What is the Outlook?

Little Seneca Reservoir

ICPRB’s Section for Cooperative Water Supply Operations on the Potomac (CO-OP) was established to serve as a cooperative technical center on water resources in the Potomac basin. CO-OP staff publish the Water Supply Outlook on a monthly basis between April and October of each year. It provides an update on the possibility of water supply releases from the area’s reservoirs based on long-term precipitation data, flows, and other information for the Potomac basin.

Download: Water Supply Outlook – April 2026

Summary/Conclusions

There is an above-normal probability of releases from the Washington metropolitan area’s back-up water supply reservoirs for the 2026 summer and fall seasons. Typically, the use of the Jennings Randolph and Little Seneca reservoirs is triggered by low river flows resulting from a combination of low summer precipitation and low groundwater levels. The Potomac basin upstream of Washington, D.C., received 3 inches of precipitation in March, which is 0.5 inches below normal. As of April 1, the 12-month cumulative precipitation is 37.6 inches, or 2.2 inches below normal. Streamflow and groundwater levels remain below normal. According to Middle Atlantic River Forecast Center, the Water Resource Outlook for the Middle Atlantic is “fair.” Although some improvement has occurred over the past several months, moderate drought to abnormally dry conditions persist across the Potomac basin. Precipitation is expected to be near normal over the next couple of weeks. At the same time, higher-than-normal temperatures are expected to increase evaporative demand. The seasonal drought outlook is more favorable, with drought conditions expected to improve through June.

At present, there is sufficient flow in the Potomac River to meet Washington metropolitan area’s water demands without augmentation from upstream reservoirs. If low‐flow conditions worsen, the metro area is well‐protected from a water supply shortage through well-established contingency plans.

ICPRB’s Low Flow Outlook

There is a 14 to 21 percent conditional probability that natural Potomac flow will drop below 600 to 700 million gallons per day (MGD) at Little Falls through December 31 of this year. At these flow levels, water supply releases from Jennings Randolph and Little Seneca reservoirs may occur. Releases occur when predicted flow is less than demand plus a required environmental flow-by. Drinking water demand ranges from 400 to 700 MGD during the summer months and the minimum flow-by at Little Falls is 100 MGD. Note that natural flow is defined as observed flow at the Little Falls gage plus total Washington metropolitan Potomac withdrawals, with an adjustment made to remove the effect of North Branch reservoir releases on stream flow.

The conditional probability is estimated by analyzing historical stream flow records and considering recent stream flow values, precipitation totals for the prior 12 months, and current groundwater levels. Years with watershed conditions most similar to current conditions are weighed more heavily when determining conditional probability. In contrast, the historical, or unconditional, probability is based solely on the long-term record without adjustment for current conditions. The 14 to 21 percent conditional probability compares to the 8 to 15 percent historical probability and is considered the more reliable indicator.

Adjusted Daily Flow at Little Falls for 2026 (black line), 1999, and 2002 as of April 14, 2026

• USGS Potomac real-time gages

Drought Status:

• US Drought Monitor
• CO-OP Drought Monitoring Updates (link updated July 2022)
• Maryland Drought Status
• Virginia Drought Status
• West Virginia Drought Monitor
• Pennsylvania Drought Status
• Forecasted U.S. Conditions Summary

Recent precipitation and forecasts:

• CO-OP’s Potomac Basin Precipitation Map
• Precipitation maps from the MARFC
• Quantitative Precipitation Forecast, MARFC 3 days
• Quantitative Precipitation Forecast, NWS 1-5 days

Groundwater Conditions:

• Basin groundwater levels

Water Supply Outlook Archive:

2026: April | May | June | July | August | September | October

2025: April | May | June | July | August | September | October

2024: April | May | June | July | August | September | October | November*

2023: April | May | June | July | August | September | October | November* | December*

2022: April | May | June | July | August | September | October

2021: April | May | June | July | August | September | October

2020: April | May | June | July | August | September | October

2019: April | May | June | July | August | September | October

2018: April | May | June | July | August | September | October

2017: April | May | June | July | August | September | October 

2016: April | May | June | July | August | September | October

2015: April | May | June | July | August | September | October

2014: October

*Due to the persistent dry conditions, the publication period for the Water Supply Outlook was extended.

Please contact us if you would like to be notified when new Water Supply Outlooks are posted.