Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 061156
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
330 AM PST Wed Dec 6 2017

.SYNOPSIS...A strong upper ridge will bring dry weather to Western
Washington for at least the next week. The air will be stable and
increasingly stagnant by the end of the week. High clouds may arrive
over the weekend as a weak weather system passes well to the north.

&&

.SHORT TERM...Satellite imagery and obs indicate some low clouds
along the Strait of Juan De Fuca and around parts of the southwest
interior. Fog and freezing fog remain limited in coverage, mostly
around the southwest interior and a few patchy spots elsewhere.
Decent offshore gradients continue with about -10mb SEA-EAT across
the Cascades and roughly -5mb PDX-BLI giving the northerly
component. The offshore flow has allowed drier air to filter down to
the surface with wider dew point spreads than the past couple
mornings. Based on the trends, will keep fog/freezing fog limited to
the southwest interior/far south Sound. Enough mixing should occur
today to allow fog and clouds to scatter out. Most the area should
have sunshine most the day with highs mainly in the mid to upper 40s.

The upper ridge dominates the region Thursday and Friday with dry
weather and lighter pressure gradients. This would usually support
more widespread night/morning fog over the lowlands with strong
radiational cooling. The temperature inversion will persist, but the
offshore flow has cleared out much of the surface based moisture.
Without a new influx of moisture, radiational cooling and fog
formation will mostly develop in the typical low lying areas of the
southwest interior and south Puget Sound. Ventilation will become
poor and air quality could start to deteriorate by the end of the
week with some hazy skies.

.LONG TERM...The GFS now resembles the ECMWF which shows a weak
system moving over the ridge well to the north of WA over the
weekend. This system poses no threat of precipitation or even
significant cloud cover, but might spread some high cloud across the
area at times. It remains to be seen on how much these clouds will
interfere with radiational fog formation. Kept fog limited to mostly
the southwest interior for now. Clouds were increased but partial
sunshine is still expected. Highs will be little changed in the 40s.

Models indicate the upper level ridge will strengthen again and
maintain the dry, stagnant pattern into early next week. There will
be areas of fog, especially in the typical fog prone locations. Air
quality issues are possible through the Saturday to Tuesday period.
Mercer

&&

.AVIATION...An upper level ridge will remain over the area through
the weekend. Northerly flow aloft becoming light tonight.
Offshore flow will continue at the lower levels. The air mass is
stable and dry except for patchy low level moisture.

Clear skies across most of the the area early this morning. The
exception being some stratus around the lower slopes of the
Olympics. Strong offshore and northerly gradients with nearly
-5 mb KUIL-KBLI and KOLM-KBLI gradient still around -2.5 mb. With
 the gradients this strong little if any fog will form this
 morning leaving clear skies except near the Olympics where the
 stratus deck around 1000 feet will continue into the afternoon
 hours before dissipating.

KSEA...Clear skies. North northeast wind 10 to 15 knots into the
evening hours.

&&

.MARINE...High pressure will remain over the area through the
weekend with low level offshore flow. Small craft advisory
easterlies will continue at times near the west entrance to the
Strait of Juan de Fuca and over most coastal waters near gaps in
the coastal terrain through Thursday evening. Felton

&&

.HYDROLOGY...There will not be any river flooding in the next
week.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 10 PM PST Thursday 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 Point Grenville To Cape Shoalwater 10
     To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater Out 10 Nm-West Entrance U.S. Waters Strait Of
     Juan De Fuca.

&&

$$

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