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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 220409

Area Forecast Discussion
National Weather Service Seattle WA
809 PM PST Mon Jan 21 2019

.UPDATE...A warm occlusion entering the offshore waters will move
onshore on Tuesday. Then a weakening low will move by to the north
of the area Tuesday night. These systems will give rain to the
area Tuesday into Tuesday night. Breezy to windy conditions will
be limited to the coast and northern interior areas late tonight
into Tuesday with windy conditions spreading into the central
Puget Sound and south interior locations Tuesday night. Rainfall
over the Olympics with the incoming system appears insufficient to
cause to Skokomish River to flood, so the Hydrology section is
updated below. Public forecasts are in good shape this evening.
The Aviation and Marine sections below. Albrecht


.AVIATION...A frontal system about 250 nm offshore will move into
Western Washington Tuesday morning. This front will give lowering
ceilings and developing light rain as it approaches tonight into
Tuesday morning and will give increasing southeasterly winds to
the coast and northern interior locations. The flow aloft is
westerly. The air mass will be stable and increasingly moist. 

Conditions were clear enough early this evening that radiational
cooling allowed temperatures to reach their dewpoints in some of
the valleys of the south interior. KOLM is LIFR in fog. Expect the
fog to thin out after midnight as mid and high clouds move in and
thicken. Conditions across most of the area will remain VFR
tonight, then MVFR conditions will dominate Tuesday afternoon
onward as low level moisture increases with the incoming front.
Some IFR conditions can be expected Tuesday afternoon in low
ceilings in a few locations like KPWT and KHQM. Albrecht

KSEA...High clouds will thicken overnight through Tuesday morning,
but expect VFR conditions to continue into late Tuesday morning.
MVFR ceilings will accompany the front Tuesday afternoon and will
likely continue into Tuesday evening. Light SE wind to 6 kt
tonight will increase to S 8-12 kt Tue afternoon and S 14-18G25 kt
Tue evening. Albrecht


.MARINE...A frontal system is entering the offshore waters this
evening. It will push through the coastal waters early Tuesday
then into the inland waters Tuesday afternoon. A quick look at
some 00Z model guidance shows the current forecasts are on track.
Gale warnings are in effect for later tonight into Tuesday over
the coastal waters and for very late tonight into Tuesday for the
eastern entrance to the strait, the northern inland waters, and
Admiralty Inlet. 

There remains a threat that gales will spread into Puget Sound and
Hood Canal Tuesday evening as a weakening low moves by to the
north of the waters and flow aloft interacts with the Olympics. A
small craft advisory remains in effect for Puget Sound and Hood
Canal late tonight into Tuesday, while a gale watch remains in
effect for those areas Tuesday night. 

High pressure over the waters Thursday through Saturday will give
generally light winds to the area. 

Minor coastal flooding remains likely along the Pacific coastline
in Grays Harbor county around the high tide early Tuesday
afternoon. The astronomical high tide is expected to be within
0.25 feet of HAT -the point where minor flooding begins- and tidal
anomalies are expected to be just large enough to result in some
minor flooding around the time of high tide. A coastal flood
advisory is in effect for the shoreline of Grays Harbor County.


HYDROLOGY...Rainfall Tuesday into early Wednesday will cause the
Skokomish River to rise. But at this time, the river is expected
to remain about 0.5 feet below flood stage. Elsewhere, river
flooding is not expected for the next 7 days. Albrecht


.PREV DISCUSSION... /issued 305 PM PST Mon Jan 21 2019/ 

SYNOPSIS...Weak high pressure will produce a break in 
precipitation into tonight. A pair of frontal systems will bring the 
return of rain at times Tuesday into Wednesday. A ridge of high 
pressure aloft will bring about another period of dry weather 
Thursday into early next week.

SHORT TERM...Still a fair amount of clouds over W WA this 
afternoon...although definitely some breaks out there for the sun to 
peek in from time to time. Temperatures area-wide have climbed into 
the 40s...with UIL almost reaching 50. Current radar is clear. All 
of this thanks to the upper ridge overhead...but this ridge will 
continue to diminish this evening and tonight as the next system 
approaches from the west.

