Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 272302

Area Forecast Discussion
National Weather Service Seattle WA
302 PM PST Mon Feb 27 2017

An active and unsettled pattern with chances of precipitation
will continue this week. Cooler than normal weather for late
February will persist for a few more days. Weak low pressure
systems will bring a chance of lower elevation snow through
Tuesday. This will give way to a warming trend and rain in the low
elevations with snow in the mountains by mid-week. Unseasonably
cold air will return by next weekend with snow levels dipping


Forecast focus remains on possible low elevation snow this
afternoon and evening with a developing convergence zone. This
will continue to be a tricky forecast. Still feeling that low
elevations, including most of the city of Seattle will see little
or no snow as it will be too warm.

Above 300-500 feet, should cool just enough for a possibility of
accumulating snowfall. An inch or two is possible in the higher
elevations with 3-4 inches if there are heavier showers within the
convergence zone. It is not possible to predict exactly where
those heavy showers would form, if they do.

This is not a high confidence forecast overall. The later in the
day the convergence lingers, the better the chance of snow
accumulation. To summarize, above 300-500 feet has the best shot
at accumulating snow. Temperatures will be a bit too warm near the
water and below 100 feet elevation for much accumulation if any.

For tomorrow morning, lingering showers will end early and
temperatures will remain near or above freezing in most areas to
help limit threat of freezing conditions. A surface trough passing
through the area later in the day will bring an increasing chance
of showers however the snow levels will be rising to 1000 feet and
higher through the afternoon. So rain will become the predominant
lowland precip type once again with any accumulating snow confined
to the mountains by late day Tuesday. Temperatures will remain
below normal though, primarily in the low to mid 40s. It will
remain showery and cool overnight Tuesday but lows in the upper
30s with snow levels remaining higher. Wednesday will be a
transitional day, as the upper trough slides east and a flat mid
level ridge builds into the area. Showers will continue but
lowland temps will moderate to the upper 40s to lower 50s for
highs, closer to normal. By Thursday a deep upper low approaching
northwestern BC will continue to push heights higher, and push a
more significant slug of moisture towards Western Washington.
Showers will continue with heavy mountain snow during that time
period. Temps near normal for the lowlands. Johnson

Friday should be a wet day across the area with a large mid level
low sliding south towards the area. Heights will fall some and
this will push temps back slightly below normal on Friday. Heavy
mountain snow will continue during this time period and will total
in the feet of accumulation with snow levels at 2000 feet and
higher. The upper low will continue its southward push into
Saturday and Sunday, bringing falling heights to Western
Washington and shoving temperatures back into the below normal
range, with falling snow levels yet again. 1000-500 mb thicknesses
will be back near 520dm again by Sunday. Johnson


.AVIATION...A weak upper level trough over W WA at 22Z will move
over the Cascades around 06Z allowing shower activity to diminish. A
convergence zone over north Puget Sound will move S over Seattle 23Z-
01Z then move over the Cascades and dissipate around 04Z. The lower
air mass will remain moist so MVFR to IFR conditions will
predominate over the greater Puget Sound region and the SW interior,
although there will probably be brief localized areas of VFR
conditions. The present MVFR to IFR conditions over the NW interior
should lift to VFR this evening.

Cool N-NW flow aloft will continue over the region through Tuesday
night. A weak front will move across the area Tuesday afternoon
through Tuesday night for an increase in showers.

KSEA...A convergence zone over north Puget Sound is expected move S
to the KSEA area 23Z-01Z. This will generate more showers with snow
mixed in if the showers are heavy enough. The convergence zone
should move E over the Cascades by 04Z. MVFR to IFR conditions are
likely with the convergence zone, becoming mainly MVFR the rest of
tonight. Surface winds will remain southerly 6 to 12 KT but may
become variable or even brief northerly when the convergence zone is
in the area. Kam


.MARINE...A weak surface low will remain over W WA through tonight
while a weak upper level trough moves S across the area this
evening. Moderate W-NW flow will continue tonight over the coastal
waters with 15-25 KT SCA winds through early Tuesday morning. After
a brief lull, onshore flow will strengthen Tuesday night as a weak
front moves across the area for another period of SCA winds over the
coastal waters. SCA winds may also develop over parts of the inland
waters prone to S or SE flow. A weak warm front will brush the
coastal waters on Wednesday which should allow winds to shift to S-
SW. A stronger cold front will approach the area Thursday then move
inland Thursday night or Friday morning. Kam


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


WA...Winter Weather Advisory until 9 PM PST this evening for Bellevue
     and Vicinity-East Puget Sound Lowlands-Everett and Vicinity-
     San Juan County-Seattle and Vicinity-Western Skagit County-
     Western Whatcom County.

PZ...Small Craft Advisory until 10 AM PST Tuesday 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 for rough bar until 10 AM PST Tuesday for
     Grays Harbor Bar.



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