Flood Potential Outlook
Issued by NWS Sterling, VA

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Winter/Spring Flood Potential Outlook
National Weather Service Baltimore MD/Washington DC
218 PM EST Thu Feb 16 2017

...2017 WINTER/SPRING FLOOD POTENTIAL OUTLOOK...NUMBER 4

...River Flood Potential Outlook through March 2nd 2017...

Introduction:
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 2nd 2017.

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.

Two week river flood potential outlook:
In the Baltimore/Washington HSA, the river flood potential is
near zero for the first week of the outlook period (through
February 23rd), and slightly below average for the second week of
the outlook period (through March 2nd).

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

Current flooding:
None.

Recent precipitation:
So far in the month of February, there has been generally less
than a half inch of liquid equivalent precipitation over a large
portion of the HSA, with almost all areas seeing less than one
inch. This is below normal everywhere. Longer-term precipitation
deficits also exist after a dry autumn. Only December and January
were seasonably wet.

Snow conditions:
The only snow on the ground in the area is at the highest
elevations and west of the Allegheny Front. There, snow depth is
3 inches or less. This is below normal and will not add to
hydrologic concerns.

River ice:
No river ice is being observed currently. Given expected weather
conditions over the next two weeks, it is unlikely that any
further river ice will form this season.

Streamflow conditions:
Streamflow is currently extremely below normal due to the
precipitation deficits. Some locations are at or near record lows
for this time of year. The exception to this is in western
Maryland and on the mainstem Potomac River, which are both near
to above normal at this time.

Soil moisture:
Soil moisture has dropped significantly in the last two weeks, and
is now below normal over most of the area, especially in the DC
metro area. The exception again is in western Maryland, where soil
moisture is actually above normal.

Groundwater conditions:
Groundwater conditions, as they often seem to, vary widely across
the area. Some of the shallower aquifers are well below normal,
but deeper groundwater levels are near or even above normal. The
issue currently is that groundwater levels are dropping when they
should typically be rising at this time of year. Overall, some
groundwater recharge is still needed after the dry autumn.

Expected weather:
Dry weather is expected through the entire first week of the
outlook period with above normal temperatures, and even many
nights above freezing. A more active weather pattern is possible
for the second week of the outlook period, but it is unclear this
far out whether that would bring enough rain to cause any
flooding concerns.

Probabilistic/Ensemble River Forecasts:
The Meteorological Model Ensemble River Forecast System (MMEFS)
indicates there is virtually zero potential for river flooding
based on current meteorological forecasts through February 22nd.

The longer-range Advanced Hydrologic Prediction Service (AHPS)
probabilistic forecasts indicate a below normal potential for
river flooding across the area through mid March. This includes
an approximately 15 percent probability of minor flooding on
Opequon Creek near Martinsburg and on the Potomac River at
Shepherdstown. The probabilities are lower at all other locations.

Summary:
The river flood potential is near zero through February 23rd, then
slightly below average through March 2nd.

Water supply outlook:
Assuming near normal precipitation during the next few months,
water supply is expected to remain normal through the spring.
Moderate drought conditions are ongoing in communities such as
Washington DC, Fairfax, Manassas, Bethesda, Columbia, Baltimore,
Westminster, Bel Air, Culpeper, and Charlottesville. See the
Drought Information Statement for additional details.

Next issuance:
The next issuance of this product will be March 2nd 2017.

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

$$



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