Flood Potential Outlook
Issued by NWS Sterling, VA

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Winter/Spring Flood Potential Outlook
National Weather Service Baltimore MD/Washington DC
948 AM EDT Thu Mar 29 2018

...2018 WINTER/SPRING FLOOD POTENTIAL OUTLOOK...NUMBER 7...

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 April 11th, 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
near average.

Longer term flood potential outlook:
The longer-term flood potential for the spring season is
currently near normal.

Current flooding:
None.

Recent precipitation:
Precipitation in the second half of March has generally been
between one and three inches, which is near to above normal. This
is in contrast to the first half of March, when very little
precipitation fell. This leaves the area below normal for March.

Snow conditions:
No snow remains in the forecast area as of March 29th. This is
slightly below normal for late March at the highest elevations.

River ice:
The river ice season in our area has now concluded.

Streamflow conditions:
Thanks to recent rainfall/snowmelt, streams are generally near to
slightly below normal, having improved from well below normal
conditions in early March.

Soil moisture:
Soil moisture is near normal across most of the area, but remains
below normal to well below normal south of Washington DC.

Groundwater conditions:
Groundwater conditions continue to improve following recent rain
and snowmelt.

Expected weather:
Unsettled weather will continue for much of the first week of this
outlook period. However, none of the precipitation is expected to
be particularly heavy, leading to total rain of an inch and a half
or less for the week, with the heaviest amounts to the west.

The 8-to-14 day outlook for week two from the Climate Prediction
Center favors continued below normal temperatures, and favors
above normal precipitation, with an unsettled weather pattern
continuing. There are some signals for potential heavy rain
toward the end of the outlook period, specifically starting the
weekend of April 7-8. However, it is too soon to label the flood
threat as anything above average.

Probabilistic/Ensemble River Forecasts:
The Meteorological Model Ensemble River Forecast System (MMEFS)
indicates there is less than a 10 percent potential of reaching
flood stage at any of our forecast point locations through April
4th. All other streams have a less than 10 percent chance.

The longer-range Hydrologic Ensemble Forecast System (HEFS)
probabilistic guidance shows most areas have a slightly greater
than usual probability of flooding through late April.

Summary:
In the Baltimore/Washington HSA, the river flood potential is
average through April 11th. The longer-term flood potential for
the spring season is currently near normal.

Water supply outlook:
Assuming near normal precipitation during the next few months,
water supply is expected to remain normal through the spring.

Next issuance:
This is the final regularly-scheduled Flood Outlook for the 2018
season. If the flood threat increases during the month of April, a
special supplemental outlook will be issued.

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

$$

JE



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