Flood Potential Outlook
Issued by NWS Sterling, VA

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WINTER/SPRING FLOOD POTENTIAL OUTLOOK
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
900 AM EST THU MAR 5 2015

...2015 WINTER/SPRING FLOOD POTENTIAL OUTLOOK...NUMBER 5

...River Flood Potential Outlook through March 12th...

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 12th 2015.

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
average through March 12th.

Current flooding:
None currently across the mid-Atlantic region.

Recent precipitation:
Over the last two weeks, one to three inches of liquid-equivalent
precipitation has fallen across the area, with the heaviest amounts
along the shores of the Chesapeake Bay. These amounts are above
normal east of the Blue Ridge and Catoctin Mountains, and near to
slightly below normal west of the Blue Ridge/Catoctins. On a one-
month timescale, precipitation is generally below normal west of
Interstate 95. On longer timescales, precipitation is generally
near to above normal, except for below normal conditions in the
Potomac Highlands.

Snow conditions:
With snow falling at the time of this writing, the entire hydrologic
service area has snow on the ground as of March 5th. Snow depths
were generally six inches or less as of the morning of March 5th,
but increasing due to the ongoing snow. These snow depths are
unusually high for early March.

Snow water equivalent of the existing snowpack is estimated to be
less than an inch, except along the Allegheny Front, and portions
of Carroll County Maryland, where amounts are one to two inches.
Generally speaking, this is hydrologically insignificant on its own,
but still above normal for early March.

River ice:
Despite some warming, thin river ice remained extensive prior to
the rain and snow of March 4 and 5. This caused ice effects on area
streams and even some minor flooding in a couple spots.

Although the ice is moving, another shot of bitter cold air will
try to briefly lock it back up this weekend, before warmer air
begins to break the ice next week. River ice is expected to
significantly decrease during the outlook period.

Streamflow conditions:
Most streams in the Baltimore/Washington HSA have been affected
by ice, but the recent rain and snowmelt have brought streams
to above normal levels, with many even at highs for this time
of year. These levels will decrease back to normal by next week.

Soil moisture:
Soil moisture is below normal across most of the mid-Atlantic
region. Given the recent bitter cold and snowpack, moisture
may not be as likely to permeate deep into the soil for the
time being.

Groundwater conditions:
Groundwater levels are generally below normal across the
hydrologic service area, but observed levels have increased,
in some cases substantially, since March 1st.

Expected weather:
Ongoing snow today will end by this evening. No additional
precipitation is currently expected during the outlook period.
Temperatures will be below normal through the weekend, then
at or above normal through the rest of the outlook period.
This will promote gradual melting of the snowpack.

Probabilistic/Ensemble River Forecasts:
The ensemble river forecasts using the experimental short-term
hydrologic ensemble forecast indicate a small potential of
flooding due to snowmelt next week. This potential will be
monitored.

The longer-term probabilistic forecasts on the Advanced Hydrologic
Prediction Service indicate a near normal probability of
reaching flood stages through late March, less than 20 percent at
all river forecast points during the one month period.

Summary:
The river flood potential is average through March 12th.

Water supply outlook:
Assuming near normal precipitation during the next few months, water
supply is expected to remain normal through the spring. No drought
conditions currently exist in the region. Development of drought
conditions is not currently expected.

Next issuance:
The next issuance of this product will be early in the evening
of March 12th 2015.

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

$$

JE



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