Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 262252
Area Forecast Discussion
National Weather Service Seattle WA
300 PM PDT Wed Oct 26 2016
.SYNOPSIS...Rain will taper to showers later this evening as a front
gradually pushes east of the Cascades. A weak split upper trough
over the area will maintain a threat of showers across the area
Thursday. High pressure briefly builds on Friday with mostly dry
weather. The large scale trough reloads off the Pacific Northwest
coast by Friday. A series of systems will maintain wet and unsettled
weather through the middle of next week.
.SHORT TERM...IR/WV imagery shows a feed of moist southwest flow
still aimed at Western Washington this afternoon. Another wave off
the Oregon coast appears to be lifting northeast toward our area.
Radar is not showing much precipitation off the coast so it is
possible much of this may be mid and high cloud. Deeper moisture
associated with the warm front still over interior Western
Washington continues to produce bands of rain lifting up from the
south. Precipitation will continue through the evening before
tapering to showers but should be lighter than earlier today.
Generally, the southwest interior and hood canal/south Olympics
received the most rainfall so far today with well over an inch in
many spots. Even parts of Puget Sound are approaching an inch with
areas north of about Everett a half inch or less. Another tenth to
one quarter inch could fall through this evening across the interior
and Cascades. See the hydro section for details on areas rivers.
Showers linger into Thursday as a weak trough remains over the
region. Models lift a weak short wave across the area Thursday
afternoon and again Thursday night so likely pops were kept in the
forecast. Models still show a brief ridge on Friday. A weak warm
frontal system approaching the coast but most models still show this
feature dissipating so the forecast remains dry at this point.
High temperatures were also bumped up a couple degrees with some
afternoon sunshine in the forecast.
The large scale trough becomes established again over the eastern
Pacific by Saturday. This will keep a moist southwest flow pattern
in place over the Pacific Northwest. Expect periods of light rain or
showers starting over the weekend.
.LONG TERM...A trough will move inland with split flow aloft taking
the system mainly south of the area Sunday. There is some threat of
light rain but the bulk of precipitation appears headed for
There may be a lull in rain or showers Monday but another system
will move into the western U.S. on Tuesday but dissipate rapidly.
Global models are still hinting at a more consolidated flow pattern
possibly developing around the middle of next week. That could usher
in some stronger fronts with more appreciable rainfall. Mercer
.AVIATION...An upper level trof of low pressure will remain
offshore thru tonight. Contd strong SW flow aloft. A weak occluded
front offshore will move across the area by 0700 UTC THU. Ely low
level flow will become sly. Expect areas of LLWS to end once the sfc
winds become sly.
Meanwhile, CIGs will be problematic thru tonight. Expect generally
MVFR CIGs/VSBYs with precipitation, with CIGs/VSBYs bouncing up/down
btwn VFR and MVFR categories. The mtns will be obscd.
KSEA...LLWS should end after 0300 UTC when winds at the terminal
become sly and increase. Meanwhile, CIGs/VSBYs will be a challenge
thru tonight. In general, expect MVFR CIGs/VSBYs with the heavier
.MARINE...A 993 mb low about 180 nm west of Cape Shoalwater will
continue moving north, eventually moving across northern Vancouver
Island late tonight. The associated occluded front will move across
the area this evening. Expect low pressure systems to mainly impact
the offshore waters Thursday through Saturday.
.HYDROLOGY...The highest rain amounts observed along the south facing
slopes of the Olympics are around 1.5 to 1.75 inches so far through
3 pm today. Total 12 hour amounts will likely not pass 2 inches and
precipitation already appears to be lighter on radar. Only slight
rises have been observed on the Skokomish river and flooding is not
expected. Up to 2 or locally 3 inches could fall in some Cascade
basins but the southerly flow pattern is not favorable for flooding,
just sharp rises tonight. Flooding is not expected on any area
rivers over the next 7 days. Mercer
.CLIMATE...Record October rainfall for Seattle is 8.96 inches set
in 2003. That month had the daily record 5.02 inches day on October
20th. Seattle has recorded roughly 8.47 inches so far this month (up
through 3pm pdt today) so it will take another .49 inches to break
the record. Months with 9 inches or more of rain have rarely
happened in Seattle. Even in the wettest month of the year,
November, it has occurred only nine times in 71 years of records (13
percent of the time ). Other months where 9 inches or more has been
recorded, December and January five times, and February and March
PZ...Gale Warning in effect until 3 AM Thursday for the coastal
waters and west entrance to the Strait Of Juan De Fuca.
Small Craft Advisories are in effect for the remaining
Small Craft Advisory for rough bar in effect until 3 AM
Thursday for the Grays Harbor Bar.
You can see an illustrated version of this discussion at