Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 151614
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
915 AM PDT Tue Aug 15 2017

.SYNOPSIS...Northwest flow aloft coupled with low level
onshore flow will maintain seasonably mild summer weather through
this weekend. The northwest flow will bring a couple of weakening
cold fronts to the area Wednesday and Friday resulting in more
morning marine cloudiness. Lower pressure aloft is expected to
arrive this weekend with a small threat of showers.

&&

.SHORT TERM...A weak but broad upper trough resides over the
northern Rockies and Great Basin this morning with NW flow aloft
over Washington. A quite weak shortwave moving into British Columbia
embedded in the NW flow will bring limited higher level cloudiness
across Western Washington today.

Surface gradients were light onshore this morning. Satellite imagery
shows marine stratus and fog along the north coast and east through
the Strait to the Salish Sea, Whidbey Island, and south to the
northern end of the Kitsap Peninsula. The KUIL raob showed tops near
1000 ft. Expect the marine cloudiness to dissipate by midday.
Overall, a lot of sunshine today with seasonable temperatures.

The progs all show rising heights aloft through Thursday with an
upper ridge axis moving ashore for a gradual warming trend.
Increasing low level onshore flow late today and again late
Wednesday should help temper the warming though.

No changes to the current forecast at this time. Buehner

.LONG TERM...From the previous discussion...
As the upper level ridge exits...upper level flow does not really
change too much...becoming more westerly. Friday night and into
Saturday morning will see the associated upper level low swing
quickly through BC which may allow for a slight chance for precip to
persist into Saturday. The main impact from this system will be to
nudge temps back down to where they were for much of the week.
Another upper level ridge building over the Pacific looks to make
the late weekend and the start of next week to feel like deja vu all
over again as said ridge will exert a very similar type of influence
as described in the short term.  SMR

&&

.AVIATION...An upper level ridge centered offshore will continue
providing the region with NW flow aloft. The low level flow will
remain onshore. Expect areas of LIFR CIGs/IFR vsbys (over mainly the
coast) to dissipate by 1900 UTC.

KSEA...VFR. Winds will generally be northerly 10 kt or less.

&&

.MARINE...
Onshore or westerly flow of varying strength will persist the rest
of this week due to high pressure offshore with lower pressure east
of the Cascades. There is a possibility wind speeds could briefly
reach gale force over the central Strait of Juan de Fuca this
evening. The probability of gale force winds over the central and
eastern Strait of Juan de Fuca will be higher Wednesday evening.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 6 PM this evening to 5 AM PDT
     Wednesday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle/

An illustrated version of this forecast discussion can be seen at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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