NASA is helping out with tracking Earl's behavior by sending both its remotely operated Global Hawk aircraft and its DC-8 "flying laboratory" into and over the hurricane on Thursday.

Controlled by pilots at NASA's Dryden center in Edwards, Calif., Global Hawk was finishing up a 24-hour flight Thursday during which it took pictures of Earl from 60,000 feet. It flew repeated patterns over the storm while its suite of instruments took measurements.

"It turned out that Earl is a well behaved storm, with cloud tops generally well below flight altitude," said NASA's Bjorn Lambrigsten. "As a result, we have been able to make multiple passes straight across the eye, with several bulls-eyes."

"This has been a very good sortie so far, and the pilots are gaining experience and confidence with flying over a storm like this," he added.

The DC-8 crew went up for a second straight day, flying directly over the eye of the hurricane several times.

The NASA flights are in addition to flights by four aircraft operated by the National Oceanic and Atmospheric Administration and the National Science Foundation. All three groups are partners in a six-week campaign dubbed GRIP -- for Genesis and Rapid Intensification Processes.

The U.S. Census Bureau on Thursday issued some updated data on coastal communities that could be impacted by Earl.

The big number: "26 million people in 68 counties, stretching from North Carolina to Maine, could face hurricane and tropical storm conditions within the next 48 hours."

Another interesting stat: 83 percent of housing units in Massachusetts' Barnstable County, where Cape Cod is located, were built before 1990. That could become important if Earl, as expected, spins close to the Cape Cod area. Older homes tend to be less resilient in storm conditions.

NBC's Washington affiliate, WRC, has set up a chat about Hurricane Earl with two of its forecasters.

Running from 4 p.m.-6:30 p.m ET today, the chat is happening at www.nbcwashington.com. Head over early to post questions in advance.

If Hurricane Earl stays on its projected track, the biggest problems will be from winds and from what meteorologists warn are rad-sounding but seriously deadly threats: rip currents and storm surges.

Along the East Coast, meteorologists add lifeguards are urging beachgoers to stay out of the water. If you choose to ignore them, they say, you're on your own if you go in above your knees.

In a hurricane advisory, the National Weather Service warns that "rip currents can become life threatening to anyone who enters the surf," adding that it's "highly recommended that if traveling to the beach, you stay out of the water entirely."

So what exactly are storm surges and rip currents?

When a hurricane skitters over the ocean, its winds pile water up higher than sea level. Eventually, this pile of water tumbles ashore a big, monstrous rush. That's a storm surge, and it can cause serious flooding and beach erosion.

The National Weather Service says a rip current, by contrast, defined by speed, not size.

They're caused when waves traveling from deep to shallow water break near the shoreline. That causes the water to swirl in a narrow, fast-moving "circulation cell."

Rip currents typically move at 1 to 2 feet per second, but speeds as high as 8 feet per second have been measured. "This is faster than an Olympic swimmer can sprint," the weather service says.

But both rip currents and storm surges are major hazards.

"A lot of people have no prior knowledge about how to survive in the ocean, so when you throw in a storm like this, it just becomes harder," Noah Rosenthal, a lifeguard supervisor for Lack's Beach Service, tells NBC station WMBF of Myrtle Beach, S.C., where waves as high as 9 feet are expected.

Although Earl has weakened to a Category 3 storm, it's still "a large and powerful hurricane," the National Hurricane Center warns.

At 2 p.m. ET, Earl was about 245 miles south of Cape Hatteras, N.C., and 720 miles south-southwest of Nantucket, Mass. It's moving north at 18 mph and packing 125-mph maximum sustained winds.

Hurricane warnings are in effect for:

• Bogue Inlet, N.C., northeastward to the North Carolina-Virginia border, including the Pamlico and Albemarle sounds.
• Westport, Mass., eastward around Cape Cod to Hull, including Martha's Vineyard and Nantucket Island.

Hurricane watches are in effect for:

• North of the North Carolina-Virginia border to Cape Henlopen, Del.
• Nova Scotia in Canada from Medway Harbour to Digby.

Tropical storm warnings are in effect for:

• Cape Fear to west of Bogue Inlet, N.C.
• North of the North Carolina-Virginia border to Sandy Hook, N.J., including Delaware Bay south of Slaughter Beach and the Chesapeake Bay south of New Point Comfort.
• The eastern part of Long Island, N.Y., from Fire Island Inlet to Port Jefferson Harbor.
• New Haven, Conn., to west of Westport, Mass., including Block Island.

Tropical storm watches are in effect for:

• North of Hull, Mass., to Eastport, Maine.
• The coast of Long Island west of Fire Island Inlet and Port Jefferson Harbor.
• Nova Scotia from Ecum Secum to Medway Harbour and from Digby to Fort Lawrence.
• New Brunswick from just west of Fort Lawrence westward to the U.S.-Canada border.

