Melting snows from last Friday's massive storm pose little immediate threat of flooding in the Washington area, but weather forecasters cautioned yesterday that possible rain on Thursday--if it's heavy--could send some streams over their banks by Friday.

As temperatures shot up into the mid-50s yesterday, the local river forecast section of the National Weather Service began monitoring small streams in the area but reported little change in flow levels so far.

The U.S. Geological Survey, which monitors the Potomac River, however, reported that the daily volume of the river just above Washington jumped from 3.9 billion gallons to 5.3 billion gallons between Monday and yesterday, but that is still little more than half the average volume of 10.3 billion gallons for this time of year.

In fact, because of below-normal precipitation and extended dryness in the region for the last several months, most rivers and streams are well below their normal flow levels, and government hydrologists welcomed last Friday's blockbuster snowstorm as a way to replenish the area's diminishing water supply.

"We think the snow will have some beneficial effects," said U.S. Geological Survey spokesman Don Kelly. "The river is down. Ground water levels are down . . . . Of course, the slower the melt-off, the better."

Bob Clark, director of the weather service office of hydrology, noted that the 16-to-20-inch snowfall in the Washington area last Friday has a "total water equivalent" of only 2 to 3 inches when melted down, but only half to three-quarters of an inch of that amount is expected to melt by Thursday.

"Normally, even a 2- or 3-inch rain will not cause you too much of a problem," he said, "especially with the dry soil conditions around here right now."

The melting snow thus will likely "just go straight down into the ground," he said.

"But if we get some heavy rain on top of the snow," he added, "then we could have some trouble." Rain would accelerate the melting process as well as add its own runoff, he said.

He noted that the "flash flood index" for the Potomac River, for example, is 2 1/2 inches. That is, he said, "you would have to have 2 1/2 inches of rain within a 3-to-6-hour period to produce bank-full stages on the river." Thus, melting snow and heavy rains later this week could conceivably cause flood conditions, Clark said, but it is far too early to tell.

As for the forecast, weathermen say temperatures should be in the upper 40s today with increasingly cloudy skies, and a low pressure area over Florida could bring rain, possibly heavy at times, on Thursday.