Flood Potential Outlook
Issued by NWS Sterling, VA

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Winter/Spring Flood Potential Outlook
National Weather Service Baltimore MD/Washington DC
800 AM EDT Thu Mar 15 2018


Each winter and early spring, the National Weather Service office
serving the Baltimore/Washington area issues a series of routine
flood potential outlooks. These outlooks estimate the potential
for river flooding (not flash flooding) across the
Baltimore/Washington Hydrologic Service Area (HSA). This area
includes the entire Potomac, Shenandoah, and Rappahannock River
basins, as well as drainage basins west of, but not including,
the Susquehanna in the Upper Chesapeake Bay.

During this time of year, contributing factors to river flooding
come from recent precipitation, soil moisture conditions, snow
cover and snow water equivalent, river ice, antecedent
streamflow, expected weather conditions, and other factors. This
outlook is valid for the period through March 29th, 2018.

In the Mid-Atlantic region, heavy rainfall is the primary factor
which leads to river flooding. Heavy rain can rapidly cause river
flooding at any time of the year, even when river flood potential
is considered to be low or below average.

One week river flood potential outlook:
In the Baltimore/Washington HSA, the river flood potential is near
average, but trending upward toward the end of March.

Longer term flood potential outlook:
The longer-term flood potential for the spring season is
currently near normal.

Current flooding:

Recent precipitation:
Precipitation for the first half of March has been below normal
across almost all of the Baltimore/Washington service area. The
only exception is in eastern Harford County, Maryland, where more
than two inches of rain and melted snow has fallen in the first
half of the month. In a region including Harrisonburg, Front
Royal, Culpeper, and Dulles Airport, only a quarter inch or less
of liquid equivalent precipitation has occurred so far this month.

Snow conditions:
Recent snows have left 6 to 9 inches of snow on the ground at the
higher elevations near and west of the Allegheny Front. There is
also still some minimal snow on the ground further east in central
Virginia but that will be melting in the coming days. The snow
that is on the ground is generally more than is typically expected
in this area by mid March. This snow is estimated to contain
around an inch or less of snow water equivalent.

River ice:
The river ice season in our area has now concluded.

Streamflow conditions:
The recent dry conditions have allowed the still-lagging
groundwater situation to affect baseflows; therefore, streamflows
are below to much below normal across northern Virginia and West
Virginia; and near to slightly below normal in Maryland. A few
sites in the Washington DC metro are at their lowest levels for
this time of year on record.

Soil moisture:
Soil moisture is near normal across most of the area, but remains
below normal south of Washington DC.

Groundwater conditions:
Groundwater conditions have either leveled off (deeper aquifers)
or are dropping again (shallower aquifers). As mentioned above,
this lower water table is having an impact on groundwater-fed
base streamflows.

Expected weather:
A storm system will cut to the south of the region on Saturday
(March 17th) and could bring some generally light precipitation
(less than a quarter inch). Another stronger system is likely to
affect the region late on Monday (March 19th) potentially through
Wednesday (March 21st). This could bring up to one inch of liquid
equivalent precipitation, with locally higher amounts possible. In
the first week of the outlook period, this is the only timeframe
with any potential of flooding.

The 8-to-14 day outlook for week two from the Climate Prediction
Center strongly favors continued below normal temperatures, and
favors above normal precipitation. At the time of this outlook, it
is too soon to have any certainty, but there are signals that
indicate a more active pattern sometime in the second week of the
outlook period which could promote a slightly enhanced risk of

Probabilistic/Ensemble River Forecasts:
The Meteorological Model Ensemble River Forecast System (MMEFS)
indicates only Opequon Creek in the West Virginia panhandle has a
greater than 10 percent potential of reaching flood stage through
March 22nd. All other streams have a less than 10 percent chance.

The longer-range Hydrologic Ensemble Forecast System (HEFS)
probabilistic guidance shows most areas have a slightly greater
than usual probability of flooding through early April.

In the Baltimore/Washington HSA, the river flood potential is
average through March 29th, but increasing during the second week
of the outlook period. The longer-term flood potential for the
spring season is currently near normal.

Water supply outlook:
Assuming near normal precipitation during the next few months,
water supply is expected to remain normal through the spring.

Next issuance:
The next River Flood Outlook will be issued on Thursday, March
29th, 2018.

For additional hydrologic or weather information, visit our
website at weather.gov/baltimore or weather.gov/washington.


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