Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 011733
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
933 AM PST Wed Mar 1 2017

.SYNOPSIS...Showers diminishing today as weak upper level ridging
builds over Western Washington. A more organized weather system will
begin to affect the region by late Thursday, bringing more rain and
mountain snow through Thursday night into Friday. An upper trough
will develop off the coast this weekend for cooler and showery
weather.

&&

.SHORT TERM...A flat upper ridge will build in for the rest of
today, with the flat ridge axis crossing Western Washington this
evening. This will lead to easing gradients, winds, and precip
coverage through this evening. Winter Weather Advisories for the
Cascades from I-90 on north will expire at 10 am, with only an
additional inch or two of snow this afternoon at Snoqualmie and
Stevens Passes. The flatness of the ridging will still support
low-level onshore flow through today, along with a few showers
and lots of cloudiness. This will be more of a lull than a true
break in the weather.

On Thursday, westerly flow aloft will increase, and upper heights
will start to fall again in response to a deep upper trough
digging southward from the eastern Gulf of Alaska. This will
deepen the moisture spreading onshore into Western Washington, and
precipitation will gradually expand and increase.

A better defined shortwave and surface cold front will quickly
move onshore on Fri PM. This will cause precip intensity and
coverage to peak on Fri afternoon, with breezy or windy
southerlies peaking in the hours closest to frontal passage.
Haner

.LONG TERM...From Previous Discussion: While activity may fizzle
out some early Saturday morning...W WA is not slated for a break
any time soon. Upper level low pressure will make its way into the
offshore waters mid morning Saturday and...not surprising
anyone...will result in more precip for the area. This low will
bring snow levels down gradually...with 1000 to 2000 ft expected
Saturday night and under 500 ft Sunday night...resulting in the
potential for wintry precip yet again for the lowlands Sunday
through Monday morning. Even though the low moves on Sunday
night...a broad trough remains over the western US and as
such...wet westerly to northwesterly upper level flow looks to
remain in place and...once again surprising no one...a wet start
to next week. SMR

&&

.AVIATION...Moderate west to northwest flow aloft will prevail
today and tonight in the wake of a front. At the surface, onshore
flow will gradually ease today then become southeast tonight ahead
of an approaching frontal system. The air mass is moist and
stable.

MVFR low clouds will be fairly widespread this morning coast and
central and south interior. The north interior will have mid level
clouds. The low clouds this morning should gradually lift today
with conditions mostly becoming VFR this afternoon and evening.

KSEA...MVFR low clouds should gradually lift to VFR this
afternoon. Gusty south wind 10-15 knots this morning should
gradually ease this afternoon and evening. Schneider

&&

.MARINE...Onshore flow will ease today in the wake of a front with
small craft advisory winds ending this afternoon or evening. A
frontal system will move through the waters Thursday with another
round of small craft advisory winds most waters. The Coastal
Waters could see a few hours of gales Thursday afternoon and
evening. Moderate onshore flow will follow the front on Friday. A
surface low will move into the Offshore waters on Saturday then
move onshore as a trough on Sunday. Schneider

&&

.HYDROLOGY...No river flooding is expected for the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 10 AM PST this morning for
     Cascades of Snohomish and King Counties-Cascades of Whatcom
     and Skagit Counties.

PZ...Small Craft Advisory until 4 PM PST this afternoon for Central
     U.S. Waters Strait Of Juan De Fuca-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-East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory for rough bar until 10 PM PST this evening
     for Grays Harbor Bar.

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

     Small Craft Advisory for hazardous seas until 4 PM PST this
     afternoon for West Entrance U.S. Waters Strait Of Juan De
     Fuca.

     Small Craft Advisory until 1 PM PST this afternoon for Puget
     Sound and Hood Canal.

&&

$$

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