Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA

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Water Supply/Spring Flood Outlook
National Weather Service Seattle WA
1233 PM PDT Fri June 12 2020

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

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NOTICE: This seasonal text product is being considered for
elimination in lieu of the content being available on the internet
from the same official sources. If there are any comments or
questions about this product being eliminated, please contact Brent
Bower at the Seattle National Weather Service Office at
brent.bower@noaa.gov.

##################################################################

Overview: The latest forecasts of water supply for the summer were
for above normal to much below normal for western Washington rivers.
Volume forecasts have increased over last month for most locations
by up to 5 percent. This was mostly due to dry late winter and early
spring changing to a much wetter late spring into early June. Actual
volumes will not vary that much from forecasts after without a major
change in the weather through the summer.

Flooding in western Washington is unlikely during the period of
mountain snowpack runoff, which peaks from April through June. This
year, spring river flows will be at normal low levels or even below.
Based on the current snowpack and expected precipitation and
temperatures, the threat of spring snowmelt flooding in western
Washington is very low. However, heavy rain from a late season
rainstorm can occasionally produce minor flooding, which almost
happend the first of the month on the Skagit River.


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

After a dry start to spring, May brought near to above normal
precipitation for all of Washington state.  For many places across
the state, May 2020 will be in their top ten wettest Mays on record.
As for the Water Year, most of Western Washington is near or above
normal except for the Olympics and Southwest Interior which are
below normal.  Most of Eastern Washington is below normal for the
Water Year except for the Northeast region, and the Palouse and Blue
Mountains, which are near normal.

For Western Washington, the monthly percentage of normal for
precipitation ranged from 90 percent on the Coast to 144 percent in
the Puget Sound Lowlands.  The greatest amount of precipitation at
the climate stations for the mountains, coast and interior lowlands
was 10.10 inches at Cedar Lake on the western slopes of the
Cascades, 4.52 at Quillayute, and 5.89 inches at Mayfield 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
2019 and ends 30 September 2020.

                          May     Water year      Past 3       Past 12
                          2020      to date       months        months
Western Washington
  Coast                    90           95           63            97
  Olympics                108           86           63            89
  Northwest Interior      130          113           89           114
  Puget Sound Lowlands    144          100           83           103
  Southwest Interior      121           86           79            90
  West Foothills Cascades 127          104           88           105
  Cascades West           134          108           83           108

Eastern Washington
  East Slopes Cascades    127           81           53            86
  Okanogan                159           71           79            82
  Central Basin           194           71           92            75
  Northeast               183           93           92            94
  Palouse and Blue Mtn    293           99          125            93


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

The snowpack was near normal for most of western Washington, but
there were some exceptions. As of June 12, the water content of the
mountain snowpack in the river basin groups ranged from 92 to 109
percent of normal.


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

Streamflows on western Washington rivers for last month were mostly
normal or above, with scattered locations below normal. Isolated
gage locations reported much above normal and also much below
normal. This is a relecition of the above normal rainfall in May.


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

Storage for Ross Reservoir for June 1 was at 107% of average.


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

The outlook for June and beyond for Washington state...for the next
two weeks the outlook is for normal to below normal precipitation.
The monthly outlook for all of June is for greater chances of above
normal precipitation equal chances of above, below, or normal
temperatures. The three month outlook for July through September is
for greater chances for below normal precipitation and above normal
temperatures.


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

Long range hydrologic models are forecasting mostly near normal
river flows and water supply for western Washington rivers through
this spring and summer. Water supply forecasts for Western
Washington ranged from a low of 59 percent for the Newaukum River to
111 percent for the Skagit River near Concrete. The lowest and much
below normal forecast volumes were for the Olympic Peninsula and
southwest region while the highest and near normal forecast volumes
were for east of the Puget Sound and especially in the northern half.

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 12.


                           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      1131     1159      98

Skagit River
   near Concrete (regulated)      Apr-Sep      6598     5934     111

Samish River
   near Burlington                Apr-Sep        38       43      89

Baker River
   Upper Baker Reservoir Inflow    Apr-Sep       881      806     109

Sultan River
   Spada Lake Inflow               Apr-Sep       194      189     103

Pilchuck River
   near Snohomish                 Apr-Sep        84       96      88

Tolt River
   Tolt Reservoir                 Apr-Sep        42       46      92

Issaquah Creek
   near Issaquah                  Apr-Sep        26       25     104

Cedar River
   Chester Morse Lake Inflow       Apr-Sep       143      152      94

Green River
   Howard Hanson Dam Inflow        Apr-Sep       228      260      88

Nisqually River
   Alder Reservoir Inflow          Apr-Sep       358      378      95

Deschutes River
   near Rainier                   Apr-Sep        28       41      68

Cowlitz River
   Mayfield Reservoir (regulated)  Apr-Sep      1609     1835      88

Chehalis River
   near Grand Mound               Apr-Sep       246      390      63

Newaukum River
   near Chehalis                  Apr-Sep        58       80      59

Calawah River
   near Forks                     Apr-Sep       139      158      88

Elwha River
   McDonald Bridge                Apr-Sep       398      472      84

Dungeness River
   near Sequim                    Apr-Sep       130      145      89

Wynoochee River
   Wynoochee Dam Inflow            Apr-Sep        75       98      76

NF Skokomish River
   Cushman Dam Inflow              Apr-Sep       140      191      73


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

There was near normal to slightly above normal snowpack as of June
12 at the SNOTEL sites. There may be less snow than normal in lower
elevations. This will result in a spring runoff in the rivers that
will be near to below normal and little chance of flooding without
substantial rain. At this point in the summer with much of the snow
melt having occurred, there is even less chance of any flooding
during what is left of the melt season.

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 snowpack, 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
and Baker Lakes are 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 12. 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            20.0 ft to 20.0 ft

Stillaguamish River
  at Arlington            14.0 ft             4.3 ft to 5.1 ft

Snoqualmie River
  near Snoqualmie        20000 cfs          5100 cfs to 5900 cfs

Cowlitz River
  at Randle               18.0 ft             8.6 ft to  8.6 ft

White River
  at R Street             6500 cfs          2600 cfs to 2600 cfs

SF Skokomish River
  near Union                                 340 cfs to  590 cfs

Elwha River
  at McDonald Bridge      20.0 ft            11.4 ft to 11.4 ft

Dungeness River
  near Sequim              7.0 ft             4.2 ft to 4.5 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 1.

$$

weather.gov/seattle
jbb


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