NOAA 00-R203
Contact: Robert Chartuk


The potential for river and stream flooding in the eastern United States is below normal for this time of the year, according to a special flood potential outlook issued by the National Weather Service, which focuses on snowfall shortages as a key factor in the decreased flood potential.

"We can thank La Niña for conditions that add up to reduced flooding risks along the Ohio River and its tributaries, the rivers draining into the Great Lakes and St. Lawrence, and all rivers east of the Appalachians Mountains," said Sol Summer, chief hydrologist for the National Weather Service Eastern Region.

"Historically, the most devastating river floods experienced in the Northeast during the winter have been associated with a combination of rapid snow melt, heavy rainfall, saturated soil, and/or river ice jams," Summer continued. "None of these flood conditions are expected in the next two weeks."

According to the hydrologist, colder ocean temperatures in the eastern Pacific—La Niña—have induced generally warmer temperatures and a precipitation deficit that has existed for the past 12 months. "While the tropical storms in September 1999 helped to alleviate a prolonged drought in many parts of the East, moderate to severe drought conditions are still present in various parts of the country," Summer noted.

In the northern reaches of the eastern United States, both snow cover and the rate of river ice formation are well below normal. And, until this past weekend, most rivers in the East were free flowing, with no little or no river ice that could induce flooding by blocking the flow of water.

Over the weekend, below-normal temperatures settled in across the eastern United States. With the arrival of these colder temperatures, river ice will quickly form and thicken. Still, with the combination of dry antecedent conditions, a very limited snow pack, and no thaw expected in the next two weeks, a below-normal river flood potential is expected.

"While these below normal conditions minimize the potential of river flooding in the East, many heavily populated areas rely on wintertime snow packs to recharge their reservoirs," explained Laurie Hogan, NWS hydrometeorologist. "If the La Niña conditions continue to persist as expected and associated rainfalls in the East remain below normal, the missing snowpack of winter 1999-2000 may be a precursor to water supply shortages returning to parts of the Eastern Region in spring and summer 2000," she said.

Regional Breakouts

The National Weather Service Office in Buffalo, N.Y., reports January snow covers at 30-70 percent of normal. Indeed, a very mild first half of the winter in western and central New York has left hydrological conditions more like November than January.

In northeast Pennsylvania and central New York, the first widespread snow event came on Jan.12, bringing below-average totals to the Wyoming Valley, Western Finger Lakes, upper Susquehanna River Basin, and western Mohawk Valley

Southern New England has experienced a mild and dry period since late October with not only below-normal rainfall, but also much-below-normal snowfall. Rainfall has been 55-70 percent of normal across most of the Connecticut River Valley and northeast Connecticut.

In southern Maine and New Hampshire, little, if any, river ice is occurring and is not thick enough to support the usual winter recreational activities of the St. John and Allagash Rivers.

In Vermont, unseasonably warm temperatures have kept precipitation in the form of rain, which has cleared ice from area rivers. Minimal snowfall in Vermont has left the ground bare in the lower elevations of the St. Lawrence, Champlain and Connecticut River Valleys. Heavy early snows in the northern Adirondacks and Green Mountains of Vermont have pretty much melted.

Central Park in New York City is currently 260 heating degree days below normal, which has impeded river ice formulation in the New York metropolitan area. The region experienced below-normal precipitation in each of the past three months since Hurricane Floyd caused record flooding this summer.

The Ohio River Basin is also reporting minimal snow fall and below-average rainfall.

Unusually low snow cover amounts for this time of the year exist in the upper Potomac Basin, with lower portions of the basin showing no snow at all.

Limited snow has occurred across much of West Virginia, including the eastern and southern mountains, southeast Ohio, southwest Virginia and eastern Kentucky.

Beyond 14 days, the National Weather Service's climate outlook calls for below normal temperatures throughout the East, with an increase of stormy weather patterns. This will create near-normal precipitation for northern parts of the Eastern Region, but should not increase the chance for flooding due to below-normal snowpacks.

The flood potential outlook covers longer-term river and stream flooding, not flash flooding, which can occur when heavy rain suddenly inundates an area.

National Weather Service River Forecast Centers and Weather Forecast Offices in the Eastern Region will continue to monitor and forecast the 2000 winter/spring flood potential, and will issue public updates every two weeks through April.

For more information, flood potential statements from the various National Weather Service field offices can be accessed through