Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA
Issued by NWS Seattle/Tacoma, WA
576
FGUS76 KSEW 101702
ESFSEW
Water Supply/Spring Flood Outlook
National Weather Service Seattle WA
1002 AM PDT Fri May 10, 2024
...Western Washington Water Supply and Spring Snowmelt Flood
Potential...
Summary: The latest forecasts of water supply for the summer were
lower than last month`s forecasts and were for below to much below
normal for western Washington rivers. In addition, the water supply
forecasts could drop even further through the spring.
The snowpack was much below normal as well. As a result, there is
little to no chance of snowmelt flooding in western Washington as is
typical.
PRECIPITATION SUMMARY
For Western Washington in April, precipitation was well below
normal. The percent of normal precipitation was generally 30 to 70
percent for the Cascades, 20 to 80 percent for the Olympic
Peninsula, and 30 to 70 for the western Washington lowlands with a
few areas near 100 percent north of King County.
Snowpack Conditions
-------------------
The snowpack was much below normal for western Washington as of May
10. West of the Cascades, the basin average water content of the
snowpack ranged from 60 to 67 percent of the median.
Snow depths for Northwest Avalanche Center locations in western
Washington as of May 1 ranged from 33 to 98 percent of normal, with
all but White Pass around 80 percent or lower.
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 May as a percentage of
period of record median:
Ross Reservoir 123%
Upper Baker Reservoir 106
Howard Hanson Reservoir 125
Mayfield Lake 100
Weather Outlook
---------------
The outlook for May and beyond for Washington state: for the next
two weeks and the month of May, month of May is for equal chances of
above or below normal precipitation and temperature. The three month
outlook for June through August is for greater odds of below normal
precipitation and above normal temperatures.
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 once again lower than last
month`s forecasts and was generally a few percent to 10 percent. The
forecasts could continue this trend and drop further during the
spring. The forecast Water supply forecasts for Western Washington
ranged from a low of 65% percent for the Issaquah Creek to 83
percent for the Snoqualmie 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
May 10.
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 904 1171 77
Skagit River
near Concrete (regulated) Apr-Sep 4042 5955 68
Samish River
near Burlington Apr-Sep 35 46 76
Baker River
Upper Baker Reservoir Inflow Apr-Sep 613 823 74
Sultan River
Spada Lake Inflow Apr-Sep 147 190 77
Pilchuck River
near Snohomish Apr-Sep 73 101 73
Tolt River
Tolt Reservoir Apr-Sep 38 46 82
Issaquah Creek
near Issaquah Apr-Sep 17 26 65
Snoqualmie River
near Carnation Apr-Sep 864 1040 83
Cedar River
Chester Morse Lake Inflow Apr-Sep 111 136 81
Green River
Howard Hanson Dam Inflow Apr-Sep 173 262 66
Puyallup River
at Puyallup Apr-Sep 861 1092 79
Nisqually River
Alder Reservoir Inflow Apr-Sep 277 395 70
Deschutes River
near Rainier Apr-Sep 27 41 65
Cowlitz River
Mayfield Reservoir (regulated) Apr-Sep 1477 1864 79
Chehalis River
near Grand Mound Apr-Sep 292 406 72
Newaukum River
near Chehalis Apr-Sep 62 85 73
Calawah River
near Forks Apr-Sep 118 154 77
Elwha River
McDonald Bridge Apr-Sep 355 465 76
Dungeness River
near Sequim Apr-Sep 101 145 69
Wynoochee River
Wynoochee Dam Inflow Apr-Sep 79 99 79
NF Skokomish River
Cushman Dam Inflow Apr-Sep 135 183 74
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, and in combination 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.
Here are the peak flow forecasts for May 10 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 16.0 FT TO 19.0FT
STILLAGUAMISH RIVER
AT ARLINGTON 14.0 FT 3.8 FT TO 5.7 FT
SNOQUALMIE RIVER
NEAR SNOQUALMIE 20000 CFS 5890 CFS TO 10500CFS
WHITE RIVER
R St. Bridge 2770 CFS TO 4160 CFS
COWLITZ RIVER
NEAR RANDLE 18.0 FT 9.1 FT TO 9.4 FT
S.F. SKOKOMISH RIVER
NEAR UNION 450 CFS TO 770 CFS
DUNGENESS RIVER
NEAR SEQUIM 7.0 FT 4.2 FT TO 4.4 FT
$$
Forecasts are selected from those prepared by the NWRFC.
For further details, graphics, and statistics regarding the water
supply forecasts visit:
https://www.nwrfc.noaa.gov/ws
https://www.nwrfc.noaa.gov/natural
For further details, graphics, and statistics regarding the peak
flow forecasts visit:
https://www.nwrfc.noaa.gov/peak/
The next water supply for western Washington will be issued the week
of June 3.
$$
weather.gov/seattle
jbb