Models remain pretty uniform with regard to the timing of the next 
system...making its way to the coast early Tuesday morning bringing 
the usual mix of wind and rain to the area. Winds will be breezy to 
locally windy for the area...however speeds do not look to encroach 
on headline worthy thresholds at this time. This initial system has 
a hard time staying together as it moves eastward and as such precip 
amounts look to be initially light. A secondary system looks a bit 
wetter though as it follows right on the heels of the first 
one...impacting the area Tuesday afternoon and evening. This will 
allow for the breezy/windy conditions to persist into 
Wednesday...but will also result in rising snow levels. This system 
does not look to linger very long...being east of the Cascades just 
after sunset Wednesday. The rising snow level combined with the 
short stay of the system shakes confidence a bit with regards as to 
whether any snow headlines will be needed for the mountains. At this 
time...leaning toward not doing anything...but will advise next 
shift that forecast could go either way. Models remain on track for 
upper level ridging to start building into the area overnight 
Wednesday and into Thursday. 

Temperatures may drop a degree or two Tuesday...but bump back up 
Wednesday with lowland temps nearing 50 degrees and really not 
deviating much from that for Thursday.  SMR

LONG TERM...The aforementioned upper level ridge looks to be the 
main weather feature for the long term once it sets up Thursday. 
Models attempt to bring a system into the area Thursday night into 
Friday...and while the means differ...the GFS keeps the system and 
any associated activity to the north while the ECMWF simply has the 
system fizzle out...the bottom line remains the same as dry 
conditions look to remain in place. As the ridge axis and center of 
high pressure gradually moves eastward throughout the second half of 
the week and into the weekend...those dry conditions look to 
persist. There is the slightest inkling that the tail end of a 
system might nick the area Sunday...but models almost seem to be 
advertising their retreat from this solution with any potential 
precip being limited to the Cascades and very minimal at best.  SMR

AVIATION...A frontal system will move into the area tonight and
Tuesday with westerly flow aloft. At the surface, weak high pressure 
over the area will shift inland tonight with increasing south to 
southeast flow into Tuesday as the frontal system nears.
The air mass is stable.

Areas of ceilings 4000-6000 feet should break up this afternoon and 
evening. At the same time, high clouds will increase this evening 
with ceilings lowering to a mid level deck later tonight.
VFR low clouds and rain will develop late tonight and Tuesday 
morning. Ceilings will continue to lower during the day Tuesday with 
MVFR or IFR conditions developing by afternoon.

KSEA...Ceilings 5000-6000 feet should break up in the next few hours 
with mostly high clouds this evening. Ceilings will lower again 
tonight and Tuesday morning as a front moves onshore and rain 
develops. Southerly wind 3-6 knots will increase to 10 to 15 knots 
Tuesday afternoon. Schneider

MARINE...A frontal system will move into the area tonight and 
Tuesday and then inland Tuesday night and Wednesday. Southerly small 
craft advisory winds are likely most waters during this period. 
Marginal gales are possible at times Coastal Waters, entrances to 
the Strait, and Northern Inland Waters. 

Winds will be relatively light again Thursday through Saturday with 
with weak high pressure over the area. 

Minor tidal overflow is possible at the coast early Tuesday 
afternoon due to high astronomical tides and slight pressure 
anomalies. A similar round of minor tidal overflow is possible for 
the inland Waters late Wednesday morning. Schneider

HYDROLOGY...From Previous Discussion...With the expected pair of 
fronts passing through the area Tuesday and Wednesday as well as 
considering how easily the Skokomish river made it up to flood stage 
with the previous weather system...the forecast for the river will 
certainly merit watching. For now, progs have the river starting 
upward again on Tuesday and do not have a forecast for Wednesday--
and that would be the day to watch to see if models keep up this 
trend. Elsewhere, river flooding is not expected over the next 7 


WA...Coastal Flood Advisory from noon to 4 PM PST Tuesday for Central 

PZ...Gale Warning from midnight tonight to 2 PM PST Tuesday 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.

     Gale Warning from 4 AM to 4 PM PST Tuesday for Admiralty Inlet-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Northern 
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory from midnight tonight to 4 AM PST Wednesday 
     for West Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Watch from Tuesday evening through late Tuesday night for 
     Puget Sound and Hood Canal.

     Small Craft Advisory from midnight tonight to 7 PM PST Tuesday 
     for Puget Sound and Hood Canal.