Washington State Forecast Discussion

Western Washington


736
FXUS66 KSEW 180402
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
902 PM PDT Thu Oct 17 2024

.UPDATE...No changes to the overall forecast. Please see the
aviation and marine sections below for updates to those forecasts.

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.PREV DISCUSSION... /issued 324 PM PDT Thu Oct 17 2024/

.SYNOPSIS...Weak surface high pressure will build Thursday night
into Friday morning providing a brief break from precipitation.
Significant rainfall is expected this weekend as an atmospheric
river is set to impact the region. Clear skies and drier
conditions with below normal temperatures return next week.

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.SHORT TERM /TONIGHT THROUGH SUNDAY/...Radar and satellite show
partly cloudy skies and a few lingering light showers moving
through the region. A weak high pressure will build Thursday night
into Friday morning giving way for a temporary break from
precipitation.

The first significant weather system of the season is set to
occur this weekend as an atmospheric river is forecasted to impact
the region Friday and through the weekend. Ensemble guidance has
the bulk of the moisture situated to the north in B.C on Friday
and Saturday. This will limit the majority of the heaviest
precipitation to the northern coast, Olympic Peninsula, and
Northern Cascades. The northern coast and Olympic Peninsula can
expect to see around 4-8 inches. Rain amounts in the Cascades and
northern interior are forecasted to receive up to 2-4 inches. An
upper level trough gliding down from Alaska will gradually push
the atmospheric river southward and bring precipitation down to
the interior regions on Sunday. The interior regions can expect to
see 1-2 inches of rainfall.

The main concern with this system will be the winds. Very gusty
conditions are expected with this system on Friday and Saturday
with gusts up to 30-40 mph expected for the northern coast and
north interior, while other locations may see gusts up to 15-30
mph. In addition, some of the mountain peaks may receive a couple
inches of snow. A light increase in temperature can be expected
this weekend with highs in the low to mid 60s.

.LONG TERM /MONDAY THROUGH THURSDAY/...Rain coverage will begin
to decrease in our area as an upper level trough moving east will
begin to push the atmospheric river out of the region on Monday.
Models show a cut off low developing Tuesday morning off the coast
of California leading to split flow over our region promoting
drier conditions through mid-week. The majority of models are
hinting at an upper level ridge building over the west CONUS.
Temperatures will remain below normal through mid-week in the 50s.


The next round of precipitation seems to be setting up
to arrive possibly as early as Thursday night as ensemble
guidance is hinting at the upper level ridge shifting east as an
upper level trough approaches the west coast. However, at this
time there is uncertainty on the timing of this event.

MGF

&&

.AVIATION...North to northwesterly flow will persist aloft
tonight as an upper level ridge nudges into the region and an
upper trough continues to push off to the east. Light onshore
flow will persist in the low levels through tonight. Flow will
then become southerly and increase Friday as a warm front
approaches the region.

VFR conditions will continue into Friday morning. Rain associated
with the warm front looks to approach the coast between 12-15Z
and gradually move inland into the interior between 17-19Z.
Visibilities may decrease to MVFR at times in rain, however
ceilings generally will remain VFR into Friday afternoon.
Southerly winds will increase Friday afternoon and evening, with
gusts to 20-25 kt possible. Terminals north of KPAE could see
gusts to 35-40 kt at times.

KSEA...VFR conditions to continue into Friday morning. Rain
looks to approach the terminal between 17-19Z Friday. Ceilings
will likely remain VFR through the afternoon and trend to MVFR
during the evening. Light and variable wind this afternoon will
become southerly again this evening, increasing to 8-12 kts Friday
afternoon. Gusts to 20-25 kt Friday afternoon and evening will be
possible. 14

&&

.MARINE...A weak system will continue to dissipate over the
region this afternoon as high pressure builds into the coastal
waters. Winds across the coastal waters have eased, but with seas
still hovering between 8-10 ft, have opted to keep the small craft
advisory in place through the remainder of the afternoon hours.
The high pressure system over the coastal waters will weaken
tonight as a warm front approaches and moves into the northern
portion of the area waters on Friday. The warm front will then
stall into Saturday, before lifting north as the trailing cold
front approaches the waters on Sunday. The cold front will then
move through the area waters Sunday night. High pressure then
looks to rebuild over the coastal waters early next week.

Gale force winds are expected with the warm front over the
coastal waters as well as the entrances to the Strait of Juan de
Fuca, Admiralty Inlet, and the Northern Inland waters Friday
afternoon into early Saturday morning.

Seas hovering at 8-10 ft this afternoon will subside to 6-8 feet
tonight. Seas will rise again over the weekend towards 11-14 ft
before subsiding back towards 6-8 ft late Sunday into Monday.
14

&&

.HYDROLOGY...The first atmospheric river of the fall season is on
track to bring periods of moderate to heavy rainfall to western
Washington over the weekend, which will cause sharp rises to area
rivers. Forecast models continue to show potential for 3 to 6
inches of rainfall over the Olympic Peninsula and North Cascades,
with 1 to 2 inches across the lowlands. Despite the significant
incoming rainfall, area rivers are not expected to rise into
action stage or reach minor flood thresholds, though river
conditions will continue to be monitored closely.

Lindeman

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Wind Advisory from 2 PM Friday to 5 AM PDT Saturday for
Admiralty Inlet Area-Everett and Vicinity-San Juan County-
Western Skagit County-Western Whatcom County.

Wind Advisory from 11 AM Friday to 5 AM PDT Saturday for North
Coast.

PZ...Gale Warning from 6 PM Friday to 2 AM PDT Saturday for West
Entrance U.S. Waters Strait Of Juan De Fuca.

Gale Warning from 2 PM Friday to 2 AM PDT Saturday for Admiralty
Inlet-Coastal Waters From Cape Flattery To James Island 10
To 60 Nm-Coastal Waters From Cape Flattery To James Island
Out 10 Nm-Coastal Waters From James Island To Point
Grenville 10 To 60 Nm-Coastal Waters From James Island To
Point Grenville Out 10 Nm-Coastal Waters From Point
Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters
From Point Grenville To Cape Shoalwater Out 10 Nm-East
Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland
Waters Including The San Juan Islands.

Small Craft Advisory from 2 PM Friday to 2 AM PDT Saturday for
Puget Sound and Hood Canal.

&&