Forecast Discussion for SEW NWS Office
000
FXUS66 KSEW 221028
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
228 AM PST Fri Nov 22 2019

.SYNOPSIS...Dry weather into early Saturday with high pressure
over the area. Pattern changes by later Saturday into next week as
several weather systems usher in chances for precipitation and
cooler air.

&&

.SHORT TERM /TODAY THROUGH SUNDAY/...Mostly clear skies, lingering
moisture, and light winds have allowed for the redevelopment of
fog around the central + south Sound and southwest interior
overnight. Most locations reporting visby of a 1/4 mile or less so
have hoisted a dense fog advisory through the morning commute.
Temperatures in the south Sound and other outlying areas have
dropped below the freezing mark as well so expect patchy freezing
fog and some slick spots on roadways this morning. Otherwise, dry
weather continues today with afternoon sun and highs near 50.

Cloud cover increases tonight so expect overnight lows to be
several degrees warmer with less fog coverage. Pattern change
then gets underway for the weekend as a more organized system
drops south from BC later Saturday into Sunday morning. Suspect
most of the day will be dry with a few showers approaching the
northern Olympic Peninsula and northern interior by the afternoon
to evening hours. Precipitation will then overspread the rest of
the area from NW to SE Saturday night into Sunday morning. Might see
a bit of a rain shadow around portions the Sound with heaviest
amounts expected in the mountains. As gradients tighten, could
also see some breezy winds across the area but doesn`t look like
anything too impactful at this point. Snow levels will start out
initially above 5500 feet but will steadily drop to 3000-4000
feet by Sunday afternoon. Latest guidance shows showers continuing
over the Cascades through Sunday night with a few inches for the
higher passes. Higher res guidance also shows the possibility of a
convergence zone forming Sunday afternoon-evening somewhere invof
of Snohomish/King counties. Elsewhere, will see drier conditions
by the afternoon hours. Afternoon highs will remain around the 50
degree mark.

CEO

.LONG TERM /MONDAY THROUGH THURSDAY/...Next system approaches the
area on Monday morning and pushes through during the day with
upper level low Monday night into Tuesday. This will bring
another round of rain + mountain snow and a shot of cooler air
from the north. Snow levels will drop to around 2500 feet on
Monday and possibly down to 1000-1500 feet on Tuesday. At the
lower end of the range, will likely see a rain-snow mix with
little to no accumulation. At this time, precipitation amounts
don`t look super impressive so passes will see another few inches
of snow. Early holiday travelers whose plans include the mountains
will want to keep an eye on the forecast as there could be a few
minor impacts. Afternoon highs on Monday will be in the mid to
upper 40s and low to mid 40s for Tuesday as cooler air arrives.

Models diverge past Tuesday with track of the upper level low.
Both GFS and ECMWF track the low south from around Vancouver
Island down the coast into California but vary in exact position
of low and strength. Ensembles are in better agreement with track
down the coast so have gone with blended approach for the
forecast. Showers will likely linger into Tuesday night then
generally taper off Wednesday morning outside a few lingering
showers across the Cascades. Regardless of exact track, cooler
temperatures look like a safe bet.

Past Wednesday, drying trend continues with models in agreement
for a mostly dry but chilly Thanksgiving into Friday. Highs on
Thursday may top out in the upper 30s to low 40s with overnight
lows below freezing for much of the area.

CEO

&&

.AVIATION...Fog, locally dense in spots, has returned to terminals
near Puget Sound and towards the coast. These terminals include
KSEA, KBFI, KPWT, KOLM, and to some extent KHQM. Temperatures are
also at or just below freezing at KPWT and KOLM. Freezing fog is
likely at these airports but will also be closely monitoring KSEA
and KBFI where temps are expected to bottom out in the mid 30s but
could approach the freezing mark near daybreak. Elsewhere there is
nothing more than some passing cirrus clouds. Fog is expected to
burn off through the morning hours, but given continued light winds
in the South Sound and lingering fog thru the day yesterday, will
need to watch KOLM today. Winds should generally be from the south
under 10kts today. Should see cloud cover increase tonight into
Saturday morning ahead of a cold front.

KSEA...Dense fog will continue through the morning hours and
gradually burn off into the afternoon, yielding clear skies and VFR
conditions. Will need to watch temps near daybreak for a brief
period of freezing fog but for now have kept temps in mid 30s. Cloud
cover will increase late tonight. Winds generally out of the SE
under 10kts.

Kovacik

&&

.MARINE...The flow will be light onshore today with high pressure
nosing into the coastal waters. Fog has again plagued Puget Sound up
into Admiralty Inlet this morning and will likely linger through the
morning hours. These will impede any marine travel this morning with
low visibility. In addition, patchy freezing fog will be possible
across Puget Sound, especially across the southern part of the
Sound. This threat should ease by mid-late morning as temps rise.

Out across the coastal waters, swells in the 11-13 ft range are
expected as a strong area of low pressure moves into the Gulf of
Alaska. With winds speed fairly light, hazardous seas will be of
concern so have kept the SCA going through today and tonight. Winds
will pick up on Saturday and although seas will still be hazardous,
the SCA for hazardous seas will need to be replaced by a SCA for
wind. Keep in mind swells will likely be stronger on Saturday at 13-
15 ft. Will also likely need SCA for some of the interior waters
(Puget Sound north into the Northern Inland Waters) on Saturday as
southerly winds pick up ahead of an approaching cold frontal
boundary.

Behind the cold frontal boundary, a strong westerly surge of wind is
anticipated down the central and eastern Strait of Juan de Fuca.
Based on model guidance, the concern for gale force wind exists.
This will need to be monitored by the next several shifts. Then
another enhanced surge of westerly wind appears reasonable down the
Strait on Sunday night, tho not near as strong. Large swell will
still likely be a concern across the coastal waters into early next
week.

Kovacik

&&

.HYDROLOGY...Precipitation moving into the region over the weekend
into next week will lead to some rises on area rivers, but river
flooding is not expected for the next 7 days. Snow levels will
also drop with each system with snow falling at higher elevations.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Dense Fog Advisory until 9 AM PST this morning for Hood Canal
     Area-Seattle and Vicinity-Southwest Interior-Tacoma Area.

PZ...Small Craft Advisory for hazardous seas until 10 AM PST Saturday
     for Coastal Waters From Cape Flattery To James Island 10 To
     60 Nm-Coastal Waters From James Island To Point Grenville
     10 To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm.

     Small Craft Advisory for rough bar until 4 PM PST this afternoon
     for Grays Harbor Bar.

     Small Craft Advisory for hazardous seas until 7 AM PST Saturday
     for Coastal Waters From Cape Flattery To James Island Out
     10 Nm-Coastal Waters From James Island To Point Grenville
     Out 10 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater Out 10 Nm.

     Small Craft Advisory for hazardous seas from 6 AM this morning
     to 10 PM PST this evening for West Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$

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NWS SEW Office Area Forecast Discussion