Flood Potential Outlook
Issued by NWS Sterling, VA

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Winter/Spring Flood Potential Outlook
National Weather Service Baltimore MD/Washington DC
259 PM EST Thu Jan 4 2018

...2018 WINTER/SPRING FLOOD POTENTIAL OUTLOOK...NUMBER 1

...River Flood Potential Outlook through January 18th 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 January 18th, 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 January 18th.

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, and much of what has fallen has been in the form
of light snow. Deficits also exist on a longer-term basis,
especially in the Virginia Piedmont and Central Virginia Blue
Ridge regions.

Snow conditions:
Thanks to the coastal storm pulling away from the region at the
time of this outlook, snow on the ground is virtually the opposite
of what you would expect it to be. Snow depths at the highest
elevations near the Eastern Continental Divide are between 2 and 8
inches, while places such as Winchester and Charlottesville have
little or no snow at all. Then the coastal storm has brought 1/2
to 6 inches of snow, generally east of US Highway 15, with the
highest amounts in southern Maryland. All the snow currently on
the ground will remain for several days, but it should all
gradually melt or sublimate during the week of January 8th as
temperatures warm, except perhaps in the mountains.

River ice:
In many places, temperatures have not been above freezing since
December 26th, and not above 40 degrees since December 24th. This
sustained cold weather has allowed relatively substantial ice to
form on area rivers. Any flood potential in the next two weeks
probably would need some sort of partial ice blockage. Upcoming
more seasonable temperatures should allow for some decrease in
river ice coverage.

Streamflow conditions:
Streamflow is currently well below normal in much of the Potomac
Basin due to the precipitation deficits, although the measurements
are somewhat problematic due to the river ice.

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

Groundwater conditions:
Groundwater conditions are below normal to well below normal
across the area, and have continued to fall into early January
when they normally are increasing by this point in the season.
Some in northern Virginia and central Maryland are even at the
lowest levels observed for this time of year in at least a decade.

Expected weather:
The deep freeze that ended 2017 and started 2018 is expected to
come to an end during the week of January 8th. There are some
signs it could return toward the end of that week, but that is
uncertain. There is a chance for precipitation (possibly rain,
possibly freezing or frozen) on Monday, January 8th. This
precipitation is currently expected to total between 1/4 and 3/4
of an inch liquid equivalent. If the higher amounts are realized,
this would be near normal for a one-week period this time of year.

The 8-to-14 day outlook from the Climate Prediction Center favors
above normal precipitation with near 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 11th.

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 January 18th.

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, January
18th, 2018.

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

$$

JE



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