The season's first tropical storm is expected to brush the mid-Atlantic coast just in time for the Fourth of July, but it is a cold front helping steer the system offshore that could deliver storms and heavy downpours before a cooler, drier holiday Friday.
Hot, humid air over the region for the middle of the week is forecast to fuel storm chances through Thursday night. Oppressive levels of moisture in the air prompted Baltimore City officials to declare the summer's first "code red" heat advisory Wednesday. Cooler, drier air pushing through Thursday night could wring several inches of rain from the atmosphere over the region.
Tropical Storm Arthur, potentially at hurricane force by the time it passes off the coast Friday, could meanwhile add more rain to the mix on the Eastern Shore and stir up strong surf and rip currents for holiday weekend beachgoers.
But in the Baltimore area, skies are expected to be mostly clear and temperatures and humidity to drop in time for parades and fireworks shows Friday afternoon and evening. The cooler air should stick around amid sunshine for the rest of the holiday weekend.
With Arthur still developing off the coasts of Florida and Georgia on Tuesday, meteorologists were nonetheless watching the system for signs of spoiling the busy travel and tourism weekend.
"There's still some potential for it to move farther east or farther west or not strengthen as much," said Dan Proch, a meteorologist with the National Weather Service in Wakefield, Va., which covers forecasting for the Delmarva peninsula.
Arthur got the hurricane season off to its slowest start in a decade when it reached tropical storm status Tuesday morning, with 40 mph sustained winds about 100 miles southeast of Cape Canaveral, Fla. The storm was slowly shifting and strengthening, with winds up to 50 mph by Tuesday evening, before models predicted it would turn northeast and speed up to pass North Carolina's Outer Banks by late Thursday, according to the National Hurricane Center.
While hurricane center forecasters project it could reach hurricane strength Thursday, with maximum winds of at least 74 mph, meteorologists said the current track projections are far enough out to sea that it could mean winds of closer to 40 mph along with a few inches of rain in Ocean City.
However, the Outer Banks were bracing for possible tropical storm or hurricane warnings.
"With the storm meandering off the Florida coast here, putting significant information this far out is a little difficult," Proch said.
In the meantime, the season's strongest heat and humidity so far surged into the northeastern United States, threatening to bring rain and storms before any possible impact from Arthur.
Air temperatures are forecast to reach the mid-90s Wednesday afternoon, and with oppressive humidity levels not yet seen this summer, it is expected to feel as hot as 103 degrees, according to the National Weather Service's Sterling, Va., office, which covers the Baltimore region. Dew points could reach the mid-70s, an air moisture measurement seen on only the muggiest days of summer.
Officials will offer water and air conditioning at six senior centers around the city. Interim city health Commissioner Dr. Jacquelyn Duval-Harvey urged residents to stay hydrated and to check on elderly and vulnerable friends, family and neighbors. Heat has been a factor in two deaths in Maryland so far this summer.
Storms are possible Wednesday and more likely on Thursday, with the potential to bring at least 2 inches of rain and possible flash flooding.
"What we have right now is a summertime pattern where you have lots of warm, humid air moving up from the Gulf Coast," said Howard Silverman, a forecaster at the weather service's Sterling office. "Whenever you have that much heat and that much humidity, things can be a little bit unstable."
Even without the influence and moisture of Tropical Storm Arthur, the conditions are ripe for heavy rain and thunderstorms. But the tropical cyclone could exacerbate the situation, in part because it could help stall the oncoming cold front and slow down any storms it produces, Silverman said.
By Friday night and through the weekend, though, humidity is expected to plummet and skies to clear. Weekend highs are forecast in the 80s.