Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 131625

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PST Tue Feb 13 2018

.SYNOPSIS...High pressure will keep the area dry today. A frontal
system will bring rain and mountain snow tonight with decreasing
showers Wednesday. Rain chances will gradually increase Thursday
and Friday. A vigorous frontal system will bring wet and windy
weather Friday night and Saturday. Cool showery weather will
settle over the area Sunday with low snow levels.


.SHORT TERM...Yet another dry and chilly morning across much of the
area this morning. Lows dipped down close to 20 in the coldest
pockets and upper 20s to lower 30s in the main Settle area. High
pressure will break down today ahead of an upper trough arriving
from the northwest by this evening. Mid and high clouds will start
to increase late this morning and afternoon. Highs in the 40s.

Rain will increase ahead of the front with a few hour shot of decent
shot of pre-frontal snow. Snow levels may rise a bit later today but
should fall tonight and mostly snow will fall at the passes,
including Snoqualmie, White, and and Stevens. A half inch to around
1 inch liquid are shown by models tonight. Onshore post-frontal
orographic snowfall should bring another few inches Wednesday
morning, then ease by afternoon. Snow levels near 1500 to 2000 feet.
Lowland rain showers will fall across the lowlands with
central/north Puget Sound partially shadowed off the Olympics.
Probably not much rain in the Seattle metro area to Everett behind
the front Wednesday. Those planning on pass travel tonight or
Wednesday should call 5-1-1 for current road conditions. A winter
weather advisory remain in effect for the Cascades and passes from
Lewis county north to Whatcom county including all passes.

Showers taper off fairly quickly by late Wednesday afternoon and
evening. Models vary on the timing of the next weaker system. The
ECMWF indicates light spotty rain could begin north and west of
Puget Sound during the day. Other models hold off until late
Thursday into Friday.

.LONG TERM...Previous discussion...An upper short wave in the
northwest flow aloft will bring a chance of rain Friday. A much
stronger weather system will bring rain and windy conditions Friday
night and Saturday. This also looks like a good pattern for a lot of
snow in the Cascades with relatively low snow levels and good
westerly orographic.

A cold upper trough will settle over the area on Sunday. Snow
levels will drop below 1000 feet by Sunday night bringing a threat
of snow showers to parts of the lowlands. However, as is often
the case, things will be drying out by the time the snow levels
get low enough for lowland snow. Monday looks mainly dry and cool
although a few showers might linger. Schneider


.AVIATION...Air mass is dry and stable this morning with northerly
flow aloft. Flow aloft will back to west-southwest by 00Z in advance
of approaching upper level trough moving down the B.C. coast. High
and mid level moisture will gradually increase through the day with
rain developing after 05Z. VFR ceilings will lower to MVFR across
the region by 09Z-12Z Wednesday.

KSEA...VFR with gradually increasing high and mid level clouds
through the day. Rain developing by 06Z with ceilings lowering to
MVFR 09Z-12Z. Light southeast wind this morning becoming southerly
this afternoon and increasing to 7 to 12 knots after 03Z.  27


High pressure along and just off the WA coast will result in
light onshore flow today. Expect a 1009 mb low and associated weak
cold front to move southeast across the area late tonight through
early Wednesday. Moderate onshore or westerly flow will prevail
in the wake of the front on Wednesday. Expect the flow to weaken
Wednesday night but remain onshore.

Beyond Wednesday, it still looks like the weather pattern will
become more active. The Saturday system may result in gale force
winds over parts of the area, the central and eastern Strait of
Juan de Fuca in particular.


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


WA...Winter Weather Advisory from 7 PM this evening to 10 AM PST
     Wednesday for Cascades of Pierce and Lewis Counties-
     Cascades of Snohomish and King Counties-Cascades of Whatcom
     and Skagit Counties.



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