Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 211637
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
936 AM PDT Fri Jul 21 2017

.SYNOPSIS...A couple of small upper level troughs will brush Western
Washington today through Saturday bringing clouds to all of the area
and a chance of showers to the coast and north part. An upper level
ridge will gradually build northward over the region late Saturday
and Sunday and onshore flow will return for dry and warm weather
through most of next week.

&&

.SHORT TERM...Westerly flow aloft prevails over the PacNW this
morning. Water vapor imagery shows a couple of upper level shortwave
troughs offshore embedded in the W flow aloft, and headed toward
southern B.C.  Both of these troughs, with weak low level warm
advection, will brush W WA for a chance of light rain over the coast
and north part through Saturday. The interior from about Everett
southward should be dry. Clouds from the first approaching trough
are already spreading across W WA this morning. At this point it
looks like there will be enough cloud cover to keep the area mostly
cloudy today. RADAR is also picking up showers off the N coast,
which is covered in the current forecast.

The strong upper level longwave ridge currently over the central
U.S. will be shifting back over the SW U.S. on Saturday, and in the
process gradually building northward over the PacNW. The departing
shortwave trough and the building ridge will end the light rain over
the area by Saturday evening and start a minor warming and drying
trend. The GFS doesn`t really clear out the low level moisture until
Sunday afternoon the air mass will have already warmed enough for
highs slightly above normal. Kam

.LONG TERM...The GFS and ECMWF are in fair agreement for early next
week. Weak W-SW flow aloft will prevail over the region through the
period. Monday starts off with weak W flow aloft as a weak upper
level shortwave ridge is crossing the area. On Tuesday, and upper
level low approaching the central B.C. coast will shift the flow to
SW, with lowering 500 MB heights indicating a little air mass
cooling. Both models keep the low over the B.C. coast through
Thursday and the GFS is a little stronger with the low than the
ECMWF. The surface ridge just offshore will maintain low level
onshore flow all next week. There will probably be a strong marine
push sometime this week but since the models are not completely in
phase the timing of the push is still uncertain. Kam

&&

.AVIATION...Moderate westerly flow aloft will continue through
Saturday. A weak warm advection pattern today will support plenty
of clouds in the 100-200 layer today, and some rain will brush the
northwest Olympic Peninsula and possibly the far north interior
near BLI this afternoon and evening. Ceilings of 030-060 will be
common on Saturday. The air mass will be stable and a little
moist today, becoming more moist late tonight and Saturday.

KSEA...Mainly thin mid-level cloud layers today between 100-200. A
moister low-level air mass will arrive late tonight, leading to
cig development in the 030-050 range after midnight and continuing
through Saturday morning. Next TAF issuance will show a SW wind
today, then a north wind shift for the evening hours. However,
there is conflicting guidance on this matter, so the north wind
shift is not assured this evening. Haner

&&

.MARINE...A weak warm front will brush across the offshore waters
this afternoon and tonight, with an increase in southeast wind
through this evening over the coastal waters. This pattern will
lead to only weak onshore flow through the Strait this evening.

Once the warm front dissipates on Saturday, a more typical
summertime pattern of offshore high pressure and lower pressure
east of the Cascades will resume. This will mean moderate onshore
flow will for Saturday and Sunday, with small craft advisory
northwest winds at times over the coastal waters. Onshore flow
will increase on Monday and become strong on Tuesday, by which
time westerly gales will be possible through the Strait. Haner

&&

.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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