Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 211141

Area Forecast Discussion
National Weather Service Seattle WA
341 AM PST Tue Feb 21 2017

.SYNOPSIS...Another frontal system moving across Oregon and
northern California will bring light precipitation to the south
half of Western Washington today. Cool weather with scattered
light showers of rain or snow will prevail Wednesday through
Friday, as a series of upper lows move south over the offshore
waters. Cool dry weather is likely this weekend.


.SHORT TERM...Southwest flow aloft continues over Western
Washington early this morning. Precipitation associated with the
system that moved through the forecast area on Washington`s
Birthday ended most everywhere around midnight, and now the
northern edge of precipitation with the system moving inland
across Oregon and northern California is about to reach our
southern border. At 3 am temperatures were in the upper 30s to mid
40s with cloudy skies and light patchy fog, except Forks was 31
degrees under clear skies.

The system over Oregon and northern California will move east-
northeast today. Its surface low center -- weakening near Eugene
early this morning -- will weaken further as it tracks across
southeast Washington and into the northern Rocky Mountains late
today. The north side of this system will bring a little
precipitation to mainly the southeast part of the forecast area
today. The heaviest will probably be over Mount Rainier, and that
should only amount to around 0.25 inch. The north interior
lowlands, the Strait of Juan de Fuca, and the north coast should
remain dry.

Westerly flow will develop ahead of an offshore trough as the
frontal system moves well east, with precipitation switching to
showers late today. A weak Puget Sound convergence zone will
probably develop, likely extending into the Cascades around
Snoqualmie and Stevens Passes. The snow level will be around 3000
ft today and fall to 1500 ft tonight. The Cascades will pick up
another 1 to 3 inches of snow total for today and tonight.

An offshore upper low will be off the central British Columbia
coast early Wednesday; the low will dig south over the Washington
offshore waters late Wednesday and then inland and across the
Cascades Wednesday night. This will put the forecast area under a
cool, moist and weakly unstable air mass with light onshore flow
at low levels. Showers should increase during the day and then
decrease after dark. Highs will be a couple degrees lower than
today -- in the lower to mid 40s, and the lows Wednesday night
will be in the lower to mid 30s. The snow level will be around
1000 ft with 1 or 2 inches of new snow.

Northwest flow aloft will prevail over the region Thursday and
Thursday night. Moisture will be limited, but the air mass will
remain slightly unstable. So there should be scattered light
showers mainly in the afternoon and evening, but precipitation
should amount to 0.10 inch or less. The air mass will be cooler
yet, with 850 mb temperatures around -6C to -8C and the snow level
in the 500 to 1000 ft range. Showers could fall as snow over the
higher terrain of the Western Washington lowlands. Daytime highs
should still get into the lower to mid 40s. Snow accumulations, if
they occur at all, should be well under an inch and limited to
terrain over 500 or maybe even 1000 ft. McDonnal

.LONG TERM...The GFS and the ECMWF are both forecasting another
deep upper low to dig south into the Washington offshore waters on
Friday. This will likely spawn some showers, especially near the
coast. The upper low will continue to move south on Saturday,
giving dry cool weather with some sunshine on Saturday. Another
upper low will probably follow close to the same track on Sunday,
and again it looks like the area should remain generally dry.
Dry weather looks likely to continue in northwest flow aloft on
Monday. Despite some sunshine, highs should be mainly in the lower
to mid 40s with lows in the lower to mid 30s. McDonnal


.AVIATION...Southwesterly flow aloft will continue today before
becoming more westerly late this afternoon and into Wednesday. Low
level moisture from earlier rains have allowed spotty fog to develop
in isolated locations and may linger into mid morning before
dissipating. That said...cigs running full range of conditions over
the area. Still the potential for some VFR spots but should start to
see cigs drop once more as next system makes its way over the area
by late this morning or early this afternoon. Best chances for
precip today look to remain confined to southern portions of the
area and along the Cascades.

KSEA...Cigs expected to stay VFR for the next hour or two before
starting to fall into MVFR conditions by 12Z...although improvement
back to VFR is possible for afternoon. Rain looks to remain south of
the terminal today. Winds northerly around 10 kts becoming southerly
this afternoon.  SMR


.MARINE...A surface low center will cross the south Oregon coast
this morning and track northeast to SE Washington later today. This
will bring another round of northerly gradients this morning to
Western Washington, followed by westerly gradients later in the day.
Models remain consistent on winds picking up over the waters
throughout the day have put up small craft advisories
through Wednesday morning for the coastal waters. Strait...northern
inland waters and admiralty inlet start up a bit later...and all
except the strait die off after midnight. Coastal waters will likely
need an extension into at least Wednesday evening while winds in the
strait will have to be re-evaluated on next shift. Onshore flow will
then continue Wed but weaken. A weak surface low will develop near
Cape Flattery on Wed night and then slide southward through the
coastal waters on Thu. Light to moderate offshore flow is expected
on Friday. Haner/SMR


.HYDROLOGY...No flooding is expected during the next week. USGS
landslide guidance remains above the threshold at which landslides
typically occur. This means there will continue to be an elevated
threat of landslides for at least the next few days.


PZ...Small Craft Advisory until 4 PM PST Wednesday for 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 4 PM this afternoon to 4 PM PST
     Wednesday for Coastal Waters From Cape Flattery To James
     Island 10 To 60 Nm-Coastal Waters From Cape Flattery To
     James Island Out 10 Nm.

     Small Craft Advisory from 7 AM this morning to 4 PM PST
     Wednesday for Coastal Waters From James Island To Point
     Grenville 10 To 60 Nm-Coastal Waters From James Island To
     Point Grenville Out 10 Nm.

     Small Craft Advisory from 4 PM this afternoon to 4 AM PST
     Wednesday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 4 PM this afternoon to midnight PST
     tonight for Admiralty Inlet-Northern Inland Waters
     Including The San Juan Islands-West Entrance U.S. Waters
     Strait Of Juan De Fuca.



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