Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 091137
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
338 AM PST Fri Dec 9 2016

.SYNOPSIS...A warm front will continue to lift northward over the
area this morning with snow turning to rain along the front. Rainy
weather is expected to continue for much of the weekend as an upper
level low will pass over Western Washington Saturday and into
Sunday. Temperatures will remain mild this weekend before becoming
colder for the first half of next week.

&&

.SHORT TERM...Transition from snow over to rain is occurring rather
slowly this morning...but is occurring as obs from Chehalis and
Centralia show rain. Current radar shows that precip activity is
becoming a bit more scattered...particularly over southern third of
the CWA. Main forecast issue this morning is almost more of a
nowcasting issue...anticipating the speed of the transition over to
rain for portions of CWA that are still seeing snow. HRRR seems like
a good tool for this...putting the transition to rain in OLM between
3 and 4 AM...in Seattle between 4 and 5 AM and fairly widespread by
7 AM this morning.  All that being said...inherited timing looks to
be on track and will opt not to alter until reality says otherwise.

Once precip becomes rain it looks to stick around for the weekend.
This particular round of rainfall will linger for much of the day
today and into tonight before becoming more scattered by Saturday
morning. This will result in continued snow accumulations in the
mountains. Given the expected pacing of accumulations...inherited
winter weather advisory for snow looks good...as 12 hour
accumulations fall right in step with 12 hour advisory thresholds.

Upper level low pressure looks to move toward Western Washington
Saturday afternoon...bringing another round of organized precip to
the area. The low itself appears to be fairly progressive...but
lingering shortwave disturbances will keep rain in the forecast
through Sunday afternoon before things start to scatter out.  SMR

.LONG TERM...A frontal system looks to move through Western
Washington Monday with a return to colder temperatures behind it.
Although the origins of the system are over the Pacific...making it
seem mild...northeasterly surface pressure gradients look to open
the gates for more arctic air thanks to Fraser outflow. Current
model solutions hint at the prospect of tightest surface pressure
gradients occurring as the upper level system moves
through...raising some questions as to what for precip might take
during this period. With arctic air entering...as opposed to having
already been in place...leaning more toward rain at this time.
Granted...it may be a moot point...as this alignment seems to have
a pretty small window since chances of precip diminish in the
evening and gone by Monday night. Cold and dry conditions are
expected for the first half of next week as subfreezing low
temperatures look to return.  SMR

&&

.AVIATION...Weak high pres aloft will move slowly east, allowing
an upper level Low to approach the British Columbia coast. An
occluded front over far southwest WA will continue moving slowly
north-northeast, and is anticipated to reach Whatcom County late
tonight. Wly flow aloft will strengthen during the day. The low
level flow will grdlly become southerly across the entire area by
late tonight.

Meanwhile, expect widespread MVFR CIGS (areas of MVFR VSBYS) today.
There will be areas of -FZRA til about 1800 UTC (10 AM PST) and
localized IFR conditions (with the heavier snow).

KSEA...It looks like periods of light snow will will continue before
transitioning to rain about 1500 UTC (7 AM PST). There is a
possibility of brief -FZRA or sleet before the switch to just plain
rain. Additional snowfall amount is anticipated to be less than an
inch. Meanwhile, expect generally MVFR conditions today.
winds will be east or southeast near 10 knots or less.

&&

.MARINE...
An occluded front extended from a 992 MB Low off the northern tip of
Vancouver Island southeast to the southwest Washington coast.
This front will continue moving slowly north, reaching the U.S.
Georgia Strait late tonight. The flow will become southerly behind
the front.

A trough or weak cold front will move across the area on Saturday
for increased onshore or westerly flow. Onshore flow will persist on
Sunday before becoming northerly on Monday.

&&

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


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 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 until 3 AM PST Saturday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties-Olympics.

     Winter Storm Warning until 8 AM PST Friday for Hood Canal Area.

     Winter Weather Advisory until noon PST Friday for Western
     Whatcom County.

PZ...Small Craft Advisories are in effect for all waters except
 the Puget Sound/Hood Canal.

  Gale Watches are in effect for the coastal waters from Cape
 Flattery to Cape Shoalwater from 10 NM out to 60 NM and
 for the east entrance to the Strait of Juan de Fuca for
  Saturday.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



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