As a sign of early spring, the fear of snowmelt flooding around here is almost as reliable as the ever-swelling chorus of birdsong at daybreak.

But this year, on the eve of April, the snow line has retreated well north of Boston, and flooding is not even a remote threat.

Despite the weekend rain, this has been one of the driest first three months in 137 years of record-keeping, marking only the fifth time that fewer than 6 inches of precipitation have fallen between Jan. 1 and March 31. As for those two big snows this winter, the liquid content amounted only to about an inch.

Stream flows are way down, and at a time when the region should be banking water for the higher-demand days just ahead, that's a "disturbing sign," said Doug LeComte, the drought expert with the government's Climate Prediction Center, outside Washington.

"This is a big recharge period for groundwater and lakes," he said.

For the last 90 days, rainfall has been below normal in every county in Pennsylvania, New Jersey, Delaware, and Maryland. And nationally, the period from Jan. 1 to Feb. 28 was the driest ever. As of last week, the government had classified 57 percent of the continental United States as "abnormally dry."

New Jersey and Pennsylvania officials say they are not quite ready to invoke the "d-word." The Upstate New York reservoirs - an important source of the Philadelphia region's drinking water - remain in decent shape, thanks to a recent run of wet years. And the dryness pattern evidently has relented, at least temporarily.

But the government's long-term outlook into June foresees "developing" drought in the Philadelphia region and throughout the Mid-Atlantic.

Since Jan. 1, rainfall has been less than half of normal in the immediate Philadelphia area, and in the last month, stream and river flows have fallen precipitously, said Delaware River Basin Commission spokesman Clarke Rupert.

The flow of the Schuylkill at Philadelphia on Friday was about 30 percent of the long-term average, and the Delaware at Trenton, near 40 percent. The Pennypack was at 44 percent, and the Neshaminy, 25.

So far, no serious agricultural stress is evident. In fact, the weather has been good to farmers, who have not had to wade through mud to seed their crops, said Andrew D. Frankenfield of the Pennsylvania State University agricultural extension in Montgomery County.

The burgeoning vegetation, however, is about to get thirstier, and it will not be long before the strengthening sun and toastier weather increase evaporation from the soil, drought experts warn.

"When you have dryness that develops during the winter, that really sneaks up on people," said Michael J. Hayes, director of the National Drought Mitigation Center, in Lincoln, Neb. "Once you get into the growing season, the demands are going to go up pretty rapidly."

Why has it been so dry? For weeks, a big part of the nation was cut off from the Gulf of Mexico's generous moisture supply, meteorologists say.

A succession of polar fronts and pulses of dry continental air had a "scouring" effect on Gulf air during the winter, said Greg Carbin, a meteorologist with the weather service's Storm Prediction Center, in Norman, Okla.

And in general, a large area of high pressure, or heavier air, in the south-central United States has been a major obstacle to the northward progress of those Gulf air masses that are laden with water vapor, LeComte said. The high has exacerbated dry conditions from Texas to the Mid-Atlantic.

As of Thursday, the soil was abnormally dry from the Mexican border of Texas to the Canadian border of New York.

Last week, as the Philadelphia humidity was rivaling that of Glendale, Ariz., the National Weather Service was issuing fire-danger advisories. "In March, that is something," LeComte said.

The pattern has shown signs of easing, at least in the short term, with more rain possible late this week.

It is unclear, however, how long the atmospheric faucet will keep running, and the Philadelphia region might well be due for a drought.

On average, droughts recur here about every three years. It has been more than six years since the last one, making it the longest stretch between droughts in the 30-year period of records.

Droughts can take hold in a hurry.

Rupert pointed out that in April 2001, the Upstate New York reservoirs were sloshing over at 102 percent of capacity.

Eight months later, they were at 23.4 percent, an all-time low.

Contact staff writer Anthony R. Wood at 610-313-8210 or twood@phillynews.com.