Winter/Spring River Flood Potential Outlook
NWS Middle Atlantic River Forecast Center (MARFC)
State College, PA
4:34 pm EST Wed, February 17, 2021
Outlook Number 21-04 - February 17, 2021
This Winter/Spring River Flood Potential Outlook is valid for the two-week period February 18 - March 4, 2021.
This outlook estimates the potential for river flooding (not flash flooding) to develop during the next two weeks across the Middle Atlantic River Forecast Center`s (MARFC) area of responsibility (Mid- Atlantic Region) based on a current assessment of hydrometeorological factors which can contribute to river flooding. Across the MARFC area these factors include future weather conditions, recent precipitation, soil moisture, snow cover and snow water equivalent, river ice, streamflow and other factors. This outlook does not address the severity/extent of any future river flooding. Remember, in the Mid-Atlantic region heavy rainfall is the primary factor which leads to river flooding. Heavy rainfall can rapidly cause river flooding any time of the year, even when overall river flood potential is considered to be low or below average.
TWO-WEEK RIVER FLOOD POTENTIAL - ABOVE AVERAGE TO AVERAGE
The river flood potential during the next two weeks (through March 4, 2021) is above average for late February across southern and eastern portions of the MARFC area of responsibility, and average elsewhere. The line separating the two categories runs roughly from southeast NY southwestward into west-central PA. Factors which contribute to this assessment of river flood potential are discussed in some detail below.
CURRENT RIVER FLOODING - NONE
As of this afternoon (Wednesday, February 17, 2021) minor flooding was occurring along a portion of the lower James River in VA. Flooding will end here this evening.
RECENT PRECIPITATION - WET SOUTH, NORMAL/BELOW NORMAL ELSEWHERE
During the last 30 days (January 18 - February 16, 2021) observed precipitation across the MARFC region has been above normal to much-above normal across far southern portions of the MARFC service area, including portions of central VA and the lower half of the Delmarva Peninsula. In this area amounts have been 1-2.5 inches above normal which is 25 to 75 percent above average. Otherwise, most of the rest of the service area has received below-normal to normal precipitation, 50-125 percent of average. Precipitation departure data is found at www.weather.gov/marfc/Precipitation_Departures.
SNOW CONDITIONS - NORMAL TO MUCH-ABOVE NORMAL
As of this afternoon (February 17, 2021), snow covers the ground across roughly 70 percent of the MARFC service area. The snow/no snow line runs roughly from near/along the WV/VA border, east through central MD and into central/southern NJ. The current snow cover extends somewhat further east than what is typical for mid February. In addition, moderate amounts of new snow are expected across much of the MARFC service area during the next two days. Within the MARFC service area, snow conditions are above normal to much-above normal for this time of year across much of NY, much of PA, northern NJ and portions of WV and MD. In this region snow depths this morning were generally in the 10-20 inch range, with scattered greater depths. Corresponding snow water equvialent values in this same region range generally from 1-4 inches, with isolated values of 4.5 inches. Snow depths in the remaining areas where snow exists are generally 1-8 inches, with 0.25-1.0 inches of snow water equivalent. These conditions are about normal for mid February. Little or no snow remains across most of VA, the Delmarva Peninsula and southern NJ, which is also about normal. But to repeat, moderate snowfall is expected across much of this region during the next two days. Snow information can be found at www.weather.gov/marfc/Snow and www.nohrsc.noaa.gov.
RIVER ICE - AVERAGE TO SOMEWHAT BELOW AVERAGE
Currently most streams and rivers in NY, much of PA, portions of NJ, western MD, eastern WV and northwestern VA have at least some ice in them. This is a fairly typical area for river ice to be observed in the second half of February. The most river ice is found in streams and rivers throughout the Susquehanna and Delaware River Basins, which is also typical for mid-late February. The river ice is not being reported as unusually thick or extensive for this time of year. Some additional river ice formation is possible for at least the first week of this outlook period. Then a gradually milder weather pattern appears likely to evolve later in this outlook period, preventing additional river ice formation.
STREAMFLOW CONDITIONS - HIGH SOUTH AND EAST, OTHERWISE NORMAL
The latest data from the United States Geological Survey (USGS) indicates current streamflow conditions within the MARFC region are much-above median across roughly the southeastern two-thirds of the region. Streamflow conditions then generally gradually decrease heading west and north from this area and are generally fairly close to median for mid February. For current streamflow conditions please visit waterdata.usgs.gov/nwis/rt.
