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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
000
FXUS66 KSEW 161057
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
257 AM PST Wed Jan 16 2019

.SYNOPSIS...A return to damp conditions is expected, beginning
today, and lingering through at least early next week. Rain will 
increase in coverage later today as a strong closed low moves 
east across the northern Pacific. Rain will continue through 
Thursday and into Friday with breezy conditions. Another round of 
rain is set for Saturday and Sunday, with showers likely through 
early next week. By the latter half of next week, a pattern change
is likely, allowing western Washington to dry out. 

&&

.SHORT TERM...An extensive area of cyclonic flow exists across
much of the northern Pacific this morning, with several distinct 
shorter wave troughs located within. A weak ridge is in place 
across the central US, with this ridge seemingly more amplified 
further north from the northern Rockies into Alberta and British 
Columbia. Near the sfc, weak offshore flow continues across the 
Pacific Northwest as high pressure resides across the Rockies and 
low pressure exists well offshore. Upon taking a closer look at 
morning satellite imagery, particularly water vapor, a subtle 
shortwave trough can be seen lifting north just offshore the 
Washington/Oregon coastlines. Coinciding with this feature is a 
deep layer of moisture with noticeable coverage of stratiform 
rain across the offshore waters. These showers will continue to 
pull north thru the afternoon hours with locations along the coast
and the Olympic peninsula likely to see some activity today. QFP 
values remain light so do not expect noticeable impacts with this 
wave. 

Later this afternoon and into the evening, a strong closed low
across the Pacific will move closer to the Washington/Oregon
coastal vicinity. As it begins to lift north, a decent swath of
widespread rain will move into the area from the southwest. Rain
will continue into Thursday as the system continues to lift 
north, still offshore of Washington coastline, though it is 
expected to begin to weaken. Hi-res guidance suggest winds will 
become breezy across portions of the area tonight into Thursday, 
especially the coast, but wind speeds look to remain below any 
type of advisory/warning criteria. The strongest winds will remain
offshore. By Thursday evening, the low will be approaching the 
northern half of Vancouver Island, with associated sfc low 
essentially stacked underneath of it. Residual rain and wind will 
exist across much of the area, with perhaps the breeziest winds 
occurring as the system exits the region late Thursday night thru 
Friday morning. This looks to be the time for continue to monitor 
the potential for wind advisory conditions but as of now, 
confidence is low. 

Kovacik

.LONG TERM...Rain looks to taper to showers briefly Friday afternoon 
as a weak short wave ridge passes overhead ahead of the next rain
maker. Still seeing evidence of a notable atmospheric river 
associated with this system which looks to pull another round of 
moderate to locally heavier rainfall across much of the CWA even 
as the parent low lifts northeast to the central BC coast. Luckily
given the large displacement of the parent low and the 
atmospheric river by Saturday afternoon, much of the rain will 
scour out of the area. Could see another quick break in the 
widespread rain thru early Sunday but given a look upstream, 
another shortwave will approaching the coast. Another round of 
rain appears likely at this point. 

After Sunday's system, a broader ridge tries to build in. At this
point in time, the ridge appears dirty, with moisture riding over
top of it and perhaps allowing for intermittent periods of rain
thru mid week. Thereafter, a major pattern change is still
forecast to occur, one in which a high amplitude trough will 
build across the eastern US, and a high amplitude ridge will set
up along the West Coast. If you're a cold weather lover, you 
might want to head east, otherwise, western Washington will 
provide for a period of rain-free weather thru the end of the 
week.

Kovacik

&&

.AVIATION...Southerly flow aloft will continue today and tonight
with an upper trough offshore. A frontal system will spread rain
into the area from the southwest tonight as the upper trough
approaches. At the surface, easterly offshore flow will continue
with high pressure inland and lower pressure offshore. The air
mass is mostly dry and stable.

There will be patchy fog and low clouds again this morning, 
mainly over the south interior. Offshore flow and some mid and 
high clouds at times should keep fog more limited this morning
compared to yesterday however. 

Moisture will increase from the southwest later today and tonight
as a frontal system arrives. Offshore flow should keep the low 
levels dry however. Rain will spread into the area but ceilings 
are expected to remain VFR, even towards the coast. 

KSEA...Mid and high clouds at times today. Fog should remain
south of the terminal this morning with offshore easterly winds.
Ceilings will gradually lower tonight as rain develops but they 
are expected to remain VFR. Easterly wind 5-10 knots increasing to
10-15 knots tonight and becoming a bit gusty. Schneider

&&

.MARINE...South to southeast flow will increase across the region
today as a pair of strong Pacific frontal systems reach the area.
Small Craft Advisory winds are expected over all waters with 
gales over the Coastal Waters and west entrance to the Strait of 
Juan de Fuca. A third strong system will reach the area on Friday 
with seas building to 15 to 18 feet. This system will move inland 
Friday night and Saturday, maintaining strong southerly flow over 
the waters. A weaker trough may clip the area on Sunday. 33

&&

.HYDROLOGY...A return to a period of wet weather will begin today
and continue through early next week. The accumulation of rainfall
with each passing system over time is expected to spark a 
response in river levels. Given some of the higher rainfall totals
expected across the Olympics, the Skokomish may reach flood stage
by this weekend. Elsewhere, not expecting river flooding over the
next 7 days at this time.

Kovacik

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 7 PM PST this evening 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.

     Gale Warning from 7 PM this evening to 4 PM PST Thursday for 
     Coastal Waters From Cape Flattery To James Island 10 To 60 
     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.

     Gale Warning from 7 PM this evening to 6 AM PST Thursday for 
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     Coastal Waters From James Island To Point Grenville Out 10 
     Nm-Coastal Waters From Point Grenville To Cape Shoalwater 
     Out 10 Nm-West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 4 PM PST Thursday for Central U.S. 
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters 
     Strait Of Juan De Fuca.

     Small Craft Advisory from 6 PM this evening to 4 PM PST Thursday 
     for Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 7 PM this evening to 10 AM PST 
     Thursday for Admiralty Inlet-Puget Sound and Hood Canal.

&&

$$

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