Flood Potential Outlook
Issued by NWS Sterling, VA

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WINTER/SPRING FLOOD POTENTIAL OUTLOOK
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
1249 PM EST THU JAN 5 2017

...2017 WINTER/SPRING FLOOD POTENTIAL OUTLOOK...NUMBER 1

...River Flood Potential Outlook through January 19th 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.

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 19th 2017.

Current flooding:
As of January 5th, no flooding is occurring in the Baltimore/
Washington HSA.

Recent precipitation:
As of January 5th, precipitation in the last 30 days has ranged from
around two inches in the Rappahannock River basin to 4 to 5 inches
in western Maryland and the eastern West Virginia Panhandle. While
these amounts are near to above normal, they follow an extremely dry
autumn, leading to longer-term precipitation deficits.

Snow conditions:
As of January 5th, there is very little, if any, snow cover even at
higher elevations. This will be changing as colder air arrives early
in the outlook period. However, current snow on the ground is
substantially below normal at higher elevations.

River ice:
Some river ice actually formed in December during a cold snap. While
little or no ice is observed as of January 5th, another upcoming
cold snap will likely cause some ice to form during the first week
of the outlook period. Any ice coverage is likely to decrease again
during the second week of the outlook period.

Streamflow conditions:
Streams are generally running near normal for this time of year,
and actually some in the upper North Branch Potomac Basin are near
record highs for this time of year. Stream levels are likely to
decrease a bit during the outlook period, though local rain and
snowmelt could cause variable levels at times.

Soil moisture:
The recent rains noted above have helped soil moisture conditions
over a large portion of the service area. There is still an area,
centered on Washington DC, where soil moisture is low -- near the
10th percentile for the date. However, the area west of the Blue
Ridge is near normal and areas near the Allegheny Front are even
above normal now.

Groundwater conditions:
Groundwater levels vary widely across the service area, which is
fairly typical. Among sites in the USGS Real-Time Groundwater
Level Network, some are near normal and others are quite a bit
below normal -- with variety even among sites in the same county.
However, in all cases, water levels have improved since the dry
weather of the autumn.

Expected weather:
A cold snap is expected for the first week of the outlook period,
with some light snow, which will not be hydrologically significant.
The cold temperatures -- which will remain below freezing for
several days in many areas -- will likely promote the development
of river ice at least along shorelines.

Temperatures begin to warm by Tuesday January 10th, and warm
significantly by the end of the first week of this outlook period.
There is a chance for rain, and possibly a little more snow,
between the 10th and 12th. At this time, amounts do not look
significant enough to promote flooding concerns.

The second week of the outlook period currently favors above
normal temperatures and precipitation, but uncertainty and the
generally dry antecedent conditions over most of the area
mean flood potential is still below average.

Probabilistic/Ensemble River Forecasts:
The Meteorological Model Ensemble River Forecasts (MMEFS) through
January 12th indicate virtually no prospects for river flooding.
There is some chance of moderate river rises at the most
susceptible locations during the January 10th through 12th event,
but even that probability is very small.

The longer range probabilistic AHPS forecasts through early
February indicate a below normal probability of minor flooding
at every river forecast point in the service area. That chance
of flooding this early in the season is usually extremely low
to begin with (12 percent or less), and the probabilities are
currently even lower than that.

Summary:
The river flood potential is below average through January 19th
2017.

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 currently exist in areas such as
Charlottesville, Culpeper, Manassas, Fairfax, Arlington,
Alexandria, Washington DC, Bethesda, Ellicott City, Columbia,
Westminster, Baltimore, and Bel Air. These drought conditions
are likely to improve through the spring.

Next issuance:
The next issuance of this product will be January 19th 2017.

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

$$

JE



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