Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 201557

Area Forecast Discussion
National Weather Service Seattle WA
858 AM PDT Thu Oct 20 2016

.SYNOPSIS...A cold front over Western Washington this morning
will move slowly east, moving over the Cascades by mid afternoon.
Rain will diminish to scattered showers behind the front this
afternoon. A weak warm front will bring some light rain to the area
on Friday. After a minor lull Saturday, another cold front will
reach the area late Saturday and Saturday night. A large upper level
low offshore will maintain moist southwest flow aloft over the
region and an active weather pattern through early next week.


.SHORT TERM...RADAR shows the back edge of the cold front extending
from roughly Bellingham to the central WA coast at 15Z/8 AM. Rain
has pretty much ended behind the cold front, but heavy rain
continues ahead of the front. Models still move the front over the
Cascades by around 21Z/2 PM. Shower activity just offshore behind
the system appears minimal on IR imagery so it looks like just some
scattered showers over most of the area. GFS and NAM still show a
weak convergence zone forming over Admiralty Inlet this afternoon
behind the front, then possibly moving a bit south along the King-
Snohomish county line later this afternoon.

The lull in post-frontal showers will continue tonight although we
will keep low chance POPs in the forecast. There will probably even
be a little partial clearing tonight which will help low
temperatures drop back into mainly the 40s overnight.

Another weak system will reach the area on Friday. With the system
originating in the colder air coming around the large longwave low
near 50N/150W it seemed like it would be an organized band of
showers driven by an upper level shortwave trough. However, the GFS,
ECMWF, and NAM all show an area of warm advection developing ahead
of an approaching shortwave trough. This weak warm front should
spread some light rain to the coast Friday morning, then to the
interior around midday. The front should weaken over the Cascades
by Friday evening, so Friday night should be uneventful.

Models have been in fairly good agreement showing another upper
level trough approaching the region on Saturday. The details have
been a mess so confidence is only marginal. The GFS shows a little
weak warm advection ahead of the system which could brush W WA late
Saturday morning. The main cold front will probably reach the area
Saturday evening while the associated upper level shortwave trough
and compact surface low curve sharply northward toward N Vancouver
Island. Kam

.LONG TERM...Previous discussion from the 345 AM AFD...Upper level
low pressure over the Pacific waters looks to be the main driver of
weather for the end of the weekend and into the first half of next
week. Sunday looks like a weaker first wave followed by a stronger
second wave on Monday. Models agree on at least some small break
Tuesday but split regarding the length of the break and how long
precip will stick around afterward. The ECMWF shows the break being
only about 6 best...before the system brings another
round of rain extending into Thursday before pushing on. The GFS
remains a bit more optimistic/progressive allowing much of Tuesday
to be dry before another round of rain starts to muscle into the
area Wednesday morning and afternoon as the low pushes further
north. Would like to see better agreement in the have
opted to keep chance wording in for much of that time frame...but is the season of wet socks...and nothing in the model
data points to anything that would refute that.  SMR


.AVIATION...A cold front will move inland today with southwest
flow aloft through tonight. The air mass is moist and stable.

Low and mid level moisture will thin out later today behind the
front. Partial clearing could lead to patchy fog Friday morning.

KSEA...Low clouds and reduced visibilities this morning as a
front moves through. Ceilings and visibilities should improve
this afternoon behind the front. Low clouds should scatter
entirely this evening. South wind 5-10 knots. Schneider


.MARINE...A series of fronts will move through the waters over the
next several days.

Currently, a front is pushing onshore. Winds have generally eased
behind the front to below small craft advisory levels. There will
probably be a few hours of small craft advisory west winds in the
Strait behind the front later today.

The next front on Friday looks relatively weak and will probably
only bring small craft advisory south winds to the Coastal Waters.

A third frontal system this weekend looks stronger with at least
small craft advisory winds most waters and gales possible Coastal
Waters around Saturday night. Schneider


.HYDROLOGY...The frontal system moving across W WA last night and
this morning has dropped about 3 inches of rain over the Skokomish
River basin up through 8 AM this morning. RADAR suggests there is
about another 2 hours of heavy rain to fall before the cold front
crosses the area and ends the rain. The Skokomish is rocketing up as
usual and should reach flood stage around 9 AM this morning. The
flood warning was issued earlier this morning. The flood watch for
Mason county will probably be ended later this morning. Otherwise,
apart from the Skokomish River, flooding is not anticipated on area
rivers the next 7 days.


WA...Flood Watch from Thursday morning through Thursday evening for
     Mason county.

PZ...Gale Warning until 5 AM PDT Thursday 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.

     Small Craft Advisory until 11 PM PDT this evening for East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 11 PM this evening to 5 AM PDT Thursday for
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 2 PM PDT Thursday for Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory until 5 AM PDT Thursday for West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 1 AM to 1 PM PDT Thursday for
     Admiralty Inlet.



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