Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 282339
Area Forecast Discussion
National Weather Service Seattle WA
300 PM PST Mon Nov 28 2016
.SYNOPSIS...Showers will end tonight and the Puget Sound
convergence zone affecting Snohomish and King counties will
dissipate. High pressure will bring mostly dry weather for much of
the area Tuesday and again on Thursday. A frontal system will bring
some rain and mountain snow Tuesday night and Wednesday. Stronger
systems this weekend could potentially bring stronger winds, rain,
and heavy mountain snow with snow levels falling to 1000 feet or
lower by early next week.
.SHORT TERM...Water vapor satellite imagery shows the back edge of
the broad upper trough centered over the central U.S. moving
eastward late this afternoon. The flow near the surface and aloft
will quickly weaken this evening with only a few residual rogue
showers by later this evening. Radar shows the nw/se oriented Puget
Sound convergence zone still producing showers and snow above 3500
feet in Snohomish and King counties. This too should dissipate later
this evening as surface gradients weaken.
Depending on if we get some partial clearing, lows Tuesday morning
could be rather chilly in the 30s outside the urban areas. Localized
fog is likely in prone river valleys and low lying areas around the
southwest interior in the morning. High pressure aloft will shift
across the area as the upper flow remains progressive, keeping most
the area dry Tuesday with sunbreaks or partial afternoon sunshine
possible. Highs should reach near 50.
Models remain in good agreement on the next system arriving Tuesday
night. Rain will spread across the area with amounts generally under
a half inch. The best lift, strong flow aloft, and cold air
advection will be across the north Cascades where over 6 inches are
possible. An advisory will probably be needed for the Cascades of
Whatcom and Skagit counties. Snow amounts from Snohomish county
south will range from around 1 to 3 inches at lower passes like
Snoqualmie to 2 to 5 inches at higher passes like Stevens. Snow
levels may fluctuate between 3000 to 3500 feet from Snoqualmie south
during part of the storm, so mixed precipitation could hold down
snow accumulation in that pass. Colder air should filter in behind
the front with showers on Wednesday. A few more inches could occur
along favored w/nw facing slopes but the passes will probably get
less than an additional 1 to 2 inches. The flow pattern will also be
favorable for a convergence zone, similar to what we had today.
Showers and convergence zone activity will dissipate by evening.
High pressure aloft and light surface flow will give another period
of dry weather Wednesday night and Thursday. Low temperatures will
again fall into the 30s, possible near freezing in some cold pockets
like Shelton and Olympia if there is enough clearing. Patchy fog is
also a good bet which may keep lows a bit higher than if skies were
clear. The seattle metro area will probably stay near or just above
.LONG TERM...Models continue to advertise a large scale shift in the
pattern Friday and over the weekend. The upper ridge will flatten
Thursday night, allowing a cold upper low over Alaska to sag
southward setting up moist zonal flow into Western Washington
Friday. The first in a series of waves will arrive with rain
spreading into most of the area Friday with little break before the
next low arrives Saturday. A tight zonal baroclinic zone and strong
westerly jet will remain directed into the area. The mountains
should pick up appreciable snowfall in this pattern. Most models
show a deepening surface low tracking across or just north of the
area Sunday brining more rain and heavy mountain snow. Winds could
also be strong in some areas depending on the exact track.
Long range models are still showing colder air arriving behind the
Sunday system from the north with a large surface high setting up
over B.C. It is too soon to know if this cold air mass will make it
into Western Washington but with moisture in the vicinity and a
closed low over the region, snow levels could be quite low. The
forecast indicates snow levels down to 1000 feet by Monday/Tuesday.
Will need to monitor this possible shift to a colder pattern knowing
the longer range models have a well known cold bias due to complex
.HYDROLOGY...All rivers have fallen below flood stage. Any
additional rain through Saturday should not be enough to produce
flooding on any area river. A stronger storm may bring a decent
amount of rain this weekend with lowering snow levels. The
Skokomish river would probably be the only river with a slight
chance of flooding by late Saturday and Sunday. Mercer
.AVIATION...An upper ridge over the northeast Pacific will shift
over Western Washington tonight with north to northwest flow aloft.
The ridge will shift inland on Tuesday with increasing southwest
flow aloft as a front approaches from the west. At the surface,
onshore flow will ease tonight as high pressure over the offshore
waters moves over Western Washington. The air mass is moist and
Areas of low and mid clouds this evening. Low clouds should fill in
tonight, especially over the interior, as gradients go flat.
Low clouds should lift and break up a bit Tuesday afternoon.
KSEA...There will probably be a mix of low and mid level clouds this
evening, then just low clouds later tonight. South wind 5-10 knots
will ease and become more easterly tonight. Schneider
.MARINE...High pressure over the Coastal Waters will shift
inland tonight with easing onshore flow. West swell will build to 16
to 19 feet this evening then subside on Tuesday.
A vigorous front will reach the area later Tuesday and move inland
Tuesday night with gales possible for the Coastal Waters.
High pressure will rebuild over the area Wednesday and Thursday.
Another front will reach the waters on Friday. Schneider
PZ...Gale Watch from Tuesday afternoon through Tuesday 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.
Small Craft Advisory until 4 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.
Small Craft Advisory for rough bar until 7 AM PST Tuesday for
Grays Harbor Bar.
Small Craft Advisory until midnight PST tonight for Central U.S.
Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
Strait Of Juan De Fuca.
Small Craft Advisory until 4 AM PST Wednesday for West Entrance
U.S. Waters Strait Of Juan De Fuca.
You can see an illustrated version of this discussion at