Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS66 KSEW 080522

Area Forecast Discussion
National Weather Service Seattle WA
922 PM PST Wed Dec 7 2016

.SYNOPSIS...Cold dry air is over Western Washington tonight.
Easterly winds will increase through Thursday with high pressure
inland and low pressure approaching from the southwest. A warm
front will lift northward across the area Thursday night. Cool and
rainy weather is likely Friday through the weekend with a low
snow level.


.SHORT TERM...Skies are clear across much of the area this
evening, except for a band of high clouds that extends from about
Forks southeastward to the Columbia River Gorge. Temperatures are
in the mid 20s to lower 30s across the area. Coldest temperatures
are in protected valleys in the north interior and from about
Bellingham northeastward.

Easterly pressure gradients are on the rise as lower pressure
approaches the area from the southwest. Winds in the passes and at
the approaches to the passes have not yet responded, but will
increase as we move toward morning. Greenwater RAWS at 2400 feet
17 miles ESE of Enumclaw is seeing east winds 30 mph with gusts to
42 mph. A wind advisory is in effect for the East Puget Sound
Lowlands until 4 PM PST Thursday for east winds rising to 20 to 30
mph with gusts to 45 to 50 mph. Winds should peak during the
morning hours on Thursday.

Clouds will increase from the southwest on Thursday. Models are a
tad slower in bringing precipitation northward, but all show
precipitation developing in the south interior and on the central
coast during the mid afternoon period, in the greater Seattle area
between 5 and 7 PM, and in the Bellingham area around midnight
Thursday night. Precipitation will begin as snow in the interior
and change to rain from south to north during the evening south of
Seattle and after midnight to toward morning from Seattle
northward. Snowfall will last about 6 hours and will amount to 4
to 6 inches in the Hood Canal area and 1 to 3 inches elsewhere.
The NAM is a bit heavier with the snowfall amounts (locally up to
4 inches), while the drier GFS keeps most areas from Puget Sound
and I-5 eastward drier with strong easterly pressure gradients
cutting back on snow amounts to 2 inches or less. The current
forecast of 1 to 3 inches in the lowlands (with the exception to
the heavier amounts around Hood Canal) appear to be on track. A
zone update was made to adjust the timing of the onset of
precipitation later into the day Thursday in the south and mainly
in the evening for areas from Seattle northward.

Model consistency and continuity is not really too good at this
time from Friday through Sunday. Cold air will remain just north
of the Canadian border and lower pressure will be right over
Western Washington. Any shift of the surface trough or tracks of
low pressure will mean the difference between snow levels around
500 feet or 1000-1500 feet. The current forecasts generally appear
to be on track at this time.

Other than the change to the timing of the precipitation onset,
current forecasts look good.  Albrecht

.LONG TERM...After the passage of another low pressure system with
rather low snow levels on Sunday, the GFS is showing Canadian air
moving southward into the area Monday through Wednesday with a
turn to drier conditions. The 00Z GFS is rather cold showing
highs in the 30s and lows in the 20s across the area. Another
winter weather threat could reach the region around Thursday of
next week, but it is too early to get excited about that system
yet as model consistency and continuity is suffering in this
pattern. Albrecht


.AVIATION...A weak ridge of high pres aloft will prevail thru Thu
morning. Nly flow aloft will become wly. Low level offshore flow
will grdlly strengthen thru Thu morning. VFR conditions will
prevail thru Thu morning, but there may be patchy dense ground
fog in the river valleys s and w of KOLM late tonight thru
midmorning Thu.

KSEA...Low level e or nely flow will strengthen thru midday Thu,
with gusts near 25 knots expected after 1500 UTC. VFR conditions
will prevail thru Thu afternoon.

At this time, it appeared that snow will begin about 0300 UTC
Fri, and then transition over to rain after 0900 UTC. There is a
slim possibility that there may be a brief period of sleet and/or
freezing rain before the precip changes over to plain rain. At
this time, we are forecasting two inches of snow at the terminal
Thu night.


Easterly flow will strengthen through midday Thursday. A warm
front will lift north across the area Thursday night, resulting
in winds veering from the south Thursday night.

A rapidly weakening 1005 mb low will move east across the coastal
waters Friday night. A weak trough will linger over the area this
weekend. Offshore flow will return on Monday and strengthen.


.HYDROLOGY...No flooding is expected over the next week.


WA...Winter Weather Advisory from 4 PM Thursday to 8 AM PST Friday
     for Admiralty Inlet Area-Bellevue and Vicinity-Bremerton
     and Vicinity-East Puget Sound Lowlands-Eastern Strait of
     Juan de Fuca-Everett and Vicinity-Lower Chehalis Valley
     Area-San Juan County-Seattle and Vicinity-Southwest
     Interior-Tacoma Area-Western Skagit County-Western Strait
     of Juan De Fuca.

     Winter Weather Advisory from 4 PM Thursday to 3 AM PST Saturday
     for Cascades of Pierce and Lewis Counties-Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit

     Winter Storm Warning from 4 PM Thursday to 8 AM PST Friday for
     Hood Canal Area.

     Winter Weather Advisory from 4 PM Thursday to noon PST Friday
     for Western Whatcom County.

     Wind Advisory until 4 PM PST Thursday for East Puget Sound

PZ...Gale Warning until midnight PST Thursday night 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.

     Gale Warning until 3 AM PST Friday 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 from 3 AM Thursday to 3 PM PST Friday for
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 PM PST Friday for Northern Inland
     Waters Including The San Juan Islands.

     Gale Warning until midnight PST Thursday night for Central U.S.
     Waters Strait Of Juan De Fuca-West Entrance U.S. Waters
     Strait Of Juan De Fuca.



You can see an illustrated version of this discussion at is the U.S. government's official web portal to all federal, state and local government web resources and services.