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Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA
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ESFSEW

Water Supply/Spring Flood Outlook
National Weather Service Seattle WA
227 PM PDT Fri April 12, 2024

...Western Washington Water Supply and Spring Snowmelt Flood
Potential...


Summary: The latest forecasts of water supply for the summer were
for below to much below normal for western Washington rivers.

The snowpack is much below normal as well. As a result, there is
little to no chance of snowmelt flooding in western Washington as is
typical. In addition, water supply forecasts could drop even further
through the spring.


PRECIPITATION SUMMARY

For Western Washington in March, precipitation was well below
normal. The percent of normal precipitation was 35 to 45 percent for
the Cascades, 45 to 65 for the lowlands from the Puget Sound
eastward, and 50 to 85 percent for the Olympic Peninsula.


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

The snowpack was much below normal for western Washington as of
April 11. West of the Cascades, the basin average water content of
the snowpack ranged from 60 to 68 percent of the median.

Snow depths for Northwest Avalanche Center locations in western
Washington as of April 1 ranged from 52 to 95 percent of normal,
with all but White Pass below 80 percent.


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

Streamflows on western Washington rivers for last month were mostly
normal to below normal. The streams in the central Cascades were
below normal.


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

Storage for Reservoirs as of the first of April as a percentage of
period of record median:
     Ross Reservoir 130%
     Upper Baker Reservoir 130%
     Howard Hanson Reservoir 150%
     Mayfield Lake 97%


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

The outlook for April and beyond for Washington state, for the next
two weeks the outlook is for below normal precipitation.  The
monthly outlook for April calls for greater chances of below normal
precipitation.  The three month outlook for May through July is for
greater odds of below normal precipitation for Washington state.



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

Long range hydrologic models are forecasting below to much below
normal river flows and water supply for western Washington rivers
through summer. These forecasts are lower than last month`s
forecasts and could continue to drop during the spring.  Water
supply forecasts for Western Washington ranged from a low of 62%
percent for the North Fork Skokomsih River to 88 percent for the
Puyallup River

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
April 11.

Note that the Normal climatology is now the 1991-2020 period.


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


River and Gauging Site            Period   Forecast   Normal  Percent
                                                   (1991-2020)
Nooksack River
   at North Cedarville            Apr-Sep       936     1171      80

Skagit River
   near Concrete (regulated)      Apr-Sep      4423     5955      74

Samish River
   near Burlington                Apr-Sep        34       46      75

Baker River
   Upper Baker Reservoir Inflow    Apr-Sep       684      823      83

Sultan River
   Spada Lake Inflow               Apr-Sep       148      190      78

Pilchuck River
   near Snohomish                 Apr-Sep        71      101      71

Tolt River
   Tolt Reservoir                 Apr-Sep        40       46      87

Issaquah Creek
   near Issaquah                  Apr-Sep        17       26      65

Snoqualmie River
   near Carnation                 Apr-Sep       875     1040      84

Cedar River
   Chester Morse Lake Inflow       Apr-Sep       118      136      87

Green River
   Howard Hanson Dam Inflow        Apr-Sep       181      262      69

Puyallup River
   at Puyallup                    Apr-Sep       958     1092      88

Nisqually River
   Alder Reservoir Inflow          Apr-Sep       316      395      80

Deschutes River
   near Rainier                   Apr-Sep        29       41      70

Cowlitz River
   Mayfield Reservoir (regulated)  Apr-Sep      1601     1864      86

Chehalis River
   near Grand Mound               Apr-Sep       274      406      68

Newaukum River
   near Chehalis                  Apr-Sep        58       85      68

Calawah River
   near Forks                     Apr-Sep       122      154      79

Elwha River
   McDonald Bridge                Apr-Sep       378      465      81

Dungeness River
   near Sequim                    Apr-Sep       106      145      73

Wynoochee River
   Wynoochee Dam Inflow            Apr-Sep        82       99      83

NF Skokomish River
   Cushman Dam Inflow              Apr-Sep        95      183      62


Spring and Summer snowmelt and Flooding Climatology of Spring
Floods:

Climatology of Spring Floods:

Flooding in western Washington is unlikely during the period of
mountain snowpack runoff, which peaks from April through June.
Rivers west of the Cascades crest usually reach their highest peak
flows during the winter season from the heavy rain from winter
storms. The vast majority of river flooding in western Washington,
and almost all major flooding, occurs between November and March.
Heavy rainfall, rather than snowmelt, 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 snowmelt, 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.

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

Forecasts: As in most years, the threat for spring snowmelt flooding
this year is extremely low, with no snowmelt flooding expected based
on river modeling, the current snow pack, combined with the expected
precipitation and temperatures. There is less than a 5% chance of
exceeding flood stage on the major rivers in western Washington this
spring with the exception of the Samish, Snoqualmie, and White
Rivers which have only a 5% chance.

Here are the peak flow forecasts for April 11 through September 31
for some western Washington rivers. Statistically,there is a 70
percent chance that the actual spring crest will exceed the lower
value and a 30 percent chance of exceeding the higher value.

RIVER AND SITE          FLOOD STAGE        MOST LIKELY RANGE OF
                                             THE SPRING CREST
SKAGIT RIVER
  NEAR MT. VERNON         28.0 FT           18.0 FT TO  21.0FT

STILLAGUAMISH RIVER
  AT ARLINGTON            14.0 FT            4.9 FT TO  6.2 FT

SNOQUALMIE RIVER
  NEAR SNOQUALMIE        20000 CFS         7790 CFS TO 11700CFS

WHITE RIVER
  MUD MTN DAM INFLOW                       3180 CFS TO 4600 CFS

COWLITZ RIVER
  NEAR RANDLE             18.0 FT            9.3 FT TO 11.0 FT

S.F. SKOKOMISH RIVER
  NEAR UNION                                770 CFS TO 1270 CFS

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 for western Washington will be issued the week
of May 6.

$$

weather.gov/seattle
jbb
National Weather Service Weather Forecast Office
