Flood Potential Outlook
Issued by NWS Sterling, VA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4
000
FGUS71 KLWX 161456
ESFLWX
DCC001-MDC001-003-005-009-013-017-021-025-027-031-033-037-043-510-
VAC003-013-015-043-047-059-061-069-079-091-099-107-113-125-137-
139-153-157-165-171-177-179-187-510-540-600-610-630-660-683-685-
790-820-840-WVC003-023-027-031-037-057-065-071-251800-

Winter/Spring Flood Potential Outlook
National Weather Service Baltimore MD/Washington DC
954 AM EST Wed Jan 16 2019

...Seasonal Flood Potential Outlook valid through January 30th...

Supplemental Flood Outlook 2019-01A

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 January 30th, 2019.

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 30th.

Longer term flood potential outlook:
The longer-term flood potential for the rest of the winter and
spring is currently above normal.

Current flooding:
None.

Recent precipitation:
The snowstorm from the weekend of January 12th and 13th
significantly changed the situation for the month of January so
far. Many areas saw significant snowfall, bringing the monthly
liquid-equivalent precipitation totals to closer to normal. Since
November and December were both extremely wet, the region remains
above normal on precipitation at all time periods from 21 days
onward.

Snow conditions:
Following the weekend snowstorm of January 12-13, virtually the
entire area has at least a little snow on the ground. In a
reversal from the normal, the greatest snow depths are in the
western and northern suburbs of Washington DC, rather than in the
mountains. Most of the area has snow depths of 2 to 7 inches as
of the morning of January 16th. The snow water equivalent of this
snow is generally around 1/10 of its depth, ranging from 0.2 to
0.7 inches.

River ice:
Some minor shore ice has been observed in the January 13-16
timeframe, mainly on smaller streams, due to lows in the single
digits and teens. Water temperatures are mostly in the 30s, though
larger reservoirs and the tidal saltwater areas are still near 40
degrees.

Streamflow conditions:
The dry start to January helped bring streams down slightly, but
the entire area remains normal to above normal, with a few spots
well above normal.

Soil moisture:
Soil moisture remains above normal across the outlook area, but
the dry weather that preceded the snowstorm has allowed soil
moisture to decrease just a bit. The entire area remains above the
80th percentile compared to the 1916-2004 measurement period. The
absolute wettest soil conditions are indicated to be along the
Allegheny Front and in northeast Maryland, where conditions are
above the 95th percentile.

Groundwater conditions:
Groundwater levels are currently near normal in deep aquifers and
above to extremely above normal in more shallow aquifers.
Virtually none of the real-time groundwater gauges in the outlook
area are below normal.

Expected weather:
Some melting and sublimation of the existing snowpack is expected
today (January 15) with highs above freezing.

A quick-moving system will affect the outlook area on Thursday
afternoon and night (January 16). This system is forecast to bring
a quick hit of light snow, with amounts of three inches or less
expected, along with a potential period of light wintry mix or
rain.

After a brief break, another much stronger storm system will
approach the region over the weekend (January 19-20). Exactly how
this system affects the region remains uncertain as of the time of
this outlook (consult future forecasts for better details), but
the most likely scenario is for a cool rain that could change to
snow at the end of the event. Further north and west over the
Potomac Highlands, there is greater potential for the
precipitation to fall in the form of snow.

The uncertainty of the precipitation type to the west, and just
how warm it gets to the east, leads to greater than usual
uncertainty about flood potential with this system. There is some
potential in the I-95 corridor for a release of all existing snow
water if temperatures rise into the 50s, which would increase
flood potential in those areas. However, just the combination of
potential rain of around one inch and the wet ground/higher than
usual streams leads to above average flood potential.

After the weekend storm, extremely cold temperatures are expected
which could cause the formation of river ice; however, since
streams are likely to be elevated even further by the weekend
precipitation, ice formation will be slightly more difficult to
develop than if these same conditions happened in recent years.

The deep freeze will be short-lived, with rapid warming by the
middle of next week and another chance of rain by Wednesday the
23rd or Thursday the 24th. There are some early signs that this
also has above average flood potential.

The 8-to-14 day outlook for January 23-29 (which includes the
storm mentioned in the previous paragraph) favors below normal
temperatures and above normal precipitation.

Probabilistic/Ensemble River Forecasts:
The Meteorological Model Ensemble River Forecast System (MMEFS)
indicates there is at least a 10 percent chance of reaching flood
stage on streams that are more susceptible to frequent flooding,
including the South Branch Potomac, Opequon Creek, Seneca Creek,
and the mainstem Shenandoah and Rappahannock. An even higher risk
(30-40 percent) exists on Opequon Creek. On average, flooding
occurs at our river forecast points in January only once every 5
years or less. The last widespread river flood in our area in
January was in 2013.

The longer-range Hydrologic Ensemble Forecast System (HEFS)
probabilistic guidance shows most areas have a higher than usual
probability of flooding through early February. For example, on
Opequon Creek near Martinsburg (which flooded 11 times in 2018),
the usual probability of flooding in the month of January is
around 10 percent. Based on current conditions, the probability
of that location experiencing flooding during the next 30 days is
around 30 percent.

Summary:
In the Baltimore/Washington HSA, the river flood potential is above
average through January 30th. The longer-term flood potential for
the rest of winter into the spring is currently above normal.

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
until well after the growing season begins unless there is
absolutely no precipitation for a prolonged period.

Next issuance:
The next regularly-scheduled Flood Outlook will be issued on or
before January 25th, 2019.

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.

$$

JE



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.