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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 171820

Area Forecast Discussion
National Weather Service Seattle WA
950 AM PST Sat Nov 17 2018...corrected

.SYNOPSIS...A dry weather pattern with areas of night and morning
fog will persist into early next week. Overnight temperatures 
will be on the cooler side, dropping into the 30s and even upper 
20s by Sunday morning at some locations. A series of weather 
systems will bring wet weather by the middle of next week through 
the holiday weekend.


.SHORT TERM...A strong ridge of high pressure will maintain dry 
weather over the Pacific Northwest through at least Monday or 
Tuesday of next week. Offshore flow will increase today with a 
1038 mb surface high over southern British Columbia extending east 
of the Cascades. The flow is more northerly this morning with the 
PDX-BLI gradient around -5 mb. Meso models show northely winds 
picking up this afternoon, around 10 to 20 mph. Drier air is already 
filtering into the area, with only patchy fog coverage this morning. 
This will clear out over the next couple hours with mostly sunny 
skies the rest of the day. Highs will reach the low 50s. 

Little change in temperatures or the overall weather pattern is 
expected through Monday. The flow will become more easterly tonight 
through Monday, allowing the northerly breeze to ease. Radiational 
cooling will be more efficient, with temperatures falling into 
the 30s or below freezing in cold pocket locations like Olympia and 
Shelton. While drier air is filtering into Western Washington, 
cooler low temperatures should allow fog to increase in coverage the 
next couple days. However, offshore flow and lower dew points will 
hinder widespread development. Fog or freezing fog may be locally 
dense in prone valleys, especially around the southwest interior and 
the interior south Puget Sound. Highs Sunday and Monday will reach 
the low to mid 50s with any morning fog burning off by early 
afternoon. Mercer 

.LONG TERM...Previous discussion...Tuesday looks like the last dry 
day for awhile. A weather system will approach the coast during the 
day, and clouds will increase across the area. Highs on Tuesday will 
again be in the low to mid 50s. The GFS and Euro differ slightly 
about when the rain will begin, with the Euro a bit slower. It does 
seem rain will start some time on Wednesday, so have kept the 
forecast of rain likely for Wednesday.

Model timing varies quite a bit after that, and so do the forecast 
tracks of the various fronts, with the Euro sending most of the 
energy into Oregon and the GFS moving the fronts right through 
Washington. For now a the forecast is for rain at times, with high 
pops but relatively low QPF, and there will be some dry periods 
between fronts. Highs will be somewhat lower than what we have been 
experiencing, mostly in the 45 to 50 range. The snow level will fall 
to 4000 to 5000 feet, which means most ski areas and passes will get 
snow at times. Burke


.AVIATION...High pressure and offshore flow will lead to dry and
stable conditions across Western WA. The flow aloft is northerly
with E/NE winds at the surface. There is patchy/shallow fog in 
the interior this morning which will scatter out by the afternoon.
Otherwise, expect VFR conditions. Patchy low clouds will likely 
reform overnight. 33

KSEA...Patchy low clouds near the terminal this morning will
scatter out for VFR conditions. N/NE winds to 8-12kt. Dry
conditions tonight. 33


.MARINE...Offshore flow will prevail across Western WA through
early next week due to strong high pressure east of the Cascades.
Winds are N/NE over the waters with Small Craft Advisories in
effect, specifically for the West Entrance and adjacent Coastal 
Waters, Northern Inland Waters, Puget Sound and Hood Canal. Winds
will ease late this afternoon. The next frontal system will 
approach the region on Tuesday then split and weaken as it moves 
inland on Wednesday. 33


.HYDROLOGY...High pressure is expected to maintain the dry weather
pattern through around Tuesday of next week. A parade of frontal 
systems will affect the region starting around Wednesday of next
week through the holiday weekend. Models show a progressive and split
flow pattern, which will tend to weaken these systems. Rain amounts 
with each system do not appear to be sufficient to cause flooding on 
area rivers. River flooding is not expected on area rivers through 
the next 7 days.


PZ...Small Craft Advisory until 4 PM PST this afternoon for Coastal 
     Waters From Cape Flattery To James Island Out 10 Nm-
     Northern Inland Waters Including The San Juan Islands-West 
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 5 PM PST this afternoon for Puget 
     Sound and Hood Canal.