Tropical storm Arthur threatens Fourth of July on Outer Banks of N.C.

Tropical storm Arthur is moving up the East Coast. It's expected to skim the coast, but could move across North Carolina's Outer Banks July 4 and bring heavy rain to Virginia.

A tropical-storm watch has been posted for a stretch of Florida's east coast, with warnings posted for the waters immediately offshore in anticipation of tropical storm Arthur – the first named storm of the 2014 Atlantic hurricane season.

Herded by the leading edge of a deep-dipping meander in the jet stream, the storm is expected to follow a track that gradually curves northeastward until the center of the storm moves across North Carolina's Cape Hatteras as a Category 1 hurricane around 8 a.m. July 4. From there, the storm is expected to continue to veer to the northeast, skirting Nova Scotia's Cape Breton Island by Sunday.

Of particular concern are the crowds that tend to gather on North Carolina's Outer Banks for the holiday weekend, says Dan Kottlowski, a senior meteorologist at Accuweather, a commercial forecasting service based in State College, Pa.

"I've been out there for the Forth of July; it's like a huge party," says Mr. Kottlowski. "We're hoping the thing stays east of the Outer Banks."

If it does, forecasters may not have to issue a hurricane warning, which would require that everyone but permanent residents to evacuate.

As of 11 a.m. Tuesday, Arthur was centered about 76 miles northeast of West Palm Beach, Fla., with maximum sustained winds of 40 miles an hour. These tropical-storm-force winds extend 45 miles from the storm's center.

Rainfall amounts are notoriously difficult to predict days in advance, especially since storm tracks have increasingly large errors – up to 226 miles to either side of the track for five days out.

But based on the latest track forecast, Arthur is expected to dump the vast majority of its rain at sea, although sections along the southeast coast could see at least one to two inches directly from Arthur as it passes, according to the National Weather Service's Weather Prediction Center in College Park, Md.

From the standpoint of a soggy Fourth of July, of equal interest is the jet stream's dip, or trough, moving in from the west.

In addition to its role in steering storm systems, the jet stream also forms a boundary between cooler air behind it and warmer air ahead of it.

Even though the trough is expected to shove Arthur increasingly away from the coast, the jet stream's winds will be drawing warm moist air up from the Gulf ahead of it. That should add to the warmth and moisture that Arthur will be pushing toward the coast.

The clash between warm air with high moisture levels and the approaching cold front is likely to trigger heavy rain along the Interstate 95 corridor Thursday night and into Friday morning, with rainfall amounts ranging from 1 to 2 inches from Virginia up through New England, with some isolated spots receiving 3 to 4 inches, Kottlowski says.

The tropical depression that morphed into Arthur emerged from a stormy mass of clouds off southeastern Florida that had been building since June 28.

Perhaps the largest danger from Arthur for now is the heavy surf it is expected to generate along the eastern seaboard. With clear weather forecast for July 5 and beyond at many locations, holiday beachgoers are urged to pay attention to heavy surf warnings and to review advice for how to swim free of rip currents if they inadvertently get caught in them.