Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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191
FXUS66 KSEW 221616
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
916 AM PDT Fri Sep 22 2017

.SYNOPSIS...An offshore upper level ridge will bring clearing
skies today and a little warming today and Saturday. This weak ridge
will remain over the region through Tuesday but will probably allow
transient weak weather systems to bring more clouds and some spotty
light rain to mainly the coast and the northern portions of the area
from Saturday onward. The ridge will build Tuesday or Wednesday with
a return to mostly sunny skies and continued warming temperatures.

&&

.SHORT TERM...Satellite imagery shows W WA is still under the
influence of somewhat moist NE flow aloft from the large upper level
low centered over SW ID this morning. RADAR is not picking any
showers up over the area, but there was quite a bit of mid level
cloud cover earlier this morning. However, the transition to drier N-
NNE flow aloft is well underway as those mid level clouds are
decreasing rapidly enough for mostly sunny skies by late morning.
The exception will be the Cascades where there could still be some
spotty shower activity this afternoon. Highs should be a few degrees
warmer today but still remaining in the 60s.

The upper level ridge just offshore will still tilt inland over
southern B.C. Saturday and Sunday. The ridge is weak enough so that
transient weather systems will pass through the ridge bringing
clouds and a chance of spotty light rain at times. The models have
not been handling these weak systems very consistently so confidence
in timing any of them is not very high. The first system to reach
the area is a weak warm front that should develop out of the mass of
clouds currently W of the Central B.C. coast. Earlier model runs had
kept light rain from this warm front offshore, but the latest 06Z
GFS, 00Z ECMWF, and 12Z NAM now bring it to the coast and Olympics
Saturday morning. Model consensus seems good enough to add this to
the forecast this morning.

Model run-to-run consistency is still in wiggle-waggle mode for the
chance of rain on Sunday. The overnight model runs now keep precip
from the next weak system up over B.C.  I`ll wait for the 12Z suite
of models before making any changes for Sunday. Sunday night and
Monday now look like the better periods for a little light rain. Kam

.LONG TERM...Previous discussion from the 320 AM AFD...Models agree
on approaching precip for Monday...sort of. The GFS has basically
taken the rain it was expecting for Sunday afternoon from solution
of 24 hours ago and bumped it to Monday while the ECMWF remains the
drier solution of the two with the system dissipating quickly as it
approaches. The Euro solution would still allow for some
showers...but POPs would likely be in the slight chance category at
best. Coming into better alignment...models agree that beyond Monday
the ridge will amplify keeping dry conditions in place at least
until Friday morning. Temperatures during this time frame will
continue to climb...allowing many locations to see lower to mid 70s
by the second half of the upcoming week.  SMR

&&

.AVIATION...A broad upper low over the Great Basin will migrate east
over the Rockies on Saturday. High pressure will remain just off the
Pacific Northwest coast with light northerly flow aloft. The air
mass will be mostly dry with moisture near the surface. Residual
patchy low clouds and fog this morning will scatter out before noon.
Otherwise VFR skies. Patchy IFR conditions will be possible again
late tonight/Saturday in fog prone locations including KOLM and KHQM.

KSEA...VFR conditions today. Light wind, becoming NW to 5 kt after
midday. Light wind after sunset. dtm

&&

.MARINE...A large area of high pressure over the NE Pacific and over
the Washington waters will result in light onshore flow through the
weekend. Winds will generally remain below 15 kt.  A warm front will
clip the region on Monday with a chance for small craft winds,
mainly in the coastal waters. Northerly or offshore flow develops
mid week. dtm

&&

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

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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