Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 130403 CCA
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
800 PM PST Thu Jan 12 2017

.SYNOPSIS...High pressure over the region will allow cold dry
weather to continue through Friday. A weakening ridge along with
milder northwest flow aloft will allow temperatures to moderate over
the weekend. Wetter and warmer west flow aloft will bring a series
of frontal systems across the area from Monday through next week.

&&

.SHORT TERM...The B.C. surface ridge has shifted S over E WA at 1033
MB this evening. Dry low level offshore flow continues but is not
that strong. The upper level ridge over the NE Pacific remains in
place but has lost amplitude and will continue to weaken through
Sunday. A frontal system offshore near 140W continues to move
steadily E, trying to push through the intervening ridge. The ridge
amplitude is low enough so that quite a lot of high cloud cover
ahead of the front will move across the area this weekend. The
leading edge of the first batch of high clouds was just offshore at
03Z/7 PM and will spread overhead tonight. The high clouds could be
enough to moderate lows by a couple of degrees or so tonight,
keeping most places in the 20s. Temperatures at 03Z were in the mid
20s to mid 30s. Dewpoints were mainly in the lower 20s around Puget
Sound and in the teens over the NW interior where leftover dry air
from Fraser river outflow still persists.

The GFS and ECMWF have the offshore front falling apart before it
reaches W WA this weekend, keeping the area dry with high clouds and
somewhat warmer temperatures. The moderating air mass should allow
highs this weekend to reach near 40 on Saturday and the lower 40s on
Sunday. Lows should be mainly in the 20s but with some lower 30s
Sunday morning.

The NAM and Canadian models both agree with the GFS/ECMWF but in
addition showed some spotty light precip reaching the area Saturday
night and/or Sunday morning. The 18Z NAM had a little precip over
the coast, Olympics, and Cascades Saturday night and sometime on
Sunday, new 00Z NAM now has some just over the coast on Sunday. The
12Z Canadian was similar to the 18Z NAM. The current forecast has
chance POPS for that period which is fine for now. Kam

.LONG TERM...Previous discussion from the 3 PM AFD...A change in the
pattern begins around Monday. A series of wet weather systems will
reach the area, one after another. There is a chance of rain Monday
from the first approaching system. Though models differ on details,
Tuesday and Wednesday both look wet. The snow level will rise to
6000 or 7000 feet. Thursday will probably be more showery as a broad
upper trough moves over the region. Burke

&&

.HYDROLOGY...River flooding is not expected through Monday. A period
of heavy rain next Tuesday/Wednesday will cause rises on area
rivers. Minor flooding on a few rivers is possible. Note: Snow is
not a significant contributor to flood flows on western Washington
rivers. Even when there is snow on the ground, river flooding is
almost exclusively caused by runoff from heavy rain.

&&

.AVIATION...An upper ridge will build into the area through Friday.
Northwest flow aloft with increasing high clouds overnight. Surface
high pressure east of the Cascades will give light low level
easterly flow. The air mass is dry and stable. Patchy freezing fog
is possible Friday morning but should be limited to only the most
favored spots and the increasing cirrus could keep it from forming
all all. High clouds will thicken up Friday.

KSEA...Cirrus arrives overnight and will thicken up to an
overcast by afternoon. Light NE wind.

&&

.MARINE...Surface high pressure east of the Cascades will give
light offshore flow through Saturday.

A weak front will dissipate as it moves through the area Saturday
night or Sunday. A series of stronger fronts will reach the area
starting Monday or Monday night. Small craft advisory conditions are
likely over the coastal waters starting on Monday.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html


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