Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 120444
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
844 PM PST Thu Jan 11 2018

.SYNOPSIS...A lingering weak low over the coastal waters will move
inland and dissipate overnight. A weak warm front will lift
northeast across the area Friday night into Saturday morning.
Sunday will be dry and quite mild as a ridge of high pressure
moves across the area and is accompanied by low-level offshore
flow. Several fronts will move through the region next week.

&&

.SHORT TERM...The low pressure system responsible for bringing
rain and heavy mountain snow to the area today has stalled over
the northern portion of the coastal waters this evening, while the
upper level low associated with this surface feature has shifted
into Eastern Washington.

Because the upper levels cooled ahead of the incoming surface
feature, snow continued for a longer period of time than earlier
expected in the lower Cascade Passes. Precipitation has only mixed
with rain at Snoqualmie Pass this evening after 12 to 15 inches of
wet snow fell. Precipitation at Snoqualmie should be mixed through
about midnight tonight before it just comes to an end. With the
precipitation tapering off, the Winter Storm Warning for the
Cascades and the Winter Weather Advisory for the Olympics will be
allowed to expire at 10 PM this evening.

Low level offshore pressure gradients remained strong enough this
afternoon into this evening that weak Fraser outflow is being
experienced from Sumas southwestward through the San Juans and
into the Strait of Juan de Fuca. The air mass over south-central
British Columbia is quite cold (-6F at Williams Lake, 27 in Hope,
and 32 in Abbotsford) and rain has been mixed with snow at times
along the border in Whatcom County. Forecasts were updated this
evening to lower temperatures overnight in areas exposed to the
northeasterly winds. The outflow should weaken toward morning as
the surface low shifts inland and dissipates.

Some light rain will fall at times later Friday and Friday night
into the first part of Saturday (especially over the Olympic
Peninsula and northern portions of the interior) as a weak warm
front lifts northeastward across the area.

Behind the warm front, a building upper level ridge combined with
low level offshore flow will give drying later Saturday through
Sunday. Saturday and Sunday afternoons will be quite mild across
the area with the lowlands getting up into the lower to mid 50s.
Enjoy this break from the cool and dreary mid January weather!

Other than the forecast update for the tonight period, forecasts
are in good shape. Albrecht

.LONG TERM...From the previous long-term discussion: Several
fronts and low pressure systems are likely to affect the region
next week. The models differ on timing a details quite a bit, but
in general it will be wet and cool with fairly low snow levels and
good ski conditions. It will probably be pretty blustery in the
lowlands at times and there are a couple of fronts that should
muster gales for the marine areas.

&&

.AVIATION...A weak surface low/trough will push inland tonight
for additional showers across Western Washington. The flow aloft
is NW to W. The low level air mass is moist with mainly MVFR
conditions. A warm front will clip the coast and northern tier of
the CWA Friday afternoon. Otherwise, drier weather is expected
with upper level ridging over the region. 33

KSEA...IFR to MVFR conditions will continue overnight with showers
in the vicinity. S winds at the surface to 10 kt. Drier conditions
on Friday with low-end VFR conditions expected, cigs near 3000 ft.
33

&&

.MARINE...Low pressure center, around 1008 mb, off the north
coast will continue to weaken overnight then shift inland early
Friday morning. The surface pressure field is resulting is N/NE
winds through the Strait of Juan de Fuca with S/SW flow over the
Coastal Waters and Puget Sound. Meanwhile, west swells of 10 to 16
feet are over the Coastal Waters and will subside to 10 to 13
feet on Friday. Winds will be generally light on Friday with weak
high pressure over the area. A warm front will clip the coast
Friday afternoon into Saturday. A weak cold front will approach
from the west on Sunday then dissipate offshore. A stronger
frontal system is slated for Monday. 33

&&

.HYDROLOGY...River flooding is not expected across the area
through the middle of next week. Periods of rain from Monday
onward will cause rises on area rivers, and there will be an
increased threat of some minor flooding on more flood-prone rivers
as we move into the end of next week. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 10 PM PST this evening for Cascades
     of Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties.

     Winter Weather Advisory until 10 PM PST this evening for
     Olympics.

PZ...Small Craft Advisory until 4 PM PST Friday for 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.

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

     Small Craft Advisory until 4 AM PST Friday for Central U.S.
     Waters Strait Of Juan De Fuca-West Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$

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