Flood Potential Outlook
Issued by NWS Sterling, VA

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WINTER/SPRING FLOOD POTENTIAL OUTLOOK...CORRECTED
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
819 AM EST FRI FEB 20 2015

...2015 WINTER/SPRING FLOOD POTENTIAL OUTLOOK...NUMBER 4

...River Flood Potential Outlook through March 5th...

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 5th 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 5th.

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

Recent precipitation:
Over the last two weeks, less than one inch of liquid-equivalent
precipitation has fallen across almost all of the area except for
lower Southern Maryland. This is below normal areawide. On a one-
month timescales, precipitation is also generally below normal
everywhere except in the upper Shenandoah Valley. On longer
timescales, precipitation is generally near to above normal,
except for below normal conditions in the Potomac Highlands.

Snow conditions:
Most of the hydrologic service area has at least a trace of snow on
the ground as of February 20th. In an unusual twist, the areas with
the most bare ground are generally along the Mason-Dixon line from
near Hagerstown to Westminster. Snow depths are otherwise generally
around two inches, with higher depths in the Potomac Highlands and
lower Southern Maryland. The maximum snow depth observed on February
20th was 10 inches at Bayard, West Virginia, but it is likely that
snow depths over a foot exist along some portions of the Allegheny
Front. These snow depths are unusually high for late February across
the entire area.

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

River ice:
NWS personnel conducted an extensive survey of area river ice this
week. River ice is widespread across the area. The only streams
with little ice are those smallest streams fed by groundwater or
springs, where the water temperature is still above freezing and
there is consistent streamflow.

Most other streams are covered by at least a thin layer of ice
(with snow atop the ice from this week`s snowfall). In the larger
streams, this ice is not thick enough to restrict streamflow.

River ice observations from the Potomac River show more extensive
river ice than what was observed in January 2014, but again at
this point, at least some unrestricted flow is continuing through
the river system. For pictures from the Potomac River, visit our
Facebook page.

Streamflow conditions:
Most streams in the Baltimore/Washington HSA are being affected
by ice, so many readings do not represent actual streamflow but
instead are indicative of the relative height of the iced river.
Because of this, it is difficult to tell how current streamflow
conditions relate to normal. But prior to the ice, streamflows
were generally below normal, so it can be assumed given recent
dry conditions that they still would be if there were not ice.

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 have been dropping during the past two weeks,
and are now near to below normal across the hydrologic service
area.

Expected weather:
Significantly below normal temperatures will persist for the
first week of this outlook period. The next storm system to
impact the region will be on Saturday. Liquid-equivalent
precipitation amounts over one inch are possible across much
of the hydrologic service area. With much of the precipitation
expected to fall in the form of snow or sleet, this yields the
potential for several inches of wintry precipitation to occur,
especially west of the Blue Ridge. Some freezing rain is also
possible.

Initially it appeared some melting would be possible on Sunday
with a brief warmup. As the Saturday system has trended in a
snowier direction, temperatures are no longer as likely to rise
significantly, and thus melting may be harder to come by. Aside
from potentially rising above freezing on Sunday, temperatures
are currently forecast to stay below freezing even during the
day for most of the first week of this outlook period. So the
snow already on the ground, and the new snow that falls on
Saturday, will likely stay around through the end of the month.

The outlook for week two...February 27th through March 5th...
still favors below normal temperatures and above normal
precipitation...maintaining the trend of most of February.

Probabilistic/Ensemble River Forecasts:
The ensemble river forecasts using the experimental short-term
hydrologic ensemble forecast indicate a low potential of flooding
through the end of February.

The longer-term probabilistic forecasts on the Advanced Hydrologic
Prediction Service indicate a slightly below 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 5th.

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 March 5th 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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