Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA

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WATER SUPPLY/SPRING FLOOD OUTLOOK
NATIONAL WEATHER SERVICE SEATTLE WA
431 PM PDT WED MAY 23 2018

...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. Heavy rain from a late season
rainstorm can occasionally produce minor flooding.


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

After a below normal March for many, April brought much above normal
precipitation to all of Washington. For some areas, April 2018 was
in their top ten wettest Aprils.  For the water year, all of
Washington continues to be near to above normal.

In western Washington, the monthly percentage of normal for
precipitation ranged from 161 percent in the Western Foothills of
the Cascades to 209 percent in the Puget Sound Lowlands.  The
greatest amount of precipitation at the climate stations for the
mountains, coast and interior lowlands was 16.73 inches at Cedar
Lake, 12.47 at Aberdeen, and 14.78 inches at Grays River
respectively.

The table below gives precipitation figures as a percent of normal
for regions of Washington.  The current water year began 1 October
2017 and ends 30 September 2018.

                        April     Water year      Past 3       Past 12
                         2018       to date       months        months
Western Washington
  Coast                   175          109           98           105
  Olympics                162          108           80           101
  Northwest Interior      168          140          142           121
  Puget Sound Lowlands    209          118          112           111
  Southwest Interior      162          104           98           101
  West Foothills Cascades 161          119          116           107
  Cascades West           179          121          123           113


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

The snowpack was near to well above normal for all of western
Washington. As of May 22, the water content of the mountain snowpack
in the river basin groups ranged from 89 to 112 percent of normal.

Snow depth levels from the Northwest Avalanche Center ranged from 63
to 127 percent of normal as of May 1. All western Washington sites
except for Crystal Mountain were at or above normal.


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

Streamflows on western Washington rivers for last month ranged
from above normal to near daily high levels.


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

Storage for Ross Reservoir for May 1 was at 13% 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 calls for below normal precipitation.  The three
month outlook for May through July is also calling for below normal
precipitation for the 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 and Calawah
Rivers to 146 percent for the Pilchuck River. 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 22.

                           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      1282     1159     111

Skagit River
   near Concrete (regulated)      Apr-Sep      6547     5934     110

Samish River
   near Burlington                Apr-Sep        48       43     111

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

Sultan River
   Spada Lake Inflow              Apr-Sep       219      189     116

Pilchuck River
   near Snohomish                 Apr-Sep       140       96     146

Tolt River
   Tolt Reservoir Inflow          Apr-Sep        44       46      95

Issaquah Creek
   near Issaquah                  Apr-Sep        28       25     113

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      1713     1835      97

Chehalis River
   near Grand Mound               Apr-Sep       495      390     127

Calawah River
   near Forks                     Apr-Sep       150      158      95

Elwha River
   McDonald Bridge                Apr-Sep       517      472     109

Dungeness River
   near Sequim                    Apr-Sep       177      145     122

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 normal to 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. With the warm and dry May we have had, a lot of snow
has melted off. 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            19.7 ft to 20.8 ft

Stillaguamish River
  at Arlington            14.0 ft             3.8 ft to 5.2 ft

Snoqualmie River
  near Snoqualmie        20000 cfs          4760 cfs to  7400 cfs

Cowlitz River
  at Randle               18.0 ft            12.0 ft to 12.1 ft

White River
  at R Street             5500 cfs          3450 cfs to 3450 cfs

SF Skokomish River
  near Union                                 320 cfs to  570 cfs

Elwha River
  at McDonald Bridge      20.0 ft            11.8 ft to 12.0 ft

Dungeness River
  near Sequim              7.0 ft             4.6 ft to 4.9 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 4.

$$

weather.gov/seattle
jbb



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