Flood Potential Outlook
Issued by NWS Sterling, VA

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Winter/Spring Flood Potential Outlook
National Weather Service Baltimore MD/Washington DC
945 AM EST Thu Jan 18 2018

...2018 WINTER/SPRING FLOOD POTENTIAL OUTLOOK...NUMBER 2

...River Flood Potential Outlook through February 1st 2018...

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 February 1st, 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.

Two week river flood potential outlook:
In the Baltimore/Washington HSA, the river flood potential is
below average through February 1st.

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

Current flooding:
None.

Recent precipitation:
For the second fall and early winter in a row, precipitation is
below normal. Over the past 30 days, precipitation is less than
half of normal in central Virginia, the Virginia Piedmont, the DC
Metro area, and southern Maryland. Deficits also exist on a
longer-term basis, especially in the Virginia Piedmont and
Central Virginia Blue Ridge regions.

Snow conditions:
Much of the area has a little bit of snow on the ground, as of the
time of this outlook, but only an inch or less. Snow depths of 2
to 5 inches exist in the higher elevations along the Allegheny
Front; most of this fell in the prior 36 hours. These snow depths
are below normal for mid-January. Based on the current forecast,
most if not all of this snow will melt during the outlook period.

River ice:
The rain and warmer temperatures on the weekend of January 13th
and 14th broke up and flushed out much of the river ice that has
plagued the area since late December. However, there is still ice
noted on some area rivers, especially the larger ones, and a known
significant partial ice blockage on Conococheague Creek in
Washington County, Maryland. The upcoming warmup should allow for
breakup of remaining river ice, and no new ice is likely to form.

Streamflow conditions:
Streamflow remains below normal throughout the region, though last
weekend`s rainfall did contribute somewhat to baseflow recharge in
areas where ice was not a factor.

Soil moisture:
Soil moisture remains below normal across most of the area. The
exception is in the Potomac Highlands and in North Central
Maryland, where soil moisture is near normal.

Groundwater conditions:
Groundwater conditions are below normal to well below normal
across the area. Normally, groundwater levels bottom out for the
season during the autumn. This year, they continued to fall into
January, but the decreases stopped following the rains of the
weekend of January 13-14. This may signal the bottom for the
season, if somewhat regular rainfall starts to occur.

Expected weather:
After two periods of exceedingly cold weather to end 2017 and
start 2018, a sustained warmup is finally on the way, with highs
reaching the 40s and 50s daily during the first week of the
outlook period, and lows remaining above freezing on at least one
night in many areas. There is a chance for precipitation (mostly
if not all rain) in the January 22/23 timeframe. This
precipitation is currently expected to total between 1/4 and 1/2
of an inch liquid equivalent. This will still likely leave the
week below normal on precipitation.

The 8-to-14 day outlook from the Climate Prediction Center favors
above normal precipitation with above 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 January 23rd.

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

Summary:
In the Baltimore/Washington HSA, the river flood potential is
below average through February 1st.

Water supply outlook:
Assuming near normal precipitation during the next few months,
water supply is expected to remain normal through the spring.
However, the low groundwater conditions are concerning if dry
weather continues. Moderate drought conditions are ongoing in
communities such as Baltimore, Washington, and Charlottesville.
See the Drought Information Statement for additional details.

Next issuance:
The next River Flood Outlook will be issued on Thursday, February
1st, 2018.

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

$$



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