The pitiless weather pattern that has dunked the Midwest under a muddy lake and blistered the Southeast and mid-Atlantic with heat -- baking Baltimore in a record 104 degrees yesterday -- is finally breaking up, forecasters say.
Beginning as early as tonight, more civilized temperatures and lower humidity are expected to prevail for at least a week. A sip or two of imported Midwestern rain is also anticipated, helping break a recent dry spell here.
"The Midwest looks like it's starting to dry out," said Russell Martin, a meteorologist with the National Weather Service's Climate Analysis Center in Camp Springs. "And those rain and storm systems look like they're going to be pushing our way."
Communities along the upper Mississippi River basin, where flood waters began drowning fields, farms and homes almost two months ago, face a week or more of drier weather. But flooding will continue as soggy fields drain and rain-swollen streams empty.
The Southeast and mid-Atlantic in general and Baltimore in particular are expected to cool off, beginning as early as tonight.
"It ought to break the drought," Mr. Martin said. "Certainly, we shouldn't have too much in the way of heat waves."
This shift is due to the breakup of the two major weather systems that have dominated the Southeast and Midwest since June.
An unusually tenacious low-pressure system has hovered over the Pacific Northwest and northern Rocky Mountain region, Mr. Martin said, delivering rain to the Midwestern states along the system's eastern edge.
"We usually don't find [low pressure] troughs inland in the summer like they were this summer over the Rockies and Pacific Northwest," Mr. Martin said. "They usually tend to hang off the West Coast. To have them come inland so far is unusual. To have them in for so long a period of time was very unusual."
What was not unusual was a high pressure ridge that stalled over much of the Southeast for long stretches this month, bringing the region periods of withering, but fairly typical, heat and humidity.
The Southeast's high pressure ridge contributed to the Midwest's misery, Mr. Martin said, by pumping warm, moist air into the region, where it fell as redundant rain.
A high-pressure ridge is moving into the Pacific Northwest, he said. That will bring cooler, drier air trickling down from Western Canada to the Midwest and East Coast tonight or tomorrow.
After temperatures in the mid-90s today, the highs tomorrow are expected to reach only into the mid-80s. The lows tonight are expected to be in the blissful mid-70s.
"It may be the last gasp of hot weather," said Dick Diener, a National Weather Service forecaster at Baltimore-Washington Airport.
Yesterday was the 10th time since July 4 that Baltimore has baked in temperatures of 100 degrees or more. The high at BWI Airport was 97 degrees -- two degrees shy of a record set in 1952.
High temperature records for the date were set in Augusta, Ga., where a reading of 103 broke a 44-year-old record by 2 degrees; Beckley, W.Va., where a high of 92 broke a mark of 88 set in 1964; and Pensacola, Fla., where a high of 98 surpassed a record of 97 set in 1986.
Mr. Diener pointed out that despite the heat spells, Maryland's weather hasn't been all that bad.
"This weekend may be the area's third consecutive weekend of enjoyable weather. It should be beautiful," he said. "The cooler, dryer air will drop the temperatures 10 to 12 degrees lower than the 100 degrees we've been having."