Flood Potential Outlook
Issued by NWS Sterling, VA

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Winter/Spring Flood Potential Outlook
National Weather Service Baltimore MD/Washington DC
913 AM EDT Thu Mar 16 2017

...2017 WINTER/SPRING FLOOD POTENTIAL OUTLOOK...NUMBER 6

...River Flood Potential Outlook through March 30th 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 30th 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
below average through March 30th across most of the area, but is
near average for small streams in southern Maryland.

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

Current flooding:
None.

Recent precipitation:
An impressive late-season winter storm brought fairly significant
liquid equivalent precipitation to the region in the March 13-14
timeframe. Amounts greater than one inch occurred roughly east of
a line from Frederick, Maryland to Manassas, Virginia to Orange,
Virginia. Areas near the Chesapeake Bay received over two inches
of mainly rain, and a few spots in Calvert and Saint Mary`s
Counties in Maryland exceeded three inches of mainly rain. The
amounts greater than an inch are above normal for a week in mid-
March, but further west less precipitation fell, and it was mainly
in the form of snow. Thus, these areas continue to experience
below normal precipitation.

Looking further back in time, precipitation over the past 30 days
is 75 percent of normal or less everywhere except where it rained
heavily this past week, and near Cumberland, Maryland. The overall
driest area is in the I-95 corridor from Columbia, Maryland down
to Fredericksburg, Virginia, where precipitation is less than 75
percent of normal on virtually all timescales back to 180 days,
thanks to a dry autumn and dry winter.

Snow conditions:
In a mid-March rarity, most of the Baltimore/Washington HSA has
snow on the ground at the time of this outlook. Snow depths range
from an inch or two over much of the area, to 4 to 8 inches in
much of northern Maryland and the eastern West Virginia panhandle.
The snow water equivalent of this snow is estimated to be around
an inch or less. Upcoming expected weather should allow for a
fairly gradual melt of this snow.

River ice:
With the recent cold temperatures, some minor thin shore ice may
have been able to form on area rivers. River water temperatures
have fallen to near freezing in most areas where snow occurred.
However, even if ice has formed, it is not expected to be
hydrologically significant, and upcoming more seasonable weather
should allow water temperatures to rise.

Streamflow conditions:
Streamflow is currently below normal in much of the Potomac Basin
due to the precipitation deficits. Exceptions are on small streams
in Stafford and Spotsylvania Counties in Virginia, in southern
Maryland, and upstream of both Savage River Dam and Jennings
Randolph Lake. Some locations in the driest areas still have
streamflows that are at or near record lows for this time of year.
This notably includes portions of the Rapidan River, South Fork
South Branch Potomac River, North Fork Shenandoah River, and Piney
River.

Soil moisture:
Despite recent precipitation, soil moisture remains below normal
across most of the area. The exception is in the eastern West
Virginia panhandle, where soil moisture is near normal.

Groundwater conditions:
Groundwater conditions are below normal across the area. Some in
northern Virginia and central Maryland are even at the lowest
levels observed for this time of year in at least a decade. Even
where levels are closer to 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
needed.

Expected weather:
Although a busy weather pattern has taken shape across the region,
with a storm system every few days, none of the upcoming systems
really looks like it will bring significant precipitation. Total
expected rainfall through Wednesday the 22nd is a half inch or
less east of the Allegheny Front. Temperatures through Wednesday
the 22nd are expected to remain at or below normal for this time
of year.

Better chances for rain may occur in the second week of the
outlook period. The 8-to-14 day outlook from the Climate
Prediction Center favors above normal precipitation with near to
below normal temperatures. However, given the dry antecedent
conditions, the flood potential will remain lower than usual for
this time of year.

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 March 22nd.
However, upcoming rainfall could at least help the low streamflow
levels recover a bit.

The longer-range Advanced Hydrologic Prediction Service (AHPS)
probabilistic forecasts indicate a below normal potential for
river flooding across the area through early April, with the
probability for river flooding at 10 percent or less areawide.

Summary:
In the Baltimore/Washington HSA, the river flood potential is
generally below average through March 30th; though flood potential
is closer to average in lower southern Maryland due to recent
rains.

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

Next issuance:
This will be the final issuance of the routine flood outlook
product for 2017. If the river flood threat increases in the
coming weeks, a supplemental flood outlook product may be issued.
Otherwise, as long as the drought conditions persist, this office
will be issuing routine drought statements. These can be found on
our website, and under product identifier DGTLWX with WMO header
AXUS71 KLWX. They will be issued weekly as long as severe drought
(D2) conditions remain in our service area.

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

$$



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