SOIL MOISTURE CONDITIONS - WET SOUTHEAST, OTHERWISE NEAR NORMAL
Soils remain extremely moist across southeastern portions of the MARFC service area. The long-term Palmer drought severity index is useful to estimate deep soil moisture conditions. The February 13, 2021 map (seen at cpc.ncep.noaa.gov/products/analysis_monitoring/regional_monitoring/palmer.gif) suggests deep soils remain extremely moist across central/southeast VA and much of the Delmarva Peninsula. Outside of this very wet area, the map shows near-normal values. Additional soil moisture data confirms the very wet soils across southeastern portions of the area. Visit www.cpc.ncep.noaa.gov/products/Soilmst_Monitoring/ and then click on U.S. Monitoring.
GROUNDWATER - VARIABLE
Within the MARFC service area, real-time USGS groundwater monitoring wells indicate current groundwater levels generally range from below average in the far north, gradually increasing to above or even much-above normal in the south for this time of year. To see groundwater levels visit groundwaterwatch.usgs.gov/usgsgwnetworks.asp.
RESERVOIR CONDITIONS - AVERAGE
Most major reservoirs within the MARFC region are holding storages that are about average for this time of year. For example, reservoirs in the Delaware River Basin that supply NYC with water are showing near-average storages presently.
FUTURE WEATHER CONDITIONS - CHANGEABLE
A winter storm will produce widespread moderate amounts of snow, sleet and freezing rain over much of the region tonight through Friday. The system should be monitored for the possibility of producing just enough rain to cause some flooding again across lower portions of the James and Appomattox River Basins in VA. After the storm, seasonable weather conditions will likely prevail for the first week of this two-week period. However, a milder weather pattern appears likely to develop during the second week. After the storm moves out Friday, there currently are no strong indications of any widespread heavy rain events for the rest of the first week of this outlook period. The latest (February 17, 2021) weather outlooks issued by the NWS Climate Prediction Center suggest that both temperatures and precipitation will be normal or above normal when averaged over the entire nine-day period February 23-March 3, 2021. Please visit www.cpc.ncep.noaa.gov for the latest outlooks.
ENSEMBLE RIVER FORECASTS - SOME THREAT OF RIVER FLOODING
The most recent runs (February 17, 2021) of the short-term (6-10 day) ensemble river forecasts, which take into account anticipated future weather conditions including precipitation and temperatures, do show some threat of river flooding developing across southern portions of the service area. The threat is due to the possibility of locally heavy rain early in the outlook period, followed by snowmelt/additional rain later in the period. Please visit www.weather.gov/erh/mmefs. Current longer-term forecasts also suggest a somewhat increased chance for river flooding compared to normal during the next 30 days for some river basins within the MARFC service area, most notably the Potomac River Basin. This is primarily due to what is now above-average snow conditions in portions of that river basin. Any future river flooding will depend on shorter-term weather conditions, such as the widespread heavy rainfall and/or rapid snowmelt.
A winter storm is expected across most of the MARFC service area beginning tonight (Wednesday, February 17) and lasting well into Friday. The storm will add to the already substantial snow conditions that exist across roughly the northwestern half of the region, and put down some new snow across much of the southern half of the region. The storm should be monitored for the possibility of producting just enough rain across portions of the James and Appomattox River Basins to produce some additional river flooding, though that possibility seems fairly low due to the cold temperatures. Meanwhile, in the longer range, while there are currently no strong indications of widespread heavy rainfall or widespread rapid snowmelt after this first storm departs late Friday, snow and to some degree river ice conditions have increased enough to be of some concern, especially over about the northwestern half of the MARFC region. A milder weather pattern is now expected to develop sometime during the second week of this outlook period, but it is too soon to tell if/where significant snowmelt will occur, and whether or not widespread heavy rainfall will occur. Needless to say, the river flood potential is becoming more elevated with time as we get closer to March, the month when river flooding due to snowmelt and/or heavy rainfall is fairly common. As such, conditions will need to be monitored closely for the rest of this winter into early spring, until such time that most of the snow has melted.
WATER SUPPLY OUTLOOK
According to the latest (February 9, 2021) U.S. Drought Monitor (droughtmonitor.unl.edu), abnormally dry conditions persist across a small portion of southern NY and northern PA. However, assuming near-normal precipitation over the next few months, no water supply shortages are anticipated within the MARFC region through May, 2021.
Please visit www.drought.gov, www.cpc.ncep.noaa.gov and www.weather.gov/marfc/WRO for additional drought and water supply information. Please visit the NWS MARFC homepage at www.weather.gov/marfc or find us on Facebook at www.facebook.com/nwsmarfc/?REF=aymt_ homepage_panel and on Twitter @nwsmarfc.
The next winter/spring river flood outlook product will be issued by this office March 3-4, 2021.