Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA

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PROBABILISTIC HYDROLOGIC OUTLOOK
NATIONAL WEATHER SERVICE SEATTLE WA
1140 AM PDT Thu Mar 11 2024

...SPRING AND SUMMER SNOWMELT FLOOD POTENTIAL IN WASHINGTON IS LOWER
THAN NORMAL FOR WASHINGTON...

The threat for spring melt flooding is lower than normal based on
river modeling, the current low snow pack, combined with the
expected precipitation and temperatures. There is generally less
than a 10% chance of exceeding flood stage on the major rivers in
Washington this spring.

The risk for small stream and overland flooding at this time is
below normal. There is a small spring flood risk for smaller streams
in eastern Washington most years since there could be a sudden large
warmup or the occurrence of thunderstorms over those watersheds that
may result in minor flooding. There is no increased risk of ice jam
flooding on the east side. For western Washington, the flood risk is
minimal as is normal. However, is a low chance of river flooding
during the spring but that is due to moderate to heavy rainstorms
bringing rivers to flood, and not from snowmelt.

Snow pack conditions: the snowpack ranged from well below normal to
below normal for most of Washington, with a few isolated gages
showing above normal as of March 11. The average water content of
the mountain snowpack in the basins that feed the major rivers east
of the Cascade crest ranged from 63 to 89 percent of normal with
most basins between 68 and 87 percent. West of the Cascades, the
water content of the snowpack ranged from 57 to 69 percent, with the
Olympics being the lowest.

Snow depth amounts from the Northwest Avalanche Center sites in the
Washington Cascades ranged from 66 to 113 percent of normal, with
most sites below 100 percent as of March 1.


Climatology of Spring Floods:

East of the Cascade Crest

Many rivers east of the Cascade crest reach their annual peak flow
in late spring or early summer when the mountain snowpack melts and
runs off. The snowpack usually reaches its annual maximum in April,
and the rivers typically crest between mid may and mid July. As a
general rule the larger the snowpack is at the end of the season,
the higher the crests will be. When spring snowmelt flooding does
occur, it is usually the combination of a rapid warm up of hot
temperatures and a much greater than normal snow pack. But it can
happen with lower snow packs as well due to the very large amounts
of snow typically in the Washington mountains.

Flooding during the snowmelt season can occur anywhere when heavy
rain falls in a river basin if the rain is intense or lasts long
enough. Typically this can occur with thunderstorms. This is
especially true if the rain falls during a time when the streams are
running higher due to snowmelt runoff.


West of the Cascade crest

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 floods, occur 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 rare after March, moderate to
heavy rain in spring, while rivers are swollen with snowmelt
runoff, occasionally 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 spring/summer
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:

Here are the latest spring and summer crest forecasts for some
Washington rivers as of March 11. 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
SNAKE RIVER
  NEAR ANATONE            20.0 FT           12.3 FT TO 14.9 FT

PEND OREILLE RIVER
  AT NEWPORT              95.0 KCFS       52.7 KCFS TO 63.1 KCFS

SPOKANE RIVER
  AT SPOKANE              27.0 FT           23.7 FT TO 24.3 FT

SIMILKAMEEN RIVER
  NEAR NIGHTHAWK          14.0 FT            8.2 FT TO  9.1 FT

OKANOGAN RIVER
  NEAR TONASKET           15.0 FT           10.5 FT TO 12.0 FT

METHOW RIVER
  NEAR PATEROS            10.0 FT            5.8 FT TO  6.7 FT

WENATCHEE RIVER
  AT PESHASTIN            13.0 FT            8.3 FT TO  9.5 FT

STEHEKIN RIVER
  AT STEHEKIN             24.O FT           22.0 FT TO 22.7 FT

COLUMBIA RIVER BELOW
  PRIEST RAPIDS DAM       32.0 FT           19.4 FT TO 21.4 FT

YAKIMA RIVER
  AT HORLICK              36.0 FT           31.4 FT TO 31.8 FT

NACHES RIVER
  NEAR NACHES             17.8 FT           16.2 FT TO 16.8 FT

YAKIMA RIVER
  AT PARKER               10.0 FT            5.9 FT TO  6.5 FT

YAKIMA RIVER
  AT KIONA                13.0 FT            7.2 FT TO  8.0 FT

WALLA WALLA RIVER
  NEAR TOUCHET            13.0 FT            7.4 FT TO  9.2 FT

SKAGIT RIVER
  NEAR MT. VERNON         28.0 FT           19.0 FT TO 21.4 FT

STILLAGUAMISH RIVER
  AT ARLINGTON            14.0 FT            6.0 FT TO  7.9 FT

SNOQUALMIE RIVER
  NEAR SNOQUALMIE        20.0 KCFS         9.1 KCFS TO 13.8 KCFS

COWLITZ RIVER
  NEAR RANDLE             18.0 FT            9.7 FT TO 12.4 FT

S.F. SKOKOMISH RIVER
  NEAR UNION                               N/A KCFS TO 5.7 KCFS

DUNGENESS RIVER
  NEAR SEQUIM              7.0 FT            4.3 FT TO  4.6 FT

$$

Forecasts are selected from those prepared by the NWRFC.

For further details, graphics, and statistics regarding the peak
flow forecasts visit: https://www.nwrfc.noaa.gov/peak/

&&

WEATHER.GOV/SEATTLE
jbb


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