White House press secretary took a moment during his daily news briefing, which was otherwise dominated by news of the oil rig explosion in the Gulf of Mexico and the Middle East peace negotiations, to mention that President Barack Obama is "closely monitoring and aggressively preparing for the storm."

The president got an update on Earl from Adm. Craig Fugate, head of the Federal Emergency Management Administration, last night and is expected to get another update later today, Gibbs said.

The Weather Channel passes along its latest Earl forecast:

Thank goodness this "monster" will pass just east of the U.S. coast.

There should be some power outages and even scattered wind damage over parts of eastern North Carolina and the Del-Mar-VA peninsula. Earl could easily hold on as a cat 2 storm by the time it makes it to New England by late Friday. Power outages and some wind damage likely over parts of southeastern Massachusetts and the Islands. Earl should pass just southeast of Nantucket Island, but any deviation slightly westward could make conditions much worse in these areas.

New York City should be on the outer fringe of the storm with some rain and occasional wind gusts Friday afternoon and evening. It could be a little worse around Boston Friday evening with bands of heavy rain and 20-30 mph winds (with higher gusts) possible. Atlantic Canada will have to deal with Earl this weekend!

Bulletin from Reuters:

Earl weakens to category three hurricane as it approaches North Carolina coast - National Hurricane Center

Say "hurricane" and "Gulf Coast" is more likely to come to mind than the "East Coast." Earl has obviously altered that way of thinking for now, but what's not as obvious are the distinct differences between the two regions when it comes to storm impacts. We're talking differences in shoreline topography, population, infrastructure and even storm speeds.

Frank Lepore, a spokesman for the Federal Emergency Management Agency and prior to that a National Hurricane Center spokesman, has seen it from both ends: the forecasting and then the impacts. And it's very clear in his mind how distinct the coasts are.

• Topography
"The Gulf of Mexico generally has a very long sloping bottom," Lepore notes. And that makes for a stronger surge when a storm pushes water ashore. During Katrina, he adds, parts of Mississippi saw a 20-foot surge of water.

The East Coast, on the other hand, generally is steeper, making it harder to push water onto land. Still, that doesn't mean a small surge can't do damage. North Carolina's Outer Banks, the area most vulnerable to Earl right now, regularly see 3- to 5-foot surges from small storm systems that overtop parts of the state highway running through the barrier islands.

• Population
The Gulf's population is around 15 million. Compare that to the Mid-Atlantic states at 57 million, and New England at 14 million.

Apart from the logistics of dealing with millions more people, there's the differences in "collective memory" among Gulf and East Coast populations, Lepore says. Gulf residents, more accustomed to hurricanes, "tend to remember" the danger when evacuations are ordered, he adds. "New England doesn't have that historical component." Indeed, the last hurricane was Bob back in 1991, when 18 people were killed.

• Infrastructure
Hundreds of oil rigs sit in the Gulf, and any threat triggers a well-practiced routine of production closures and crew evacuations, Lepore says.

The East Coast doesn't have that problem, but it does have lots of trees — think downed power lines and blocked roads. So even if most of the East Coast sees just tropical storm-force winds from Earl, Lepore says, "it doesn't take much to gum up the works."

• Storm speeds
Storms move faster at higher latitudes, Lepore notes, so that means the East Coast sees systems move through much faster than in the Gulf.

An East Coast storm typically "does not linger" because it's often racing along at 35 mph at northern latitudes, Lepore says. "It's not dropping a lot of rain" the farther north a storm goes and the faster it moves. Often it doesn't even hit land, instead moving along the coast offshore.

Along the Gulf, on the other hand, "it's gonna make landfall."

Weather Channel meteorologist Bill Karins says the latest projections show Earl coming closest to Cape Hatteras, N.C., and the Virginia coastal cities of Norfolk and Virginia Beach about 3 a.m. ET. Earl's a sprinter, though, so those projections will likely change several times today.

From there, Earl will track northeastward parallel with the Eastern Seaboard. Projections have it closest to Long Island, N.Y., and Nantucket Island, Mass., sometime around 6 p.m. ET tomorrow.

Even though the best guess right now is that the hurricane itself won't come ashore — meaning there won't be a lot of rain — Karins says Earl's weaker backside will still pack a powerful punch, with winds near 100 mph.

"There will be damage," he warns.

For the Northeast, Earl is the biggest hurricane threat since Bob blew through with 105-mph winds 19 years ago. So even though the weather right now is calm ahead of Earl's possible arrival this week, New Englanders are hard at work getting their emergency plans in place. Katie Davis of NBC station WJAR-TV in Providence, R.I., reports from New Bedford, Mass.:

N.C. Highway 12 runs along the Outer Banks, providing the key access route back to the mainland. But early Thursday, the state Higway Patrol closed a stretch just north of Oregon Inlet to all southbound traffic heading toward Cape Hatteras. Northbound traffic is still coming out, NBC's Terry Pickard reports, but that closure means no one can get back into Cape Hatteras for now.