Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA

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WATER SUPPLY/SPRING FLOOD OUTLOOK
NATIONAL WEATHER SERVICE SEATTLE WA
222 PM PDT FRI MAY 12 2017

...Western Washington Water Supply and Spring Snow Melt
Flood Potential...

Overview: the forecasts of water supply for the summer were for
above normal to much above normal for western Washington rivers.
This is due to abundant rain this winter season and a healthy
snowpack.

Flooding in western Washington is unlikely during the period of
mountain snowpack runoff, which peaks from April through June. This
year will be similar despite greater than normal snow pack. Based on
the current snowpack and expected precipitation and temperatures,
the threat of spring and summer snowmelt flooding in western
Washington is low as is typical.


PRECIPITATION SUMMARY
---------------------

April was the third month in a row with above normal precipitation
for all of Washington state, with some regions in the eastern half
receiving greater than 200 percent of normal for the month.  For a
few places, April was in the top ten for wettest April on record.
With the continued above normal precipitation, the values for the
water year remain above normal for the entire state as well.

The monthly percentage of normal for precipitation ranged from 124
percent in the western foothills of the Cascades to 168 percent on
the Washington coast.  The greatest amount of precipitation at the
climate stations for the mountains, coast and interior lowlands was
15.88 inches at Forks in the Olympics, 14.43 at Quillayute, and
10.45 inches at Grays River in the southwest interior.

The table below gives precipitation figures as a percent of normal
for regions of Washington.  The current water year began 1 October
2016 and ends 30 September 2017.

                        April     Water year      Past 3       Past 12
                         2017       to date       months        months
Western Washington
  Coast                   168          136          163           127
  Olympics                163          128          148           120
  Northwest Interior      129          126          160           113
  Puget Sound Lowlands    159          141          193           129
  Southwest Interior      133          130          165           122
  West Foothills Cascades 124          126          170           117
  Cascades West           130          129          168           121


Snowpack Conditions
-------------------

The snowpack was near to well above normal for all of western
Washington. As of May 12, the water content of the mountain snowpack
in the river basin groups ranged from 117 to 1126 percent of normal.

Snow depth levels from the Northwest Avalanche Center ranged from 91
to 159 percent of normal as of May 1.


Streamflows Summary
-------------------

Streamflows on western Washington rivers for last month roughly
ranged from above normal to much above normal. The current flows on
area rivers as of May 12 was not quite as high at as many locations
but remains above normal to much above normal.


Reservoir Storage Summary
-------------------------

Storage for Ross Reservoir for May 1 was at 57% of average. This low
value is likely a reflection of holding space open for the
expected large spring melt inflows.


Weather Outlook
---------------

The outlook for May and beyond for Washington state...the monthly
outlook for May is calling for equal chances of above, below or near
normal precipitation for nearly all of Washington, except for below
normal precipitation for the extreme southwest corner of the state.
The three month outlook for May through July is calling for equal
chances of above, below or near normal precipitation for the entire
state.


Water Supply Outlook
--------------------

Long range hydrologic models are forecasting near normal to much
above normal river flows and water supply for most rivers through
this spring and summer. Water supply forecasts for western
Washington range from a low of 95 percent for the Tolt River to 130
percent for the Dungeness River at grand mound. Almost all rivers
have forecasts above 100% of normal.

Water supply forecasts that include regulation are used for
locations where forecasts are listed below as regulated, for all
other locations forecasts are for natural volumes. Here are the
stream flow volume forecasts for specific rivers and sites as of May
11.

