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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 151117

Area Forecast Discussion
National Weather Service Seattle WA
317 AM PST Tue Jan 15 2019

.SYNOPSIS...One more dry weather day expected as western
Washington remains in between a ridge to the east and a trough to
the west. Troughing will become a greater influence by Wednesday 
with a return to some light showers. The next Pacific storm system
will affect the area late Wednesday thru early Friday, bringing 
with it more widespread rain and breezy winds. A fairly unsettled 
pattern looks likely into early week with several additional 
chances for rain. The pattern may be on the verge of change
towards mid week, with the likely setup for the Pacific Northwest
being a ridge and a period of dry weather.


.SHORT TERM...The synoptic pattern early this morning can be 
characterized by a broad area of cyclonic flow across the eastern 
Pacific and extending into portions of the western US, with a 
ridge extending downstream of this feature along the spine of the 
Rockies. The sfc pattern essentially matches the mid and upper 
levels, with low pressure located west of the California coast and
high pressure extending up along the Rocky Mountain states.

Aforementioned Rocky Mountain mid level ridge influence once over
the Pacific Northwest is exiting this morning, leaving the local
area in between it and the trough off the coast. Water vapor
imagery clearly shows a quick moving shortwave trough migrating
south to north across the region this morning, carrying with it
nothing more than some high level clouds. Prior to the arrival of
these cirrus clouds, dense fog had developed across a good portion
of southern Puget Sound, with obs showing vsby reductions to 1/4 
mile or less. Clear skies and generally benign winds were expected
to allow the fog to spread across most of Puget Sound, where a 
Dense Fog Advisory has been issued. The high clouds along with 
evidence of an easterly gradient has somewhat disturbed the 
development of fog across portions of Puget Sound. Could very well
pan out that most fog remains confined to the south Sound but 
will leave headlines as is for now. To make matters a little more 
problematic across the south Sound, patches of freezing fog will 
be likely thru mid morning. Please use extra caution during the 
morning commute given the potential for reduced visibility and
slick roadways. As the day progresses today, fog could have a 
hard time dissipating in some locations, but overall do expect to 
see some more cloud cover than previous days given that the 
trough west of the California coast will begin to lift north.

By Wednesday, another quick moving shortwave trough will skirt the
offshore waters of western Washington as a much deeper Pacific
storm system digs closer towards the offshore waters of the
Pacific Northwest upstream of this perturbation. Hi-res guidance
indicates that this quick moving wave will bring the return of
showers to the coast and perhaps to a decent portion of the 
Olympic Peninsula, altho QPF seems minimal. Wouldn't be surprised 
if a few showers snuck into the interior by the afternoon but have
preferred to confine POPs mostly to the western portion of the 

By Wednesday night and Thursday, the stronger Pacific storm system
will move much closer to the local area. This will certainly
increase shower coverage for a larger chunk of the CWA. At this
time, it remains strongest over the offshore waters, which will
likely have wind and wave implications, but as the system moves
closer to land, it weakens noticeably and lifts towards
BC/Vancouver Island. The sfc low looks to follow suit but will
have to watch for wind potential with this system. Right now not
seeing winds look as high as the previous several systems, but
conditions will likely be breezy.


.LONG TERM...As Thursday's system continues to lift north into
Friday, rain should taper to showers with a brief ridge moving
across the area. This break will be short-lived as another system
traverses the Pacific and begins to lift north into BC. Obviously
some discrepancies exist at this point but guidance is set at 
this point on a noticeable river of atmospheric moisture, and 
with broad troughing continuing across the Pacific, shortwaves 
look to have no issue tapping into the moisture, making for what 
looks to be at least a wet start to the weekend. Luckily a 
progressive enough pattern should help carry the broad troughing 
eastward into the interior of the western US. From there the next 
ridge will set up, giving western Washington its next break from 
widespread rain. How long this will last will remain fuzzy for 
the time being but there is evidence of a pattern change, one in 
which troughing sets up across the eastern US and ridging parks 
over the western US. This would lean the forecast towards the 
drier side of things.



.AVIATION...High pressure aloft over the area will weaken and 
shift inland today and tonight as a weak system approaches from 
the southwest. The flow aloft will be southerly. At the surface, 
offshore flow will continue with high pressure inland and lower 
pressure offshore. The air mass is stable and generally dry.

There is an area of fog over the south Sound early this morning.
This fog patch was actually larger a few hours ago but the 
easterly gradients appear to be eroding it a bit on its eastern 
edge. Fog will likely linger over parts of the Southwest Interior
today perhaps breaking up for a few hours late in the day.

There are also some fog patches further north over Puget Sound 
and north into the San Juans but again the easterly winds seems 
to be keeping fog mostly near or over the water with satellite
showing a trend for decreasing fog overall in the last few hours.
There will likely continue to be fog patches over northern Puget 
Sound and the San Juans this morning but it does not look like it 
will become widespread. 

An area of Stratus has also formed over the Eastern Strait, 
probably aided by northeasterly upslope flow into the Olympics. 
This area of stratus is expected to break up later today. 

KSEA...Fog was knocking on the door a few hours ago but easterly
gradients appear to have pushed the fog a bit west and south of
the terminal. There is still a chance some LIFR fog could slosh 
into the terminal later this morning but it is looking less likely
than earlier thinking. Otherwise, high clouds at times today. 
Light wind becoming northerly 3-6 knots later today. Schneider


.MARINE...Offshore flow will continue over the waters due to
strong high pressure east of the Cascades. Highest wind and waves
are near the west entrance to the Strait of Juan de Fuca - a Small
Craft Advisory remains in effect this morning. A pair of strong 
Pacific frontal systems will reach the area Wednesday and Thursday
for increasing S/SE flow over the waters - gales are possible. A 
third strong system may reach the area Friday night or Saturday. 


.HYDROLOGY...No hydro concerns are expected in the near term. The
next chance for widespread rainfall will be Wednesday night
through early Friday. At this time, no river flooding is 
expected, although with highest QPF amounts /generally 1-2 inches/ 
expected across the Olympics, will need to monitor the Skokomish 
River. Additional rainfall will be possible into the weekend, with
highest QPF across the higher terrain.



WA...Dense Fog Advisory until 9 AM PST this morning for Admiralty 
     Inlet Area-Bremerton and Vicinity-Everett and Vicinity-
     Seattle and Vicinity-Tacoma Area.

PZ...Small Craft Advisory until noon PST today 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-West Entrance U.S. Waters Strait Of 
     Juan De Fuca.