Flood Potential Outlook
Issued by NWS Sterling, VA

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WINTER/SPRING FLOOD POTENTIAL OUTLOOK
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
1008 AM EST THU JAN 19 2017

...2017 Winter/Spring Flood Potential Outlook...Number 2...
...River Flood Potential Outlook through February 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.

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 above
average through January 26th 2017...then below average from January
27th through February 2nd 2017.

Current flooding:
As of January 19th, no flooding is occurring in the Baltimore/
Washington HSA.

Recent precipitation:
As of January 19th, month-to-date precipitation has ranged from
one and a half to three inches, which is near to above normal
everywhere except Nelson County, Virginia. Longer-term precipitation
deficits still exist, but given the time of year, they are becoming
less important than the ongoing wetter pattern.

Snow conditions:
As of January 19th, there is essentially no snow cover anywhere
within the HSA. This is unusual for this time of year (and a stark
contrast to the second half of January 2016). Snow cover is likely
to remain near zero for at least the first week of the outlook.

River ice:
Although some river ice has formed during a couple cold snaps earlier
in the season, none exists at this time, and none will form during
at least the first week of the outlook period.

Streamflow conditions:
Streamflow is all over the range presently. Heavier rain during the
past 30-45 days has allowed streamflows in western Maryland and
much of the eastern West Virginia Panhandle to rise to above
normal levels for mid-January. Streamflows are near normal close to
the Chesapeake Bay. In between these two areas, streamflows are
still below normal as full baseflow recharge has not yet occurred.
Streamflows are likely to recover significantly during the first
week of the outlook period, and even minor flooding is possible
depending on how heavy the rain ends up being.

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 the Virginia Piedmont, where soil moisture is low --
near the 10th percentile for the date. However, the area west of
the Blue Ridge is near normal.

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:
The first week of the outlook period, through January 26th, is
expected to have temperatures around 10 degrees above normal.
Rain (generally light) is expected on Friday January 20th, with
a more substantial system moving through on Sunday January 22nd
and Monday January 23rd. Total expected rain is between one and
a half and three inches, with potential for isolated higher
amounts in localized areas.

The rain will certainly be enough to cause significant within-
bank rises on area rivers, and if the heavier amounts occur,
some minor flooding is possible. See the next section for
probabilities of occurrence.

After the Sunday/Monday system, it currently appears a quieter
weather pattern takes shape, with temperatures gradually moving
closer to normal (or even below normal in the second week of
the outlook period), and below normal precipitation for the rest
of the outlook period.

Probabilistic/Ensemble River Forecasts:
The Meteorological Model Ensemble River Forecasts (MMEFS) through
January 25th indicate a higher than usual probability for river
flooding as a result of the Sunday/Monday January 22-23 rain
event. Based on meteorological model output, there is at least a
30 percent probability of minor flooding for the following river
forecast points in the January 22-25 timeframe:
* South Branch Potomac River near Springfield, WV
* Cacapon River near Great Cacapon, WV
* Opequon Creek near Martinsburg, WV
* Potomac River at Shepherdstown, WV
* Shenandoah River near Millville, WV
* Potomac River at Point of Rocks, MD
* Rappahannock River at Remington, VA

Please consult the latest river forecasts for additional detail
as this potential threat materializes.

The longer range probabilistic AHPS forecasts through mid
February, which do not take into account future weather but do
note current conditions, 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 (20 percent or less), and the
probabilities are currently even lower than that.

Summary:
The river flood potential is above average through January 26th
2017...then below average for January 27th through February 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 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 in this outlook period.

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

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

$$

JE



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