Flood Potential Outlook
Issued by NWS Sterling, VA

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Winter/Spring Flood Potential Outlook
National Weather Service Baltimore MD/Washington DC
830 AM EST Thu Feb 15 2018

...2018 WINTER/SPRING FLOOD POTENTIAL OUTLOOK...NUMBER 4

...River Flood Potential Outlook through March 1st 2018...

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 1st, 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
above average through March 1st.

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

Current flooding:
None.

Recent precipitation:
For the first time in months, we can finally say that recent
conditions have been wet. For the first half of February, rain
(combined with melted snow, where appropriate) amounts have ranged
from around two inches in the Potomac Highlands to 3-4 inches in
most areas, with a swath of 4-6 inches extending either side of a
line from Charlottesville VA to Fredericksburg VA to Annapolis MD.
These amounts are above normal across the board, and more than
four times normal in the heaviest locations.

These rains have largely eliminated winter precipitation deficits,
and have started to eat away at rainfall deficits from the fall.

Snow conditions:
There is no snow on the ground at the time of this outlook, which
is below normal for mid-February in the higher elevations.

River ice:
River ice has generally broken up, and with unseasonably warm
temperatures expected in the coming days, it appears the river ice
season in our area has now concluded.

Streamflow conditions:
Streamflow is near to above normal throughout the region, having
improved significantly due to recent rainfall. With more rain
expected, these improved conditions are likely to persist.

Soil moisture:
Soil moisture has increased due to the recent rains, but it still
remains below normal in many areas as much of the rain turned to
runoff and did not get into the soil.

Groundwater conditions:
Similarly to the soil moisture, groundwater has been slow to
recover despite the recent rainfall. With that said, though,
conditions have dramatically improved in the last two weeks, with
sites that were near decadal lows improving by several feet of
water depth. In particular, a well in Orange County, Virginia that
has been near or below record lows for much of the last year has
spiked several feet, and is at least now moving above the record
lows. Most other wells have shown similar rises in the water
table, and the shallow wells are much above normal at the moment.

Expected weather:
The active weather pattern of the first half of February looks to
continue in the second half of the month. Rain is expected tonight
(February 15) and Friday (February 16), with another system
bringing rain -- and perhaps a period of ice or snow on Saturday
(February 17). After a break on Sunday (February 18), there is at
least a chance of rain every day in the following work week
(February 19 through 23).

Rainfall totals during the first week of the outlook period,
through February 21, look most likely to be between 1 and 2
inches, but with some potential for significantly higher amounts,
especially in the Potomac Highlands. Temperatures for the first
week of the outlook period look to be generally above to much
above normal, with highs today (February 15) and Tuesday/Wednesday
(February 20-21) expected to be near or above 70 degrees.
Temperatures that warm this time of year often promote heavy
rainfall.

The 8-to-14 day outlook for week two from the Climate Prediction
Center favors continued above normal precipitation, and strongly
favors continued above normal temperatures. Brief colder snaps can
be expected during the two week period, but are not expected to be
the prevailing weather condition.

Probabilistic/Ensemble River Forecasts:
The Meteorological Model Ensemble River Forecast System (MMEFS)
indicates some small potential for smaller streams to exceed
Action Stage this weekend (February 17-18) and perhaps again early
next week (February 19-20). However, the probability of reaching
flood stage through February 20th is (as of February 15th) less
than 10 percent.

The slightly longer-range Hydrologic Ensemble Forecast
System (HEFS) probabilistic forecasts indicate a near to above
normal potential for river flooding across the area through early
March. The probability of flooding on smaller streams, including
Opequon Creek near Martinsburg, is around 10-20 percent next week,
and as much as 25 percent later in the month.

Summary:
In the Baltimore/Washington HSA, the river flood potential is
above average through March 1st, due to higher base streamflows,
and wetter soils. This also combines with a period of an active
weather pattern bringing frequent rain chances along with warm
temperatures.

Water supply outlook:
Assuming near normal precipitation during the next few months,
water supply is expected to remain normal through the spring.
Moderate Drought conditions are ongoing, as of February 15th, in
communities such as Washington and Baltimore. See the Drought
Information Statement for additional details.

Next issuance:
The next River Flood Outlook will be issued on Thursday, March
1st, 2018.

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

$$



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