Hurricane Irene around 11 a.m. Thursday morning (NOAA)

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Hurricane Irene, 645 miles south of Cape Hatteras, is less than two days away from initiating a devastating blow to a large section of the East Coast. While its intensity has held steady since last night, the risk to much of the eastern seaboard has grown larger as computer models have nudged its track westward, closer to the coast, if not slightly inland.

Not only are severe impacts likely for coastal regions from the Outer Banks of North Carolina to New England, but interior sections of the mid-Atlantic, including the Richmond, Washington, D.C., Baltimore, and Philadelphia may also experience major effects from Irene

National Hurricane Center track forecast

[Irene] potentially could be extremely destructive with massive disruptions to society and commerce along its entire track with very high winds/storm surge/ocean overwash/beach erosion/sound and bay side coastal flooding and extreme tide potential. Widespread heavy rains in the 6-10 inch range will be common with greatly increased inland flood potential.

In North Carolina, where hurricane conditions could begin within 48 hours, mandatory evacuations have been ordered for the North Carolina Outer Banks and a hurricane watch extends from Surf City to the Virginia border. A tropical storm watch extends southward to Edisto Beach in South Carolina.

“Pounding” the northwestern Bahamas (two thirds of New Providence is without power) , the storm’s maximum sustained winds are 115 mph, and strengthening may occur today and tomorrow as the Irene sits over very warm water and amidst relatively light wind shear. Although it is expected to weaken some as it approaches the Carolina coast due to increasing wind shear, it could still make landfall along the Outer Banks as a major category 3 or strong category 2 hurricane, with winds over 100 mph.

We (along with the National Hurricane Center) continue to emphasize that there is considerable uncertainty with respect to the storm’s exact track and intensity details which can have major implications on weather conditions and impacts in a given area. What follows below is a general guide of what to generally expect along the eastern seaboard but changes in the track would alter these effects...

The worst conditions should to occur near and just east of where the storm first makes landfall (assuming it does so) - most likely in North Carolina’s Outer Banks. However, due its large size, as the storm rides up the coast, hurricane or strong tropical storm conditions may well also impact coastal and tidewater areas of southeast Virginia, the Delmarva Peninsula (a Phase I emergency plan discouraging visitors and encouraging evacuation preparation has gone into effect in Ocean City, Md), New Jersey and southern New England (including Long Island and New York City). Winds gusts of 50-90 mph, storm surge flooding and six inches or more of rain are possible.

The Weather Channel made the following very important points in a recent article:

* With a population explosion along coastal areas of the Northeast during the past several decades, there is little to no precedence for a hurricane of this potential magnitude making landfall over highly populated metropolitan areas such as New York City.

* Regardless of track and intensity, confidence is growing that Hurricane Irene will cause extensive tree and power line damage. Electricity infrastructure will be greatly compromised for millions if not tens of millions of Americans.

* Recent heavy rains over parts of the Northeast, especially New Jersey, have made tree root systems highly vulnerable. Flooding rains combined with high winds will add to tree destruction.

Irene will weaken some as it moves over land and/or over cooler waters as it heads north, but the storm is very large and its wind field will expand even as peak winds weaken - increasing the geographic area affected.

For interior sections of the mid-Atlantic, including Richmond, Washington, D.C., Baltimore and Philadelphia, conditions are highly dependent on the exact track. However, heavy rain (several inches) and tropical storm force winds are possible especially along and east of I-95. A slight eastward shift in track would mean less severe conditions (more showery weather) and a westward track shift could bring damaging wind gusts, widespread power outages and significant flooding to these major metropolitan areas.

Good resource: AccuWeather summary of what to expect in different areas

Stephanie Abrams of the Weather Channel posted to Twitter the very useful timetable shown below describing when conditions will deteriorate and be the worst at a variety of locations along the East Coast.

Timing of impacts from Hurricane Irene (StephanieAbrams via Twitter )