Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA

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Water Supply/Spring Flood Outlook
National Weather Service Seattle WA
841 AM PDT June 20 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 an above normal
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
---------------------

May brought near normal precipitation for most of Washington. There
were a couple of regions on the western side of the Cascades that
saw above normal precipitation, while one region on each side of the
Cascades saw below normal precipitation for the month. For the water
year, all of Washington state remains above normal.

The monthly percentage of normal for precipitation ranged from 82
percent in the Olympic Mountains to 121 percent in the Puget Sound
lowlands.  The greatest amount of precipitation at the climate
stations for the mountains, coast and interior lowlands was 8.20
inches at Cedar Lake on the western slopes of the Cascades, 5.15 at
Quillayute, and 5.29 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.

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.

                       May       Water year    Past 3      Past 12
                      2017        to date      months       months
Western Washington
Coast                   109          135         176         130
Olympics                 82          125         145         122
Northwest Interior      108          124         147         117
Puget Sound Lowlands    121          140         171         133
Southwest Interior      114          129         153         125
West Foothills Cascades  85          124         151         120
Cascades West            96          127         161         123


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

The snowpack was near to well above normal for all of western
Washington. As of May 31, the water content of the mountain snowpack
in the river basin groups ranged from 114 to 188 percent of normal.


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

Streamflows on western Washington rivers for May roughly ranged from
above normal to much above normal. The current flows on area rivers
as of June 14 was not quite as high at as many locations but remains
near normal to above normal.


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

Storage for Ross Reservoir for June 1 was at 74% of average, with
much snow yet to melt.


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

The outlook for June and beyond for Washington state, the monthly
outlook was calling for below normal precipitation for the western
half of the state.  The eastern half had equal chances of above,
below or near normal precipitation, except for above normal
precipitation for the extreme eastern part. The three month outlook
for June through August was calling for equal chances of above,
below or near normal precipitation for the entire state.


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

Long range hydrologic models were forecasting near normal to much
above normal river flows and water supply for most rivers through
this spring and summer. The forecasts for most sites were higher
than last month by roughly 5-10%. Water supply forecasts for western
Washington range from a low of 92 percent for the Tolt River to 150
percent for the Wynoochee River inflow to the dam. Almost all rivers
have forecasts for above normal volumes.

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
June 19.

                       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       1329    1159     115

Skagit River
  near Concrete (regulated)      Apr-Sep       6712    5934     113

Baker River
  Upper Baker Reservoir Inflow   Apr-Sep        899     806     112

Sultan River
  Spada Lake Inflow              Apr-Sep        207     189     109

Tolt River
  Tolt Reservoir Inflow          Apr-Sep         41      46      92

Cedar River
  Chester Morse Lake Inflow      Apr-Sep        179     152     118

Green River
  Howard Hanson Dam Inflow       Apr-Sep        329     260     126

Nisqually River
  Alder Reservoir Inflow         Apr-Sep        472     378     125

Cowlitz River
  Mayfield Reservoir (regulated) Apr-Sep       2378    1835     130

Chehalis River
  near Grand Mound               Apr-Sep        523     390     134

Elwha River
  McDonald Bridge                Apr-Sep        495     472     105

Dungeness River
  near Sequim                    Apr-Sep        182     145     126

Wynoochee River
  Wynoochee Dam Inflow           Apr-Sep        147      98     150

NF Skokomish River
  Cushman Dam Inflow             Apr-Sep        238     191     125


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

At this point in June, there is a 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 a longer summer runoff in
the rivers with the potential for high flows if we get a hot spell.
But as we get to the end of this month further higher peaks will no
longer be likely. Despite the rivers running high, the flows will be
nowhere near a flood threat. However, with rivers full of snow melt,
they will be more susceptible to flooding from 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 June 19. 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
                                            (Probable Crest Date)
Skagit River
  near   Mt. Vernon       28.0 ft            18.8 ft to 20.2 ft

Stillaguamish River
  at Arlington            14.0 ft             3.2 ft to 5.4 ft

Snoqualmie River
  near Snoqualmie        20000 cfs          3820 cfs to  6700 cfs

Cowlitz River
  at Randle               18.0 ft             9.5 ft to  9.5 ft
                                                  (June 21)

SF Skokomish River
  near Union                                 570 cfs to  820 cfs

Elwha River
  at McDonald Bridge      20.0 ft            12.6 ft to 13.2 ft

Dungeness River
  near Sequim              7.0 ft             4.7 ft to 4.8 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 July 3.

$$

weather.gov/seattle
jbb



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