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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 220402

Area Forecast Discussion
National Weather Service Seattle WA
802 PM PST Wed Feb 21 2018

.SYNOPSIS...A weak low pressure system over Northwest Washington
this evening will bring areas of light snow to the area. This low
will move quickly south into Oregon later tonight. A break in the
weather is expected between systems on Thursday. A stronger 
system will give wet weather to the area Friday into Saturday with
low snow levels. Cool northwest flow aloft will continue into the
middle of next week, with cool wet systems around Sunday and then
next Tuesday or Wednesday.


.SHORT TERM...Satellite imagery and area radars show a surface low
just west of Hoquiam and an upper level low or vorticity maximum
just east of the Hood Canal Bridge. The system is expected to push
rapidly southward into Oregon overnight with precipitation rapidly
ending from the north between 10 PM and 1 AM. The forecast
challenge of the evening is snowfall amounts as the system quickly
moves and its structure evolves. One area of interest extends from
around Everett and Arlington westward to Port Townsend and the 
Hood Canal Bridge. A report on social media indicated that 3 
inches fell in Sequim as this feature passed that area earlier. 
How far east will this area extend as it pushes south and will it 
maintain its structure? At this point it appears that this area 
has diminished somewhat over the past hour. With the exception to 
an isolated location around Everett or Mukilteo, it appears that 
snowfall totals east of the Sound will remain below one inch while
portions of the Kitsap Peninsula (especially north) will see 1-3 
inches. Farther north, snowfall over the San Juans and around 
Bellingham is rapidly ending from the north. Another area of 
interest is from around Shelton and Olympia westward through the 
Chehalis Gap to southeast of Hoquiam and into the Willapa Hills. 
Showers moving southeast along the coast appear rather heavy on 
radar. A quick 2-3 inches can fall from these heavier showers.

Winter weather advisories are in effect for the lowlands of the
Olympic Peninsula, the interior from Olympia southward, and the
lowlands west of the Sound. The first advisory to expire is on
the North Coast where showers are coming to an end. We will likely
be able to cancel advisories a bit before the stated expiration
time from north to south as snow comes to an end overnight.

From the previous short term discussion: Upper heights will rise 
nicely on Thu morning, and dry northeast flow aloft will quickly 
clear out skies in the AM. A sunny Thu afternoon is expected, 
though the air mass will still be cool. Shortwave ridge axis will 
move southeast past the forecast Thu night. 

Northwest flow aloft will bring another shortwave down the B.C.
coast on Friday PM, with the height fall center passing turning
east-southeast near Vancouver B.C. on Fri evening. A slowly 
intensifying 1012 mb surface low will pass across the far North 
Cascades late Friday night. To its south, a turn to onshore flow 
with southerly gradients will bring a good shot of warm advection 
and lift on Friday afternoon and evening. With a cold air mass to 
begin, most lowland locales will start as snow. The coast should 
quickly change to rain soon after precip onset. Away from the 
coast, the transition to rain is more likely to occur Friday 
evening after starting with 1-3 inches of snow during the 
afternoon. A trailing cold front will move across on Sat morning,
with a few post-frontal rain and snow showers continuing. With a
cooling air mass behind the front on Sat, some more localized
accumulations are possible, especially where there is a focus for
lift such as in a Puget Sound Convergence Zone. On a larger scale,
weather should be drying out over the course of Saturday. Haner

.LONG TERM...From the previous long term discussion: Northwest 
flow pattern will continue. Another system is expected Sunday. Air
mass will not be as cold to start with, so precip will come in 
the form of rain over the majority of the lowlands. However, some 
post-frontal snow showers are possible on Sunday night. Next 
similar system expected late Tuesday or next Wednesday. Haner


.AVIATION...A weak upper level low is sliding southward through
Western Washington. Light northwest flow aloft will become
stronger north flow late tonight into Thursday night as the upper
low moves well southward and is replaced by a weak short wave
upper ridge. At the surface, a weak low is moving southward along
the Washington coast this evening. Low-level northeast flow will 
increase somewhat late tonight into Thursday as the low moves off
to the south. Then the low level flow will decrease later

The upper low is bringing MVFR to IFR conditions in light snow and
ceilings to the area. Conditions are expected to rapidly improve
to VFR from the north late tonight into early Thursday morning. 
Snowfall totals tonight will range from one to three inches at 
terminals on the Olympic Peninsula, while terminals east of Puget 
Sound will generally see less than an inch. Albrecht

KSEA...Snow showers will continue to give low end MVFR to
occasional IFR conditions to the terminal through about 08Z.
Conditions will improve to MVFR as precipitation comes to an end
after 08Z then conditions will rapidly clear with good VFR
conditions expected after 14Z. East wind 5-8 kt will become
0309-13kt after 12Z. Albrecht


.MARINE...A surface low just west of Grays Harbor will drop
southward onto the Oregon coast late tonight. Winds will increase
from the northeast as the low moves south of the area later
tonight into mainly Thursday morning. Small craft advisories begin
over the coastal waters at 11 PM this evening, while they begin in
the Northern Inland Waters and at the east entrance to the strait
6 AM Thursday. Visibility will locally be reduced in snow showers
until midnight (1/2 to 2 miles).

Winds will decrease Thursday afternoon as high pressure moves
southeast across the area. 

A stronger frontal system will move southeast across the area on
Friday. This system will give widespread small craft advisory 
conditions to the waters. A set of stronger systems Saturday and 
Sunday will affect the area and may result in gale conditions over
some of the waters over the weekend. Albrecht


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


WA...Winter Weather Advisory until 3 AM PST Thursday for Admiralty 
     Inlet Area-Bremerton and Vicinity-Central Coast-San Juan 
     County-Tacoma Area-Western Strait of Juan De Fuca.

     Winter Weather Advisory until 7 AM PST Thursday for Eastern 
     Strait of Juan de Fuca-Hood Canal Area-Lower Chehalis 
     Valley Area-Southwest Interior.

     Winter Weather Advisory until 9 PM PST this evening for North 

PZ...Small Craft Advisory from 11 PM this evening to 11 PM PST 
     Thursday 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 from 6 AM to 6 PM PST Thursday for East 
     Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland 
     Waters Including The San Juan Islands.