Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 211717
AFDSEW

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

.SYNOPSIS...Precipitation over the area will turn to showers as a
frontal system over Oregon moves inland 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 this morning. Precipitation over the Seattle metro and
the southern half of the forecast area will taper off this morning as
a frontal system over Oregon moves inland.

Westerly flow will develop ahead of an offshore trough as the
frontal system moves well east. Showers are possible in the
afternoon, and 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. JSmith/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 could 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. JSmith/McDonnal

&&

.AVIATION...Southwest flow aloft will push a surface low from
central Oregon to northwest Montana for the rest of today. As the
low exits late today, precip and deeper moisture will start to
exit, but the development of low-level onshore flow will maintain
cloud layers below 050 across Western Washington tonight. Moisture
and clouds will deepen again on Wed morning as a weak disturbance
moving down the B.C. coast causes some larger-scale lift over
western Washington and an increase in shower activity.

KSEA...North winds will continue through at least 20z (noon), and
possibly as late as 23z (3 pm). At that point, southerly gradients
will be developing along the I-5 corridor and prompting a turn
back to a southerly wind component. Southerly winds will then
continue tonight and through the day on Wed. Otherwise, slow
lifting of the low cloud cigs is expected this afternoon and
evening. A brief period of cigs in excess of 030 around 04z-08z (8
pm- midnight) this evening, before cigs come back down overnight.
Haner

&&

.MARINE...A surface low will move from central Oregon to northwest
Montana for the rest of today, causing gradients and winds over
the waters to back from a northerly to a westerly direction over
the course of today and this evening. Once the westerlies develop,
looking for SCA-strength westerlies through the Strait this
afternoon and evening.

Onshore flow will continue on Wed to the south of a weak surface
that will be sliding southeast down the west side of Vancouver
Island. The surface low will weaken and dissipate on Wed night as
it tracks south through the Washington coastal waters. Moderate
offshore flow will develop on Friday morning in response to a
strengthening 1012 mb low moving southward along 127W-128W. Haner

&&

.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.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 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-
     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 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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