According to current National Weather Service forecasts, precipitation related to hurricanes Idalia and Franklin seem to be mostly by-passing the Potomac basin.
Note that at this time, we do not see a need for a Washington Aqueduct load shift from Great Falls to Little Falls.
Flows at Little Falls are declining, and a Washington Aqueduct load shift to the Little Falls pumps may be needed today. Based on Middle Atlantic River Forecast Center forecasts, we expect ¼ to 1 inch of rainfall in the basin over the next 72 hours, with a basin-wide average of perhaps ½. The longer term forecast is not promising.
River flow at Little Falls has not decreased as drastically as projected over the past 24 hours. Current river flow at Little Falls is about 778 cfs (503 MGD) and predicted to decrease over the next 24 hours. Operations at Dams 4 and 5 are still affecting flows at Little Falls and we are closely watching whether flows will drop below 500 cfs.
Daily flows:
Little Falls gage flow 2023-08-27: 550 MGD (851 cfs)
Minimal rain fell in the basin over the past 24 hours. Expected rainfall over the next three days ranges from 0.25 to 1 inch according to the Middle Atlantic River Forecast Center. Hydropower operations continue to affect river flows. Large fluctuations in river flows are observed at the Shepherdstown and Point of Rocks gages and are expected to affect river flows at Great Falls and Little Falls.
Recent basin-wide average precipitation (above Little Falls):
River flow at Little Falls continues to steadily decline. Both our internal tools and the Middle Atlantic River Forecast Center’s flow forecast (based on the NAEFS) predict river flow at Little Falls to reach a minimum of about 500 cfs within the next 24 hours.
No rain fell in the basin over the past 24 hours. Rainfall is expected over the next 3 days according to the Middle Atlantic River Forecast Center, ranging from 0.1 to 1 inch.
Hydropower operations continue to affect river flows. Large fluctuations in river flows are observed at the Shepherdstown and at the Point of Rocks gage and are expected to affect river flows at Great Falls and Little Falls.
River flows are higher at both Point of Rocks and Little Falls due to the presence of two separate flow peaks caused by hydroelectric power generation at upstream dams (Dams 4 and 5). Flows at Little Falls are now expected to fall significantly over the next 24 – 36 hours as a flow trough – again due to hydropower operations – passes by the stream gage station. We will be working with MDE this week to find a strategy to alleviate this problem.
The recent rainfall in the Potomac basin, averaging ¼ inch, has not significantly changed streamflows. Operation of hydroelectric dams on the Potomac River, Dams 4 and 5, are resulting in high peaks and low troughs in flow, which are visible at the USGS Potomac River streamflow gages at Shepherdstown, Point of Rocks, and Little Falls. A flow peak is currently at Little Falls, but we expect a trough to arrive at water supply intakes and at Little Falls late tomorrow.
Today we are initiating CO-OP Drought Operations, per the Water Supply Coordination Agreement (WSCA) of 1982. According to the WSCA, Drought Operations is triggered when flow in the Potomac River at Little Falls, plus Washington metropolitan area Potomac withdrawals, “is projected to be less than twice the projected withdrawals for any of the next five days.”
Potomac River flows continue to slowly decline. A modest amount of rain is in the Middle Atlantic River Forecast Center’s 3-day forecast, ranging from ¼ to ¾ inch across the basin, and slightly more is predicted in the 7-day forecast.