Blame it on Bermuda.
That was the center of a high-pressure system that pumped in unseasonable heat and humidity, and set the stage for thunderstorms and showers yesterday that grounded planes at Baltimore-Washington International Airport, disrupted power for thousands of homes, and created slippery roads for drivers across Central Maryland.
Forecasters said the system, known as a "Bermuda High," pushed warm, humid air from the Southwest into North Carolina, bringing humidity from the ocean and making the region's atmosphere conducive to thunderstorms.
The unstable air mass generated sprinklings of hail and pockets of heavy showers that dumped an inch or more of rain in parts of Baltimore, and more than 2 inches in sections of Anne Arundel County.
It also washed away a huge accumulation of tree pollen that had been making allergy sufferers miserable during this week's heat wave. Temperatures reached almost 90 yesterday - slightly cooler than the Tuesday and Wednesday readings in the low 90s.
"You get cooler bay air butting up against the hot air that's over the inland and that's a good place for storms to fire," said Barbara Watson, a meteorologist with the National Weather Service in Sterling, Va.
Bolts of lightning had touched down at BWI by late afternoon, delaying departures for many scheduled flights into the evening. Several incoming flights were placed in holding patterns or diverted.
At the storm's height, between 3:30 p.m. and 4:30 p.m., the thunderstorms knocked out power for about 35,000 customers of Baltimore Gas and Electric Co, said spokesman Clay C. Perry. The utility has about 1.14 million customers.
By 10 p.m., more than 21,000 outages remained with about 13,000 in Anne Arundel County and a little more than 6,000 in Baltimore County. More than 100 BGE workers had been dispatched last night to fix broken transformers and downed power lines that were damaged by the mixture of lightning and wind gusts of up to 60 mph, Perry said.
Traffic signals throughout the metropolitan area were disrupted because of lightning strikes. On Jenifer Road in Timonium, the high winds were blamed for a downed power line that set the asphalt ablaze.
In Anne Arundel, several houses were hit by lightning, and the fire department was swamped with calls.
Standing water on parts of Baltimore-Annapolis Boulevard, Routes 10 and 100, and Edwin Raynor Boulevard caused problems for drivers trying to navigate the slippery roads and intersections where traffic lights were not working, county fire and police reported.
A wall of a recycling business in Hanover collapsed from the pressure of strong winds about 5 p.m., leveling 100 feet of concrete block, said Anne Arundel Fire Division Chief John M. Scholz.
The Arundel Recycling building in the 7500 block of Connelley Drive, in the BWI Commerce Park, remained standing because the 14-foot-high wall was not load-bearing, Scholz said, adding that none of the employees inside was hurt.
There is a chance for more thunderstorms in the afternoon today with continued highs in the 80s to lower 90s.
Sun staff writers Richard Irwin and Laura Barnhardt contributed to this article.