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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

FXUS66 KSEW 210104

Area Forecast Discussion
National Weather Service Seattle WA
504 PM PST Sun Jan 20 2019

.SYNOPSIS...Low pressure may produce some sprinkles across extreme 
southern Washington this evening before ending. A break in 
precipitation is expected Monday and Monday night. Another weak 
system tracks further north across the area Tuesday and Tuesday 
night producing more widespread light rain. A strong ridge of high 
pressure building offshore will move directly over our region 
providing mild and dry weather into next weekend.


.SHORT TERM...IR/WV satellite imagery shows a vigorous low
pressure circulation moving into Oregon/Northern CA late this
afternoon. Radar shows some of the lighter norther edge
precipitation barely reaching far southwest Washington. Radar
returns are weak and only some sprinkles with traces are expected
before the entire system shifts east by midnight. 

A break in rain is expected overnight through Monday, with all 
the rain and shower activity south of WA under a progressive 
split flow pattern. filtered sunshine should still allow highs to
reach the low 50s, several degrees above average. 

Models bring the next system in from the northwest and on a
further northward track into Western Washington on Tuesday. 
Widespread light rain will spread into the area with amounts up to
a quarter inch of rain the lowlands and 1-3 inches of snow above 
3000 feet in the Cascades of Pierce and Lewis counties. Overall,
precipitation will be light and temperatures above average. 


.LONG TERM...The GFS and ECMWF show a trailing wave riding quickly
into Western Washington Wednesday. This system could also produce
some breezy winds and bring a little snow to the mountains. 

virtually all global models build a strong ridge just offshore
which will shift directly over the Northwest Friday into next
weekend. Expect partial sunshine and near to above average highs.
Low 50s are possible in exposed locations around Puget Sound. The
ridge appears to strengthen and expand influence over the area 
next weekend with dry weather. Mercer


.AVIATION...Westerly flow aloft turning northwesterly this evening 
as an upper level trough sets up over Eastern Washington. Northerly 
gradients at the surface will become light late this afternoon into 
this evening.

Cigs a mix of VFR to MVFR conditions over W WA this afternoon with 
low clouds over most of the Puget Sound area proving to be rather 
resilient. Some additional clearing might take place into the 
evening...but not expecting much change before cigs start to lower 
due to stratus development tonight and into early Monday morning. As 
with any low level cloud development...would expect the usual fog 
prone areas to see the usual overnight reduction in 
visibility...with IFR conditions expected in such locations as OLM 
and PWT. Given how difficult it was to get clearing today... would 
not be afraid to say that any fog or low clouds that develop will 
hang on into the mid to late morning.

KSEA...MVFR conditions continue to linger. Some improvement might 
occur...but likely only to improve case scenario 
being up into borderline high-MVFR to low-VFR. Cigs expected to drop 
into low end MVFR tonight and into the morning as stratus makes its 
way into the terminal. Northerly winds 5 to 10 knots into Monday 
afternoon. Felton/SMR 


.MARINE...Swells have subsided and will wipe away SCAs for 
haz seas with the afternoon issuance. Winds at Sand Heads in 
Canadian waters currently meeting SCA have opted to 
bump up the start of the SCA for the N interior. These winds do not 
look to last much beyond inherited end time might be a 
touch pessimistic but will be retained. An upper level ridge will 
bring a brief break in the action Monday but then a series of weak 
frontal systems will affect the region Monday night through 
Wednesday. These systems look to bring another round of headline-
worthy winds to area waters. Once those are finished by mid 
week...the Thursday through Saturday period appears to be relatively 
quiet. SMR


.HYDROLOGY...The idea that a couple of weak fronts would just brush 
the area Tuesday and Wednesday is trending toward wetter solutions. 
Considering how easily the Skokomish river made it up to flood stage 
with this past front, I think now that the river forecast bears 
watching for the middle of the week. For now, I just have the river 
starting upward again on Tuesday and do not have a forecast for 
Wednesday--but that's the day to watch of the models keep up this 
trend. Elsewhere, river flooding is not expected over the next 7 



WA...Coastal Flood Advisory from 10 AM to 2 PM PST Monday for the central
     Washington coastline of Grays Harbor County.

PZ...Small Craft Advisory until 1 AM PST Monday for Northern Inland 
     Waters Including The San Juan Islands.