Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 201752
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
952 AM PST Fri Jan 20 2017

.SYNOPSIS...A broad upper level trough will remain over the
offshore waters through the weekend. A series of systems rotating
around it will bring showers and breezy conditions at times to
Western Washington along with seasonable temperatures. By early
next week, a ridge of high pressure will build into the Pacific
Northwest for the possibility of several dry days.

&&

.SHORT TERM...KLGX shows showers moving up the south Washington
coast. Expect these showers to skirt the southwest part of the
forecast area this morning. The GFS potential vorticity does not
match perfectly with the water vapor imagery for these showers and
it looks like they may turn out to be slightly more widespread
than expected. Will keep an eye on the showers and may edit the
POPs for this evening in the afternoon forecast package.

Also keeping an eye on the pressure gradients across the
Cascades. Seattle is currently 8 mb lower than Wenatchee, so
currently expecting just below advisory level east winds in the
foothills and across the Cascades. This gradient is not forecast
to increase by more than another mb or so.

An upper level trough and its associated upper lows are forecast
to spin offshore through Sunday night before the last upper low
moves to the south into the California coastal waters on Monday.
Expect seasonable temperatures and showers around portions of
Western Washington through Sunday night, typical for this time of
the year. JSmith

.LONG TERM...Extended models in good agreement this morning with
the trough diving south on Monday and dry northerly flow aloft
developing over Western Washington. Will wait for another model
run or two to totally remove the chance of showers from the Monday
forecast. For now have lowered the pops down to 20 percent Monday
afternoon. Upper level ridge nosing into British Columbia Tuesday
and Wednesday with the upper level ridge axis just offshore or right
over Western Washington. Light low level offshore flow. At this
point there isn`t any mention of fog in the forecast but if the
model trends continue given the recent heavy rains and the weak
low level offshore flow look for more morning fog to get added to
the forecast in later packages. Extended model agreement breaks
down on Thursday with the ECMWF shifting the ridge east and
bringing a weak system into the area. The GFS is a little slower
with the ridge movement east and weaker with this feature. Current
forecast more in line with the GFS. With the lack of consistency
will stay with the dry forecast for Thursday. Felton

&&

.AVIATION...An occluded front will spread light rain to the area
today, mainly the coast and southwest Washington. The flow aloft
is southerly. Surface flow is E/NE and ceilings are mainly VFR,
aside from patchy low clouds and fog in the south sound this
morning. An upper level trough will maintain isolated to
scattered showers as it moves inland tonight. 33

KSEA...N/NE surface flow to 10 kt today with VFR conditions.
Light rain in the vicinity this afternoon and evening. 33

&&

.MARINE...An occluded front with bring easterly gales to the
Coastal Waters today with Small Craft Advisory winds in the
Strait of Juan de Fuca and northern waters. Meanwhile, west swells
will rise to 16-21 ft, peaking on Saturday. A 960-965 mb surface
low will track along 130W late Saturday night with another
occluded front moving inland on Sunday. This front may produce
gale force winds over the Coastal Waters again. The flow over
Western WA will turn offshore early next week as the surface low
weakens and drifts south offshore. 33

&&

.HYDROLOGY...Upstream gages on the Skokomish River are showing
signs of receding, and the river has started to recede near
Potlatch as well. Expect the river to fall below flood stage this
evening. No additional flooding is expected at this time with only
showers in the short term forecast and then a switch to drier
conditions starting Monday.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...High Surf Advisory from midnight tonight to 6 PM PST Saturday
     for Central Coast-North Coast.

PZ...Small Craft Advisory from 4 PM this afternoon to 4 AM PST
     Saturday for Coastal Waters From Cape Flattery To James
     Island 10 To 60 Nm-Coastal Waters From Cape Flattery To
     James Island Out 10 Nm-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.

     Gale Warning until 4 PM PST this afternoon for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-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 for rough bar until 4 PM PST Saturday for
     Grays Harbor Bar.

     Small Craft Advisory until 10 PM PST this evening for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 10 AM PST this morning for Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 4 PM PST this afternoon for Admiralty
     Inlet-West Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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



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