Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6
000
FGUS76 KSEW 071643 CCA
ESFSEW

Water Supply/Spring Flood Outlook
National Weather Service Seattle WA
942 AM PDT Thu May 7 2020

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

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

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 forecasts of water supply for the summer were for near
normal to much below normal for western Washington rivers. This is
mostly due to much lower than normal precipitation in the late
winter and early spring. Forecasts for most rivers were slightly
higher than last month due to some much needed rain the very end of
April and first week of May. Actual volumes will heavily depend on
the weather through May.

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. 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.


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

The below normal precipitation continued into April for the entire
state of Washington.  All of Eastern Washington and a few regions in
Western Washington did not get above 50 percent of normal for the
month.  As for the Water Year, most of Western Washington is near
normal except for the Olympics and Southwest Interior which are
below normal.

For Western Washington, the monthly percentage of normal for
precipitation ranged from 32 percent in the Olympics to 58 percent
in the Northwest Interior.  The greatest amount of precipitation at
the climate stations for the mountains, coast and interior lowlands
was 5.19 inches at Cedar Lake on the western slopes of the Cascades,
3.56 at Quillayute, and 2.92 inches at Monroe in the Puget Sound
Lowlands.

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                    43           95          118            94
  Olympics                 32           83          103            86
  Northwest Interior       58          108          133           111
  Puget Sound Lowlands     53           98          116            97
  Southwest Interior       45           83          105            87
  West Foothills Casca     57          102          126           102
  Cascades West            53          105          137           106


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

The snowpack was near to above normal for most of western
Washington, but there were some exceptions. As of May 7, the water
content of the mountain snowpack in the river basin groups ranged
from 96 to 114 percent of normal.

Snow depth levels for western Washington from the Northwest
Avalanche Center ranged from 27 to 106 percent of normal as of May
7.


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

Streamflows on western Washington rivers for last month were mostly
below normal, ranging from near normal flows down to much below
normal flows. The lowest values were west of the Puget Sound and
southwest area.


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

Storage for Ross Reservoir for May 1 was at 79% of average.


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

The outlook for May and beyond for Washington state: for the next
two weeks the outlook is for above normal precipitation.  The
monthly outlook for May calls for equal chances of above, below or
near normal precipitation.  The three month outlook for May through
July calls for greater chances of below normal precipitation.


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

Long range hydrologic models are forecasting mostly below normal
river flows and water supply for western Washington rivers through
this spring and summer. Water supply forecasts for Western
Washington range from a low of 61 percent for the Chehalis River at
Grand Mound to 106 percent for the Stillaguamish River. The lowest
and much below normal forecast volumes are for the Olympic Peninsula
and southwest region while the highest and near normal forecast
volumes are 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
May 6.


                           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      1097     1159      95

Skagit River
   near Concrete (regulated)      Apr-Sep      5601     5934      94

Samish River
   near Burlington                Apr-Sep        37       43      85

Baker River
   Upper Baker Reservoir Inflow    Apr-Sep       808      806     100

Sultan River
   Spada Lake Inflow               Apr-Sep       188      189      99

Pilchuck River
   near Snohomish                 Apr-Sep        63       96      66

Tolt River
   Tolt Reservoir                 Apr-Sep        38       46      83

Issaquah Creek
   near Issaquah                  Apr-Sep        21       25      86

Cedar River
   Chester Morse Lake Inflow       Apr-Sep       142      152      94

Green River
   Howard Hanson Dam Inflow        Apr-Sep       218      260      84

Nisqually River
   Alder Reservoir Inflow          Apr-Sep       351      378      93

Cowlitz River
   Mayfield Reservoir (regulated)  Apr-Sep      1591     1835      87

Chehalis River
   near Grand Mound               Apr-Sep       240      390      61

Calawah River
   near Forks                     Apr-Sep       105      158      66

Elwha River
   McDonald Bridge                Apr-Sep       385      472      82

Dungeness River
   near Sequim                    Apr-Sep       136      145      94

Wynoochee River
   Wynoochee Dam Inflow            Apr-Sep        72       98      74

NF Skokomish River
   Cushman Dam Inflow              Apr-Sep       146      191      77


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

There was near normal to slightly above normal snowpack as of May 7
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.

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
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 6. 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            19.4 ft to 21.2 ft

Stillaguamish River
  at Arlington            14.0 ft             4.9 ft to 6.4 ft

Snoqualmie River
  near Snoqualmie        20000 cfs          5600 cfs to 9100 cfs

Cowlitz River
  at Randle               18.0 ft            11.2 ft to 11.2 ft

White River
  at R Street             6500 cfs          3500 cfs to 4300 cfs

SF Skokomish River
  near Union                                3130 cfs to 4540 cfs

Elwha River
  at McDonald Bridge      20.0 ft            11.9 ft to 12.2 ft

Dungeness River
  near Sequim              7.0 ft             4.5 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 June 1.

$$

weather.gov/seattle
jbb



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.