Flood Potential Outlook
Issued by NWS Sterling, VA

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Winter/Spring Flood Potential Outlook
National Weather Service Baltimore MD/Washington DC
158 PM EDT Thu Apr 18 2019

...Final Seasonal Flood Potential Outlook...

Seasonal Flood Outlook 2019-09

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 remainder of the spring.

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.

Flood potential outlook:
The flood potential for the remainder of the spring remains above
normal due to persistently wetter than normal conditions over the
last year or so.

Current flooding:
None as of April 18th.

Recent precipitation:
Since the last outlook was issued on April 4th, precipitation
amounts have ranged from less than an inch in the I-95 corridor
between Washington and Wilmington to over three inches along and
either side of the Blue Ridge Mountains, and in parts of Frederick
County, Maryland. The lesser amounts are below normal; but most
areas are above normal for the month of April so far.

On longer term scales from 60 days and beyond, precipitation is
still above normal, and in most cases more than 25% above normal,
throughout the region after a record wet 2018. The exception is
along the Allegheny Front, where precipitation has been below
normal since late 2018, until the last couple weeks.

Unbelievably, we are still preliminarily setting precipitation
records as recently as mid-April...with IAD and BWI airports
setting new 365-day running precipitation total records
(preliminary; subject to correction). DCA fell just short of the
previous record. All three sites could break the record (in the
case of IAD/BWI, again) before the end of April.

Snow conditions:
There is no snow on the ground in the region, which is typical for
this late in the season.

River ice:
No river ice formation is expected for the remainder of the
season.

Streamflow conditions:
Streamflows are near normal where precipitation has been less so
far in the month of April, but above normal everywhere else.

Soil moisture:
Soil moisture remains near to slightly above normal across the
outlook area.

Groundwater conditions:
Groundwater levels are currently generally near to above normal.

Expected weather:
A potentially significant rain event is possible early in the
outlook period (Friday April 19th and Saturday April 20th). One to
two inches of rain is likely; isolated areas could receive three
to four inches. This has the potential to cause river and stream
flooding.

A relatively unsettled weather pattern is expected to follow, with
at least a small potential for rainfall in some portion the
outlook area every day through the final weekend of April. At the
time of this outlook, it is too soon to say whether any of this
precipitation might be heavy enough to cause more flooding.

The precipitation outlook from the Climate Prediction Center for
the rest of April favors above normal precipitation, which will
keep the flood threat above average. The outlook for May indicates
equal chances of above, below, or near normal precipitation.

Summary:
In the Baltimore/Washington HSA, the river flood potential is
above average through the remainder of the spring.

Water supply outlook:
Given the extreme rainfall of 2018, water supply should not be an
issue for quite some time. Drought development is highly unlikely
unless there is absolutely no precipitation for a prolonged
period.

Next issuance:
This is the final issuance of this outlook for the spring 2019
season. The next outlook will be issued in January of 2020. In the
meantime, everyone is encouraged to monitor the Hazardous Weather
Outlook daily for any potential for flooding in the ensuing seven
day period. Flash Flood season in the mid-Atlantic is typically
from early May through late August, and Flash Flooding is a deadly
and dangerous phenomenon.

For additional hydrologic or weather information, visit our
website at weather.gov/baltimore or weather.gov/washington.
Select "Rivers and Lakes" or "Hydrology" for more information.

$$

Elliott


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