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 EDT Thu Apr 16 2020

...River Flood Potential Increasing Slightly...
...Remain Alert Entering Flash Flood Season...

LWX Seasonal Flood Outlook 2020-08 (Final)

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 time period through April 30th, 2020.

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 river flood potential is essentially zero through April 22,
2020. The potential increases beginning April 23rd through the
end of the month and is probably a bit above normal, especially
during the period of April 23-25.

CURRENT FLOODING:
As of 8 AM April 16th, there is no flooding within the
Baltimore/Washington HSA.

RECENT PRECIPITATION:
The rain this past Monday (April 13th) was a good reminder that
flooding can occur even when the river flood potential is
technically below average.

That rain also accounts for a decent portion of the rain that has
fallen since mid-March. Totals during that time range from around
three inches over the Potomac Highlands to over six inches in
parts of Augusta and Nelson Counties in Virginia and Baltimore
County in Maryland. These amounts are all at or above normal,
except in far southern Maryland and in the southern Potomac
Highlands.

For the year-to-date, precipitation totals are generally in the
10-15 inch range, which is fairly close to normal -- again, except
in the southern Potomac Highlands and along the Allegheny Front
(where upslope precipitation was much less than usual this
winter).

SNOW CONDITIONS:
A little snow actually fell between April 13th and 15th, but was
hydrologically insignificant.

RIVER ICE:
River ice season has ended. This was the first winter with
limited river ice in several years.

STREAMFLOW CONDITIONS:
Streamflows have made a solid comeback in the first half of April,
and are now near to above normal across the region, as of April
16th.

SOIL MOISTURE:
Soil moisture is generally above normal for this time of year, as
of April 16th.

GROUNDWATER CONDITIONS:
Groundwater levels have increased during the first half of April,
and are now near to above normal across the region, as of April
16th.

EXPECTED WEATHER:
Dry and unseasonably chilly conditions are expected today (April
16th). An approaching storm will spread rain across the region on
Friday (April 17th) into Saturday (April 18th), which could be
chased by a dusting of snow along the Allegheny Front at the end.
Most of the area will see less than a quarter inch of rain; the
Potomac Highlands and Allegheny Front are more likely to see
amounts between a third of an inch and one inch total.

Another rain chance comes in for Sunday night (April 19th) and
Monday (April 20th), but rain amounts with that system are
expected to be extremely light. Dry weather follows through
Wednesday, April 22nd.

Another southern US storm system approaches on Thursday, April
23rd. Although this is still more than a week away at the time of
this outlook, several of the medium-range forecast models indicate
some heavier rainfall is possible sometime during the time period
of Thursday, April 23rd through Saturday, April 25th. (It should
be noted that some indicate no rain falls at all, though this
seems much less likely). This time period should be monitored for
potential for flooding, especially given the recent wet period.

ENSEMBLE RIVER FORECASTS:
Ensemble river forecasts show zero potential for river flooding
through April 22nd, and still a fairly small potential in the
April 23-25 timeframe. Longer term hydrologic ensembles indicate a
near to below average probability of flooding through mid-May.

SUMMARY:
In the Baltimore/Washington HSA, the river flood potential is
essentially zero through April 22, 2020. The potential increases
beginning April 23rd through the end of the month and is probably
a bit above normal, especially during the period of April 23-25.

WATER SUPPLY OUTLOOK:
Given the current status and outlook, water supply concerns appear
unlikely for the next several months. However, if dry conditions
persist, minor drought cannot be ruled out.

NEXT ISSUANCE:
This is the final seasonal flood outlook for 2020. The next
scheduled issuance will be in January 2021.

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


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