Flood Potential Outlook
Issued by NWS Sterling, VA

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Winter/Spring Flood Potential Outlook
National Weather Service Baltimore MD/Washington DC
1252 PM EDT Wed Mar 20 2019

...Seasonal Flood Potential Outlook valid through April 4th...

Seasonal Flood Outlook 2019-07

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 4th, 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 April 4th.

Longer term flood potential outlook:
Despite the recent dry conditions, the longer-term flood
potential for the spring remains above normal due to persistently
wetter than normal conditions over the last year or so.

Current flooding:
None as of March 20th.

Recent precipitation:
Virtually no precipitation has fallen between March 10th and the
morning of March 20th. This is the longest stretch of completely
dry weather since early December. Most areas are, as of March
20th, below normal for the month of March and near normal for the
calendar year of 2019.

Longer term, precipitation is still above normal, and in most
cases more than 50% above normal, throughout the region after a
record wet 2018.

Snow conditions:
There is no snow on the ground in the region, which is fairly
typical for this time of year.

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

Streamflow conditions:
Streamflows remain near normal to above normal for this time of
year, except in the headwaters of the Potomac and Monocacy Rivers,
where streamflows have dropped to below normal for the first time
in quite a long time.

Soil moisture:
Soil moisture is near to somewhat above normal across the outlook
area, but soil moisture has decreased significantly during the
recent dry spell. The wettest soils are in the Shenandoah Valley,
which is above the 80th percentile compared to the 1916- 2004
measurement period.

Groundwater conditions:
Groundwater levels are currently generally near to above normal.
Virtually none of the real-time groundwater gauges in the outlook
area is below normal.

Expected weather:
Conditions have become more favorable for a fairly heavy rain
event on Thursday (March 21st). Locations especially east of the
Blue Ridge and Catoctin Mountains, and north of Interstate 66, are
likely to receive over an inch of rain, and could see two inches
or more. There is about a one-in-three chance of these heavier
rain amounts being observed, which would produce minor river
flooding on the more susceptible streams.

Dry conditions are then expected through the weekend (March
23-24). Another system affects the region Monday into Tuesday
(March 25-26), which could produce another 0.75 to 1.50 inches of
rain (possibly combined with a little snow, as temperatures cool
to below normal for late March).

The outlook from the Climate Prediction Center for the second
week of the outlook period favors above normal precipitation,
which will keep the flood threat average or above average.

Probabilistic/Ensemble River Forecasts:
The Meteorological Model Ensemble River Forecast System (MMEFS)
indicates rises on rivers, especially east of Interstate 81, from
the rain on Thursday (March 21st). Some minor flooding is possible
if the reasonable worst case scenario heavy rainfall is realized.

The longer-range Hydrologic Ensemble Forecast System (HEFS)
probabilistic guidance shows most areas have a near to above
usual probability of flooding through mid April, based on
antecedent conditions and typical spring rainfall.

Summary:
In the Baltimore/Washington HSA, the river flood potential is
above average through April 4th. 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:
These seasonal flood outlooks are often discontinued after the
start of the spring season if the flood threat appears average or
below. For this year, however, given the continued higher than
average threat of flooding, we will continue to issue these every
two weeks, at least through the month of April. Therefore, the
next regularly-scheduled Flood Outlook will be issued on April
4th, 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.

$$



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