Extended Streamflow Guidance
Issued by NWS Middle Atlantic RFC

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Winter/Spring River Flood Potential Outlook
National Weather Service Middle Atlantic River Forecast Center
(MARFC)
State College, PA
11:30 am EST Thu, Feb 21, 2019


Outlook Number 19-04 - February 21, 2019

This Winter/Spring River Flood Potential Outlook is valid for the two-
week period February 21-March 7, 2019.

This outlook estimates the potential for river flooding (not flash
flooding) to develop during the next two weeks across the Middle
Atlantic River Forecast Center`s (MARFC) area of responsibility (Mid-
Atlantic Region) based on a current assessment of
hydrometeorological factors which can contribute to river flooding.
Across the MARFC area these factors include future weather
conditions, recent precipitation, soil moisture, snow cover and snow
water equivalent, river ice, streamflow, and others.  This outlook does
not address the severity/extent of any future river flooding.

Remember, in the Mid-Atlantic region heavy rainfall is the primary
factor which leads to river flooding.  Heavy rainfall can rapidly cause
river flooding any time of the year, even when overall river flood
potential is considered to be low or below average.

TWO-WEEK RIVER FLOOD POTENTIAL - VARIABLE

The river flood potential during the next two weeks (through March 7)
varies from somewhat above-average across most of the southern half
of the MARFC service area, then decreases to near-average or even
somewhat below average across the northern half.  Factors which
contribute to this assessment of river flood potential are discussed in
some detail below.

CURRENT FLOODING - NONE

There is currently no river flooding occurring within the MARFC service
area.

RECENT PRECIPITATION - NEAR TO ABOVE AVERAGE

During the last 30 days (January 22 - February 20, 2019) observed
precipitation has been mostly average to above average across the
MARFC service area.  Portions of western and central PA have been
the wettest compared to normal, while portions of VA and the
Delmarva Peninsula have been the driest compared to normal.  To
view precipitation departure data please visit
https://www.weather.gov/marfc/Precipitation_Departures.

SNOW CONDITIONS - VARIABLE

Due to yesterdays storm, at the time of this writing snow/sleet covers
the ground across nearly all of the MARFC service area.  Only lower
portions of the Delmarva Peninsula and southeastern VA have little or
no snow/sleet on the ground.  However, most if not all of the
snow/sleet across southern and eastern areas will melt during the next
few days.  Current snow depths range from 1-10 inches while current
snow water equivalent values range from a minimum of about 0.25
inches across the eastern edge of the snow cover to a maximum of
around 2.25 inches across portions of the Chenango, North Branch
Susquehanna and Upper Delaware Basins in NY and portions of the
West Branch Susquehanna in central PA.  Snow conditions are
somewhat below average for this time of winter across NY and
northern PA.  Elsewhere, conditions are fairly close to average for this
time of winter.  The snow/sleet across the far south and east is a little
unusual, but it will melt in the next day or two and is generally
hydrologically insignificant.  Snow information can be found at
https://www.weather.gov/marfc/Snow and
https://www.nohrsc.noaa.gov.

RIVER ICE - BELOW AVERAGE TO AVERAGE

Recent observations from ice observers and satellite photos currently
indicate that little river ice exists on streams and rivers within the
MARFC region.  As such, river ice conditions are below average for
this time of year across roughly the northwestern half of the region.
Meanwhile, across the southeastern half of the region, the lack of river
ice is fairly typical for late February.

STREAMFLOW CONDITIONS - ABOVE MEDIAN TO MUCH-ABOVE
MEDIAN

The latest data from the United States Geological Survey (USGS)
indicate above normal to much-above normal streamflow conditions
across most of the MARFC region.  Streamflow conditions are
currently highest relative to normal across southern and eastern
portions of the region.  Please visit https://waterdata.usgs.gov/nwis/rt
for current streamflow data.

SOIL MOISTURE CONDITIONS - GENERALLY MUCH-ABOVE
AVERAGE

The long-term Palmer drought severity index is useful to estimate deep
soil moisture conditions.  The February 16, 2019 chart (found at
https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/regional_
monitoring/palmer.gif) suggests deep soils across most of the MARFC
service area contain moisture that continues to be much-above normal
for this time of year.  Additional more detailed soil moisture information
supports the idea that soils are extremely wet across nearly all of the
MARFC area.  Go to
https://www.cpc.ncep.noaa.gov/products/Soilmst_Monitoring/ and then
click on U.S. Monitoring to view this soil moisture information.

