Flood Potential Outlook
Issued by NWS Seattle/Tacoma, WA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2
000
FGUS76 KSEW 072244
ESFSEW

PROBABILISTIC HYDROLOGIC OUTLOOK
NATIONAL WEATHER SERVICE SEATTLE WA
244 PM PST Thu Mar 7 2019

...SPRING AND SUMMER SNOW MELT FLOOD POTENTIAL IN WASHINGTON IS
LOWER THAN NORMAL FOR MOST AREAS OF WASHINGTON STATE...

Overview:

Based on the current less than normal snow pack, and expected
precipitation and temperatures, the threat of spring melt flooding
is lower than normal in most areas of eastern Washington, with the
exception of the southeast part of the state. For western
Washington, the flood risk is minimal as is normal. There is no
single river that can be pointed out as having the greatest risk for
spring melt flooding this year. In fact, there is a 10 percent or
less chance of exceeding flood stage for rivers in Washington.

There is, as is typical, a small spring flood risk for smaller
streams in eastern Washington most years since there could be minor
flooding with a sudden large warmup or the occurrence of
thunderstorms over those watersheds. The risk for small stream and
overland flooding at this time is about normal to slightly above
normal in the southeast. This is due to the large amount of low to
middle elevation snow and a larger coverage area than usual at these
elevations. There is no increased risk of ice jam flooding on the
east side.

Snow pack conditions:

The snowpack ranged from much below to near normal for most of
Washington as of March 7. The exception was southeast Washington
that was above normal. The water content of the mountain snowpack in
the basins that feed the major rivers east of the Cascade crest
ranged from 77 to 117 percent of normal with most basins between 90
and 106 percent. West of the Cascades, the water content of the
snowpack ranged from 72 to 91 percent of normal.

Snow depth amounts from the Northwest Avalanche Center sites in the
Washington Cascades ranged from 85 to 107 percent of normal 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.

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

While flood-producing rainfall is rare after March, moderate to
heavy rain in spring, while rivers are swollen with snow melt
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 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 6. 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 CREST
SNAKE RIVER
  NEAR ANATONE            20.0 FT           13.4 FT TO 16.2 FT

PEND OREILLE RIVER
  AT NEWPORT              95.0 KCFS       49.5 KCFS TO 64.5 KCFS

SPOKANE RIVER
  AT SPOKANE              27.0 FT           24.3 FT TO 24.7 FT

SIMILKAMEEN RIVER
  NEAR NIGHTHAWK          14.0 FT            8.2 FT TO  9.2 FT

OKANOGAN RIVER
  NEAR TONASKET           15.0 FT           10.8 FT TO 12.4 FT

METHOW RIVER
  NEAR PATEROS            10.0 FT            5.8 FT TO  6.5 FT

WENATCHEE RIVER
  AT PESHASTIN            13.0 FT            8.3 FT TO  9.4 FT

COLUMBIA RIVER BELOW
  PRIEST RAPIDS DAM       32.0 FT           19.1 FT TO 21.2 FT

YAKIMA RIVER
  AT HORLICK              36.0 FT           31.3 FT TO 31.8 FT

NACHES RIVER
  NEAR NACHES             17.8 FT           15.9 FT TO 16.6 FT

YAKIMA RIVER
  AT PARKER               10.0 FT            5.7 FT TO  6.4 FT

YAKIMA RIVER
  AT KIONA                13.0 FT            6.9 FT TO  7.9 FT

WALLA WALLA RIVER
  NEAR TOUCHET            13.0 FT            7.7 FT TO  9.2 FT

SKAGIT RIVER
  NEAR MT. VERNON         28.0 FT           19.7 FT TO 22.1 FT

STILLAGUAMISH RIVER
  AT ARLINGTON            14.0 FT            7.6 FT TO 10.6 FT

SNOQUALMIE RIVER
  NEAR SNOQUALMIE        20000 CFS         8400 CFS TO 13500 CFS

COWLITZ RIVER
  NEAR RANDLE             18.0 FT           11.1 FT TO 13.6 FT

S.F. SKOKOMISH RIVER
  NEAR UNION                               1400 CFS TO 4700 CFS

DUNGENESS RIVER
  NEAR SEQUIM              7.0 FT            4.4 FT TO  4.8 FT

$$

WEATHER.GOV/SEATTLE
jbb


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.