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Forecast Discussion for Seattle/Tacoma, WA

FXUS66 KSEW 171227

Area Forecast Discussion
National Weather Service Seattle WA
427 AM PST Sun Dec 17 2017

.SYNOPSIS...Rain will increase today as a warm front spreads
inland from the Pacific. Then a cold front will stall over the
region on Monday. A strengthening low center will pass across the
area on Tuesday morning, and a strong cold front will bring wind
and heavy mountain snow. Lingering showers will continue Wednesday
morning, then an upper ridge will bring dry weather on Thursday
and Friday.


.SHORT TERM...Warm advection and strong west flow aloft around the
periphery of an upper ridge will push a warm front across Western
Washington today. The veered low-level wind profile is classic of
a warm advection pattern. Isentropic lift will increase, with rain
expanding and becoming heavier this morning. Over the Cascades,
snow levels will rise today, reaching the 4000-5000 foot range by
this evening. A Winter Weather Advisory is in effect for the
Cascades from King County north. Snoqualmie Pass at 3000 feet
should not get much snow, but Stevens Pass and Mount Baker may
exceed 6 inches today.

Then tonight, a slow-moving cold front that is oriented parallel
to jet stream winds will slowly sag southward into Western
Washington before turning up stationary. This will provide an
effective focus for precip tonight and Monday morning. Plus,
strong west flow aloft will further enhance lift over the
Cascades. This will bring heavy precip to the mountains. Later
shifts may need to issue a Winter Storm Warning for parts of the
Cascades for tonight and early Monday. For most of the night, snow
levels in the 4000-5000 foot range would lessen snowfall amounts
in the passes. Snow level should drop early Monday morning to
3000-4000 feet, just as precip rates come down some.

Rain may briefly end on Monday afternoon further north along the
Strait and up near Bellingham. Further south, Monday afternoon
will merely be a lull with lighter precip.

Way offshore between 130W and 140W, cyclogenesis will occur on
Monday along the front, with the new low center deepening quickly
to 1000 mb or less as it tracks toward southern Vancouver Island
or the north Washington coast on Tuesday morning. Deepening lows
come with lots of large-scale lift and therefore heavy precip
rates. Once the low passes east of our longitude on Tuesday
afternoon, a sharply defined trailing cold front will move
southeast, bringing snow levels back down below 3000 feet by late
Tuesday. The sharp cold front will also act as a focus for heavy
precip, which will be shifting down toward southwest Washington
late Tuesday. This setup should bring heavy snow to both the
Olympic and Cascade Mountains, where snow will be measured in feet
at most locations other than perhaps Snoqualmie Pass. Even the
interior lowlands will get some heavy rain, with locations
outside of the Olympic rain shadow getting 1 inch or more of rain
late Monday night through Tuesday afternoon. Should also have some
windy weather on Tuesday, but we may escape without land-based
wind highlights.

.LONG TERM...Strong cold front will exit southeast from the
forecast area on Tuesday night. A compact cold core aloft will
follow the front, with 500 mb temps briefly dipping to near -34C
early Wed morning. Cold air aloft will support plenty of showers.
Later Wed morning, upper heights will rise in response to an upper
ridge axis near 140W bulging onto the coast. The air mass will
stabilize, and drier air will advect into Western Washington
thanks for northeast surface wind delivering drier continental
air from the interior of B.C.. Showers will quickly taper off on
Wednesday morning and end late Wed.

Dry northwest flow aloft and upper ridging around 130W-135W will
keep the weather cool and precip-free for the rest of the week.


.AVIATION...A warm front will stall over the area today with
westerly flow aloft. The air mass is moist and stable.

KSEA...MVFR low clouds are expected today. South wind 5-10 knots
will increase to 10-15 knots and become gusty today. Schneider


.MARINE...A warm front will bring small craft advisory strength
winds to most waters today. There will be relative lull in between
weather systems Monday. A stronger frontal system will move
through the area Monday night and Tuesday. Gales are likely for
the Coastal Waters with this system. Offshore flow will develop
Wednesday and continue into Thursday. Schneider


.HYDROLOGY...River flooding is not expected the next 7 days.


WA...Winter Weather Advisory until 10 AM PST Monday for Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit

PZ...Small Craft Advisory until midnight PST tonight 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.

     Small Craft Advisory until 6 AM PST Monday for Coastal Waters
     From Point Grenville To Cape Shoalwater Out 10 Nm.

     Small Craft Advisory until noon PST Monday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 10 PM PST this evening for Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 4 PM this afternoon to midnight PST
     tonight for West Entrance U.S. Waters Strait Of Juan De

     Small Craft Advisory from 10 PM this evening to noon PST Monday
     for Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 4 PM PST this afternoon for Admiralty

     Small Craft Advisory until 4 AM PST Monday for Puget Sound and
     Hood Canal.



NWS Seattle/Tacoma, WA (SEW) Office Area Forecast Discussion

Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather