg, Brooklyn. People in the supermarket were obviously nervous and anxious, most of them filled their shopping cart up full. By the time when I went, the water was sold out.

One guy was using his traveller's suit case: it was filled with canned foods, water and crackers.

6.07pm ET: Further from Suzanne Goldenberg, who has been listening to the 5pm podcast from the National Weather Service's hurricane centre:

The biggest threat, she says, comes from storm surges, with winds piling the water towards the shore line in the Norfolk area, Delaware Bay, the Jersey shore, New York Harbour and Long Island Sound at particular risk.

Note: we have a new moon and extra high tides in any case. High tide is at 8am tomorrow morning for New York City, which pretty much coincides with the arrival of Irene.

Suzanne summarises further:

The update also notes that while Irene might be shifting a little to the east, and out over the ocean, that is unlikely to bring relief. The storm will produce between 6-12 inches of rain across a 150-mile swathe to its west. Some areas can expect up to 20 inches of rain.

The other danger, highlighted in the podcast, is downed trees, with strong winds bringing down trees and causing minor structural damage to buildings.

On cue, here is Patrick Wintour, the Guardian's British political editor, who seems to have chosen a possibly inopportune time to be vacationing in the Hamptons:

We are expecting 70 mile per hour winds over the next 24 hours and power blackouts here in Long Island south seashore. Cars are parked from trees, water in the bathtub, flashlights at the ready and life vests for the young ones. Playing stormy music, ping pong and Lego, and watching the hour but not the weather channel. Roar of the ocean can be heard from kitchen garden and is growing louder. The winds and rain get serious by midnight.

I think we see some British sangfroid there. Keep calm and carry on, Patrick.

Rikers Island prison, home to 12,000 inmates. Photograph: Seth Wenig/AP Photo

5.52pm ET: My colleague Paul Harris draws my attention to this piece from Mother Jones about the status of the Rikers Island prison, which lies in the waters between Queens and the Bronx. (Former IMF chief Dominique Strauss-Kahn, of course, was recently a guest.) The predicament of the prison's 12,000 inmates and guards has a new pertinency as Mayor Bloomberg delivered a tough message about the compulsory evacuation of 250,000 people from Zone A vulnerable areas. Mojo notes:

According to the New York City Department of Correction's website, more than three-quarters of Rikers Island's 400 acres are built on landfill – which is generally thought to be more vulnerable to natural disasters. Its 10 jails have a capacity of close to 17,000 inmates, and normally house at least 12,000, including juveniles and large numbers of prisoners with mental illness – not to mention pretrial detainees who have yet to be convicted of any crime. There are also hundreds of corrections officers at work on the island.

We were not able to reach anyone at the DOC for comment, but the New York Times's City Room blog reported: "According to the city's Department of Correction, no hypothetical evacuation plan for the roughly 12,000 inmates that the facility may house on a given day even exists. Contingencies do exist for smaller-scale relocations from one facility to another."

5.31pm ET: The figure for the number of people without power along the United States' eastern seaboard keeps rising. Suzanne Goldenbergupdates us:

About 1 million people are now without power because of Hurricane Irene – and the storm hasn't even reached the major population centres yet. As the storm moved north, power outages were reported in North Carolina, Virginia, Maryland, Delaware and Washington, DC.

The biggest outages so far are in North Carolina, where more than 500,000 households were without power by afternoon. South-eastern Virginia is also experiencing blackouts. The power company Dominion Resources said about a third of its 212,000 customers in south-eastern Virginia were without power by 4pm.

The heavy rain also took down power lines around the city of Richmond, where about 50,000 people were without power, as well as in the suburbs of Washington, DC.

Mayor Bloomberg updates New Yorkers on City's response to Hurricane Irene. Video: YouTube

5.14pm ET: Michael Bloomberg's remarks about using the law to move people out of the Zone A evacuation areas are the most telling element of this afternoon's press conference. Learning the lessons of Katrina, the New York mayor is not pussyfooting around with the mandatory evacuation of low-lying areas: the message is that if people do not go of their own accord, the police will be force them to leave.

This is a storm where if you're in the wrong place at the wrong time, it can be fatal … the thing that makes the most sense is to first comply with the mandatory evacuation.

Outside the mayor's jurisdiction, on Long Island, county and town officials are also enforcing the compulsory evacuation of some 400,000 people,reports the New York Times.

President Obama with Homeland Security Secretary Janet Napolitano at Fema headquarters. Photograph: Getty Images/Ron Sachs

4.33pm ET: The Guardian's US environment correspondent,Suzanne Goldenberg (@suzyji), has filed this report from Washington, DC about the situation facing the state of Vermont. Vermont, which is at least 200 miles north of New York City, is not normally troubled by tropical storms, but has already seen an unseasonably large amount of rain this month.

Vermont was expected to be well clear of Irene, but White House reporters who visited Fema's headquarters in Washington with Barack Obama earlier today have been told the state is at risk of severe flooding.

Vermont's governor, Peter Shumlin, declared a state of emergency on Saturday. According to the pool report, quoting an unidentified Vermont briefer:

"The weather service is saying that every river in the state will probably flood over the next two days, so we are preparing for that. Our quick water rescue teams pre-deployed. They will be in place by noon tomorrow. And the National Guard is also standing by ready to support us.''

State officials in Vermont are also warning people to be "self-reliant", and brace themselves for the possibility that they will not have power for two or three days.

The eye-catching detail there is "every river in the state" likely flooding.

3.56pm ET: What is conspicuously apparent is that Mayor Michael Bloomberg is making sure that he's out in front of the media and fully engaged. This new tweet (@MikeBloomberg) is typical of his messaging in this weather-related crisis – in marked contrast, as the New York Times pointed out, to his somewhat mysterious absence during last Christmas's snowstorm, for which he took a lot of flak:

I want to assure our city that we will get through this. We are New Yorkers, and New Yorkers have always risen to the challenge #Irene

And so here is also answering questions at this afternoon's press conference, talking about issues of flooding risk, power cuts and telecoms outtages, and also addressing the status of evacuation plans. So far, he says, there are only 1,400 people using the evacuation centres (for locations, see this pdf map):

We've prepared for a large number of people; we're not worrying about running out of capacity. We have room for 70,000 people, with cots and beds for 40,000 … we don't think it'll come remotely close.

He also reports that compliance with the compulsory evacuation of low-lying areas like the Rockaways, on the south shore of Long Island, has generally been good – 80% of inhabitants there have left. But he warns that if people refuse to leave, they will feel the force of the law:

We are going to break down doors if we have to.

The relatively low number of people so far turning up at the evacuation centres tallies with reports from my colleague Karen McVeigh (follow her at @karenmcveigh1), who has been talking to evacuees in Brooklyn:

At one evacuation centre in Brooklyn, close to the waterside neighbourhood of Red Hook, staff said that only "about 100" people had turned up so far. Niyelle Manley, 24, who arrived at the centre with her two-month-old baby Kallyah, and wheelchair-bound mother, Patricia, said that out of the 20 families in her block, seven or eight had stayed behind.

"We warned a lot of them but I guess they took it as a joke," said Niyelle.

The Manleys live in the projects, on the first floor of a block near the pier in Red Hook and left home after a volunteer arranged transport for them. "I ain't happy about it but something has to be done on account of my grand-daughter," said Patricia, 52. They were waiting outside John Jay High School in Park Slope for another vehicle to take them to another shelter better equipped for wheelchair users.

Meanwhile, New Jersey Governor Chris Christie (see Matt Wells's account if his press conference below) has tweeted the following advice