Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA

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Water Supply/Spring Flood Outlook
National Weather Service Seattle WA
344 PM PDT Mon May 20 2019

...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. For example,
https://www.erh.noaa.gov/sew/hydro/droughtsew-A1-php.
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.

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Overview: Despite water year precipitation in the 75% to 95% range,
low water supply forecasts abound due to being dry and warn since
the first of the year. Abundant precipitation early in the water
year helps little for the following summer. The forecasts of water
supply for the summer were for below normal to much below normal for
western Washington rivers. This is mostly due to lower than normal
precipitation in recent months and a very low snow pack. If the
spring and summer were to turn out to be unusually wet and cool,
that would alter the outcome.

Flooding in western Washington is unlikely during the period of
mountain snowpack runoff, which peaks from April through June. This
year, spring flooding will be even less likely than usual. Based on
the current snowpack and expected precipitation and temperatures,
the threat of spring and summer snowmelt flooding in western
Washington is extremely low. However, heavy rain from a late season
rainstorm can occasionally produce minor flooding.


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

In contrast to the dry March, April saw near to above normal
precipitation for Washington state.  For the water year, most of
Washington state was near to below normal, with a majority of the
below normal regions on the west side. Without the abundant
precipitation early in the water year, these numbers would be even
lower.

For Western Washington, the monthly percentage of normal for
precipitation ranged from 90 percent on the Coast to 143 percent in
the Northwest Interior.  The greatest amount of precipitation at the
climate stations for the mountains, coast and interior lowlands was
12.54 inches at Cedar Lake on the western slopes of the Cascades,
8.27 at Quillayute, and 6.00 inches at Clearbrook in the Northwest
Interior.

The table below gives precipitation figures as a percent of normal
for regions of Washington.  The current water year began 1 October
2018 and ends 30 September 2019.

                          APR     Water year      Past 3       Past 12
                         2019       to date       months        months
Western Washington
  Coast                    90           83           55            79
  Olympics                 95           82           53            80
  Northwest Interior      143           95           86            87
  Puget Sound Lowlands     92           82           71            78
  Southwest Interior      101           76           76            71
  West Foothills Casca    123           83           77            78
  Cascades West           127           86           76            81


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

The snowpack was below to much below normal for all of western
Washington. As of May 20, the water content of the mountain snowpack
in the river basin groups ranged from 22 to 53 percent of median.

As of May 1,  the snow depth levels for western Washington from the
Northwest Avalanche Center ranged from 0 to 75 percent of normal
with a median value of 58 percent.


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

Streamflows on western Washington rivers picked up in April to
mostly near normal. Even with the increase in precipitation in
April, a number of gages still showed well below normal stream
flows, while a small few showed above normal stream flows..


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

Storage for Ross Reservoir for May 1 was at 72% of average.


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

The outlook for May and beyond for Washington state: the monthly
outlook for May is for below normal precipitation for much of
Washington, except for equal chances of above, below or near normal
precipitation for the extreme northeast corner of state.  The three
month outlook for May through July calls for below precipitation for
Western Washington.

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

Long range hydrologic models are forecasting below normal to much
below normal river flows and water supply for rivers through this
spring and summer. Water supply forecasts for western Washington
range from a low of 58 percent for the North Fork Skokomish River to
95 percent for Issaquah Creek. Most locations had forecasts between
60% and 80% of normal. Losers way outnumbered gainers as the percent
of normal dropped for most gages over last month by as much as 15%
to 25%.

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 20.

                           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       797     1159      69

Skagit River
   near Concrete (regulated)      Apr-Sep      4538     5934      76

Samish River
   near Burlington                Apr-Sep        29       43      68

Baker River
   Upper Baker Reservoir Inflow   Apr-Sep       647      806      80

Sultan River
   Spada Lake Inflow              Apr-Sep       129      189      68

Pilchuck River
   near Snohomish                 Apr-Sep        72       96      75

Tolt River
   Tolt Reservoir                 Apr-Sep        34       46      73

Issaquah Creek
   near Issaquah                  Apr-Sep        24       25      95

Cedar River
   Chester Morse Lake Inflow      Apr-Sep       111      152      73

Green River
   Howard Hanson Dam Inflow       Apr-Sep       204      260      79

Nisqually River
   Alder Reservoir Inflow         Apr-Sep       326      378      86

Cowlitz River
   Mayfield Reservoir (regulated) Apr-Sep      1467     1835      80

Chehalis River
   near Grand Mound               Apr-Sep       235      390      60

Calawah River
   near Forks                     Apr-Sep        96      158      61

Elwha River
   McDonald Bridge                Apr-Sep       300      472      64

Dungeness River
   near Sequim                    Apr-Sep       114      145      79

Wynoochee River
   Wynoochee Dam Inflow           Apr-Sep        67       98      68

NF Skokomish River
   Cushman Dam Inflow             Apr-Sep       110      191      58


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

As of the first of May, there is less than normal snow pack. This
will result in a spring runoff in the rivers that will be below
normal and no 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 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 20, for today through September 1. The
peak flow forecasts are all substantially below flood levels.
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            16.6 ft to 18.3 ft

Stillaguamish River
  at Arlington            14.0 ft             2.5 ft to 4.0 ft

Snoqualmie River
  near Snoqualmie        20000 cfs          4053 cfs to  6348 cfs

Cowlitz River
  at Randle               18.0 ft             8.3 ft to  9.1 ft

White River
  at R Street             5500 cfs          2696 cfs to 3346 cfs

SF Skokomish River
  near Union                                 238 cfs to  571 cfs

Elwha River
  at McDonald Bridge      20.0 ft            10.9 ft to 11.4 ft

Dungeness River
  near Sequim              7.0 ft             4.4 ft to 4.7 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 3.

$$

weather.gov/seattle
jbb



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