GROUNDWATER - ABOVE NORMAL TO MUCH ABOVE NORMAL

Throughout the MARFC area, most USGS groundwater monitoring
wells continue to measure above-normal to much-above-normal
groundwater levels.  Please visit https://groundwaterwatch.usgs.gov.
RESERVOIR CONDITIONS - AVERAGE TO ABOVE AVERAGE

Most major reservoirs within the MARFC region are holding storages
that are average to above average for this time of year.

FUTURE WEATHER CONDITIONS - MILD AND WET, THEN
COOLER AND DRIER

Snowmelt is anticipated across much of the MARFC region during the
next few days, but especially across the southern half of the region.
Additionally, a moderate-to-heavy rain event seems likely this
weekend, again mainly across the southern half of the region.  This
weather pattern has a decent chance of causing river flooding to
develop during the next several days in this region, including the
Potomac, Shenandoah, Rappahannock, James and Appomattox River
Basins.  Beyond the next few days, a gradually cooler and drier
weather pattern looks likely for most of the MARFC region.  This would
act to decrease the chances for river flooding to some degree.  The
latest (February 20) longer-range weather outlook issued by the NWS
Climate Prediction Center suggests the second week of this two-week
period may see below-average temperatures and about normal
precipitation for most of the MARFC region.  Long-range outlooks can
be viewed at https://www.cpc.ncep.noaa.gov/.

ENSEMBLE RIVER FORECASTS - A FAIR THREAT OF RIVER
FLOODING DEVELOPING ACROSS THE SOUTH

The most recent runs (February 21, 2019) of the short-term (one week)
ensemble river forecasts, which take into account anticipated future
weather conditions including precipitation and temperatures, show
fairly high chances of river flooding developing during the next several
days, mainly across the southern half of the MARFC service area.
Longer-range ensemble river forecasts also show a somewhat
elevated risk of river flooding, compared to normal, for the next 30
days - again mainly for the southern half of the region.  This elevated
longer-range risk reflects the very wet soils and high streamflow
conditions that currently exist across the region.  Please visit
https://www.weather.gov/erh/mmefs for short-term ensemble river
forecasts.

SUMMARY

An elevated risk of river flooding - above average for this time of winter
- currently exists across much of the southern half of the MARFC
region.  Melting snow during the next few days, combined with a threat
of moderate to heavy rain this weekend, are helping to create the
short-term river flood threat.  Some of the flooding could reach
moderate levels.  Beyond the next few days, the river flood potential
remains somewhat above average across the south through this
outlook period ending March 7.  With the wet soils and high streamflow
in this region, any additional heavy rain events later in this outlook
period could easily result in river flooding.  Meanwhile, across the
northern half of the region, the river flood potential during this two-
week outlook period is average or even somewhat below average.
Snowmelt further north will be less impressive, and the weekend rain
event at this time looks to impact southern regions more than northern
regions.  In general, weather conditions after this weekend look to turn
gradually cooler and drier across most of the MARFC region.  The
latest long-range weather outlook issued by the NWS Climate
Prediction Center can be seen at https://www.cpc.ncep.noaa.gov/.

WATER SUPPLY OUTLOOK

According to the latest (February 19) U.S. Drought monitor
(https://droughtmonitor.unl.edu), there are currently no drought
conditions within the MARFC service area.  Visit
https://www.drought.gov, https://www.cpc.ncep.noaa.gov and
https://www.weather.gov/marfc/wro_north for additional drought and
water supply information.  Assuming near-normal precipitation for the
next few months, no water supply shortages are anticipated anywhere
within the MARFC region through at least May, 2019.

Please visit the NWS MARFC homepage at
https://www.weather.gov/marfc or find us on Facebook at
https://www.facebook.com/nwsmarfc/?REF=aymt_homepage_panel
and on Twitter@nwsmarfc.

The next Winter/Spring Flood Outlook will be issued by this office in
two weeks, on or about March 6-7, 2019.

SK


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