                           Water Supply Forecasts
                  Natural Flow Unless Otherwise Specified
                        (in thousands of acre feet)

River and Gauging Site            Period   Forecast   Normal  Percent
                                                      (1981-2010)
Nooksack River
   at North Cedarville            Apr-Sep      1286     1159     111

Skagit River
   near Concrete (regulated)      Apr-Sep      6402     5934     108

Baker River
   Upper Baker Reservoir Inflow   Apr-Sep       860      806     107

Sultan River
   Spada Lake Inflow              Apr-Sep       219      189     116

Tolt River
   Tolt Reservoir Inflow          Apr-Sep        44       46      95

Cedar River
   Chester Morse Lake Inflow      Apr-Sep       176      152     116

Green River
   Howard Hanson Dam Inflow       Apr-Sep       306      260     118

Nisqually River
   Alder Reservoir Inflow         Apr-Sep       441      378     117

Cowlitz River
   Mayfield Reservoir (regulated) Apr-Sep      2189     1835     119

Chehalis River
   near Grand Mound               Apr-Sep       495      390     127

Elwha River
   McDonald Bridge                Apr-Sep       517      472     109

Dungeness River
   near Sequim                    Apr-Sep       188      145     130

Wynoochee River
   Wynoochee Dam Inflow           Apr-Sep       124       98     126

NF Skokomish River
   Cushman Dam Inflow             Apr-Sep       246      191     129


Snow Melt
----------

At this point in May, there is greater than normal snow pack in
most areas at mid to upper elevations, with healthy amounts of water
being stored there. This will result in high spring runoff in the
rivers. Despite the rivers running high, the flows will be nowhere
near a flood threat. However, with rivers full this spring, they
will be more susceptible to flooding from spring rainstorms.

Climatology:
Rivers west of the Cascades crest usually reach their highest peak
flows during the winter season. The vast majority of River flooding
in western Washington, and almost all major floods, occur between
October and March. Heavy rainfall, rather than snow melt, is the
primary cause of these events.

The historical record does not show major flooding in western
Washington during the period when the mountain snowpack runs off.
The runoff from snow melt, even during unusually hot weather, is
small compared to the runoff during heavy winter rains. This is true
regardless of the size of the mountain snowpack. Rarely, under just
the right conditions of greater than normal snow pack, greater
coverage to lower elevations, and near record warm temperatures,
river flows might rise to near minor flood stage.

While flood producing rainfall is not common after March, moderate
to heavy rain in spring or summer, while rivers are swollen with
snow melt runoff, occasionally drive the most flood prone Rivers
above flood stage. Typically these are rivers such as the Skokomish
and Snoqualmie. Heavy rain in the spring or summer, when Ross Lake
is full, can also cause the Skagit River to flood. While these
floods are typically minor compared to the winter events, they
sometimes cause substantial damage to farm crops.


Spring and Summer Snow Melt Peak Flow Forecasts
------------------------------------------------

Here are the latest spring and summer crest forecasts for western
Washington rivers as of May 11. Statistically there is a 67 percent
chance that the actual spring crest will fall within the most likely
range.

River and Site          Flood Stage        Most Likely Range of
                                          the Spring/Summer Crest
Skagit River
  near Mt. Vernon         28.0 ft            21.2 ft to 23.4 ft

Stillaguamish River
  at Arlington            14.0 ft             5.4 ft to 6.1 ft

Snoqualmie River
  near Snoqualmie        20000 cfs          8140 cfs to 10570 cfs

Cowlitz River
  at Randle               18.0 ft            10.6 ft to 12.2 ft

SF Skokomish River
  near Union                                 903 cfs to  909 cfs

Elwha River
  at McDonald Bridge      20.0 ft            13.2 ft to 15.0 ft

Dungeness River
  near Sequim              7.0 ft             5.0 ft to 5.2 ft

Forecasts are selected from those prepared by the NWRFC.
For further details, graphics, and statistics regarding the water
supply forecasts visit:
https://www.nwrfc.noaa.gov/ws
https://www.nwrfc.noaa.gov/natural

For further details, graphics, and statistics regarding the peak
flow forecasts visit:
https://www.nwrfc.noaa.gov/peak/

The next water supply and spring flood outlook for western
Washington will be issued around the week of June 5.

$$

weather.gov/seattle
jbb



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