Flood Potential Outlook
Issued by NWS Raleigh/Durham, NC

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Winter/Spring Flood Potential Outlook
National Weather Service Raleigh, NC
830 AM EST Fri Jan 19 2018

...THE CHANCE OF RIVER FLOODING OVER CENTRAL NC FOR THE WINTER/SPRING
IS BELOW NORMAL...

Precipitation Summary

The latter half of 2017 was quite dry, with only a few counties in
the central Coastal Plain receiving normal rainfall for the 6 month
period. The dryness was most pronounced over the Piedmont, where the
headwaters of the Tar, Neuse, and Cape Fear river basins received
less than 75 percent of normal rainfall (a deficit of 6-10 inches).
There was a fairly large swath across the heart of the Piedmont
which received less than 50% (a deficit of 10-12 inches) of normal.

No tropical systems provided widespread heavy rain during the fall,
and this same rainfall-deficit pattern emerges in the analysis of
rainfall over the past 3 months - the 2018 water year).

As might be surmised, we entered the mid-late summer with no drought
conditions analyzed over central NC by the U.S. Drought Monitor
(www.drought.gov). Drought crept inexorably across the area through
fall and early winter, and moderate drought (D1) now covers all but
a narrow swath of counties in RAH`s eastern hydrologic service
area - basically the Coastal Plain.

A welcome winter storm this past week produced widespread snow with
water equivalent ranging from a few tenths in the Coastal Plain to
around an inch of liquid water equivalent in the central and west.
The Piedmont (particulary the western Piedmont), where moderate
drought has been gradually worsening, was the beneficiary of the
heaviest snowfall, with reports ranging up to double digits over the
Cape Fear river basin.

See www.water.weather.gov for detailed rainfall analysis.

Precipitation and departure from normal:

          Precipitation (inches) and departure from normal
            for the water year beginning 1 October 2017

    Month                RDU            GSO        Fayetteville
------------------------------------------------------------------

  October             4.29/ 1.04     3.03/-0.10     2.76/-0.45
  November            1.28/-1.84     0.95/-2.16     0.54/-2.23
  December            2.29/-0.78     1.60/-1.38     2.97/ 0.32
  January to date     1.16/-0.88     1.98/ 0.17     0.62/-1.11

Total precip          9.02/-2.46     7.56/-3.47     6.89/-3.47
Percent normal            79             69             67
---------------------------------------------------------------

                  Begin     Actual   Normal  Departure  Percent
                  date       Pcpn     Pcpn   from norm  of norm

RALEIGH-DURHAM INTERNATIONAL AIRPORT (RDU)

 LAST   7 DAYS   01/12/2018   0.99    0.82      0.17      121%
 LAST  14 DAYS   01/05/2018   1.10    1.62     -0.52       68%
 LAST  30 DAYS   12/20/2017   1.58    3.24     -1.66       49%
 LAST  90 DAYS   10/21/2017   7.39    9.34     -1.95       79%
 LAST 180 DAYS   07/23/2017  17.10   21.53     -4.43       79%
 LAST 365 DAYS   01/19/2017  43.85   43.34      0.51      101%

PIEDMONT TRIAD INTERNATIONAL AIRPORT (GSO)

 LAST   7 DAYS   01/12/2018   1.88    0.72      1.16      261%
 LAST  14 DAYS   01/05/2018   1.98    1.43      0.55      138%
 LAST  30 DAYS   12/20/2017   2.44    2.94     -0.50       83%
 LAST  90 DAYS   10/21/2017   6.40    9.03     -2.63       71%
 LAST 180 DAYS   07/23/2017  13.74   20.48     -6.74       67%
 LAST 365 DAYS   01/19/2017  42.55   42.20      0.35      101%

FAYETTEVILLE AIRPORT (FAY)

 LAST   7 DAYS   01/12/2018   0.40    0.66     -0.26       61%
 LAST  14 DAYS   01/05/2018   0.62    1.37     -0.75       45%
 LAST  30 DAYS   12/20/2017   1.57    2.58     -1.01       61%
 LAST  90 DAYS   10/21/2017   5.55    7.94     -2.39       70%
 LAST 180 DAYS   07/23/2017  16.67   22.06     -5.39       76%
 LAST 365 DAYS   01/19/2017  40.43   44.46     -4.03       91%

Streamflow and lake levels

Streamflow on unregulated streams and rivers across central NC are
much-below-normal, with all gages reporting flows below the 10th
percentile, which is generally less than 25% of normal for this time
of year. Some modest improvement is expected over the next few days
as snowfall from a very recent winter storm melts.

See https://waterwatch.usgs.gov for additional details.

The major water supply and flood control reservoirs in central NC
are Falls Lake and B. Everett Jordan Lake, both operated by the U.S.
Army Corps of Engineers.

Falls Lake fell below its target elevation back in July of 2017, and
has been on a slow downward trend since that time. The lake is
currently hovering around 248 feet, which is ~3.5 feet below the
target elevation of 251.5 feet. Inflow into the lake is within the
top 10 lowest for Dec-Jan to date, with inflow records dating back
90 years.

B. Everett Jordan Lake also fell below target elevation back in July,
and has been falling steadily since then. The lake elevation is
currently slightly below 212 feet, which is 4 feet below the target
elevation of 216 feet. Inflow into Jordan for the Nov-Jan time frame
also ranks within the top 10 lowest on record, which dates back 85
years.

See www.epec.saw.usace.army.mil for additional details.

Short term forecast

Temperatures will moderate in the short term, with little chance of
significant precipitation over the next week. The next system which
appears capable of producing some significant rain would be late next
weekend - around 1/27. Given the ongoing dryness, it would require
around 2 inches of widespread liquid water to cause river flooding,
and this upcoming system is unlikely to produce sufficient
precipitation to produce river flooding.

Longer term precipitation outlook

We are currently in a weak to moderate La Nina phase of the El
Nino/Southern Oscillation (ENSO) cycle, which is expected to linger
through Spring. This is the `warm` ENSO phase, which typically
produces increased chances of warmer-than-normal temperatures and
less-than-normal precipitation. Given the persistent dryness in
place and unfavorable outlook for precipitation, the longer range
chance of river flooding is considered below normal.

Additional details and discussion may be found at the Climate
Prediction Center website: http://www.cpc.ncep.noaa.gov

In summary, current hydrologic conditions are significantly drier
than normal across central NC.

There are no hints of a system in the near-term which would be
capable of producing precipitation significant enough to produce
river flooding through early February. Similarly, there are no
climatological signals pointing towards prolonged wet periods in the
longer range outlook, so the chance of flooding through early Spring
is also below normal.

The next scheduled Winter/Spring flood outlook will be issued
on February 2nd.

For additional hydrologic or weather information, visit our website
at www.weather.gov/raleigh.

MLM



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