Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 011734
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
934 AM PST Thu Dec 1 2016

.SYNOPSIS...
A progressive mid level flow pattern across the Pacific NW will
bring quick moving systems for the next several days. Ridging
today will give way to a system arriving Friday morning. A
southward push of arctic air through western Canada by Saturday
and into Sunday will bring falling heights and colder surface
temperatures into Western Washington by late in the weekend.
Enough cold air should be present by late Sunday and Monday to
produce lower elevation snow. The coldest air of the season will
linger across the area through Wednesday in advance of an
approaching Pacific storm.

&&

.SHORT TERM...
Current radar depicts a persistent NW/SE oriented convergence zone
across Snohomish and portions of King County. This will continue
through the late morning hours and should gradually dissipate.
Scattered showers will continue on the coast today as well in the
warm advection area in advance of a deepening low over the NE
Pacific. Moisture and warm advection will increase into early
Friday when showers will overspread the forecast area. Not a
significant storm system with generally a quarter inch or less in
the major metro and Puget Sound lowlands. Slightly higher amounts
in the mountains but important to note snow levels will rise late
Friday to well above pass levels by the evening hours and into
early Saturday. Moisture lingers through Saturday and then
increases as a cold front drops through the area by early Sunday
morning. This front will produce colder temperatures across the
area on Sunday as highs remain in the low-mid 40s along with
scattered rain showers in the area. 850 mb temps will drop to
around -6 to -8c by Sunday evening which will force snow levels to
drop considerably by late Sunday. Starting out around 1500-2000
feet Sunday morning, snow levels should fall more into the
500-1000 foot level by Sunday evening and bring possibility of
some wet snowflakes mixed in with the rain showers. However
moisture looks fairly limited on Sunday late, so this would just
be very showery in nature and warm surface temps would likely
preclude lowland accumulations through Sunday evening. Johnson

.LONG TERM...
Monday remains the forecast focus with the potential for lowland
snow. The consensus of deterministic models and ensemble
solutions is that there will be lowland snow in Western Washington
late Sunday and into early Monday. Cold air will continue to pour
in overnight Sunday. By Monday morning 1000-850mb thickness values
will be below 1300 across all of the forecast area. This would
indicate that it will certainly be cold enough for some in the
lowlands by early Monday morning. Surface winds however remain
southerly across the Seattle metro area which would indicate
lingering near surface temperatures above freezing. Moisture will
be present as well with a quick moving mid level trough traversing
the region. Of course, the details are what matter most. Exact
timing, locations, any potential accumulations are what matter the
most. At this junction, about 100+ hours out in time, it is too
early to make definitive forecasts on the details.

Cold air will linger in the region through Wednesday and Thursday
with highs in the 30s and lows in the 20s. Another system will
approach the area by Thursday and Friday that looks to have a
more significant amount of moisture. The eventual question will
be how quickly the warm air scours out the low level cold air as
the precip arrives. Johnson

&&

.AVIATION...There are isolated to scattered showers on the radars
from a weak disturbance aloft. These showers will mainly affect
the Cascades this afternoon as weak high pressure builds over
Western Washington. The flow aloft is N/NW. There are patchy low
clouds but most areas are VFR this morning. Rain will increase on
the coast tonight as a warm front moves in. 33

KSEA...Patchy low clouds in the vicinity this morning for brief
MVFR cigs...then becoming VFR this afternoon. Surface winds south
to 10 kt. Rain spreading inland Friday morning 12-15z. 33

&&

.MARINE...Onshore flow will ease today as weak high pressure
builds over Western Washington. Southerly winds will increase
on Friday as a warm front moves in. The Coastal Waters may see
gales with this front and a Gale Watch is in effect. The cold
front will flip winds to onshore Friday afternoon and
evening...with onshore flow continuing through Friday night. A
stronger system will bring rising winds Saturday and Sunday.
Northerly flow will develop on Monday as strong high pressure
forms over interior British Columbia. 33

&&

.HYDROLOGY...No river flooding is expected over the next week as
we head into a cooler weather pattern starting late this weekend.
Felton

&&

.CLIMATE...With an average temperature of 50.9 degrees November
2016 in Seattle was the second warmest November on record in
Seattle. The record is 51.9 degrees in 1899. The average high
temperature for the month was 56.0 degrees which was the 4th
warmest on record. The average low was 45.7 degrees which was the
2nd warmest on record. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Watch from late tonight through Friday afternoon 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 hazardous seas until 6 PM PST this
     evening 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-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory for rough bar until 7 PM PST this evening
     for Grays Harbor Bar.

&&

$$

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