Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 262327
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
315 PM PST Sat Nov 26 2016

.SYNOPSIS...Rain and mountains snow will transition to showers this
evening as a front dissipates and moves southeast of Western
Washington. Another vigorous cold front will spread rain and heavy
mountain snow into the area Sunday afternoon through early Monday.
High pressure builds Tuesday giving a brief period of mostly dry
weather. Another front arrives Wednesday with rain and mountain
snow, then a period of drier weather is possible Thursday and Friday
ahead of another weather system.

&&

.SHORT TERM...The northern stream of the 500 mb split trough is still
off the Pacific Northwest late this afternoon. WV satellite imagery
clearly shows the evolution of the system with the southern branch
already moving into CA while the northern portion is slower to push
inland as it becomes more negative tilt. The N/S oriented surface
front is directly over Western Washington and shows little sign of
movement as the system pivots in place. This will keep rain going a
bit longer before transitioning to showers later this evening. Warm
advection ahead of the system has kept snow levels relatively high,
mostly above 5000-5500 feet, except a bit lower over the north
Cascades. Mount Baker has received over a foot of snow and models
show enough QPF lingering this evening to give up to another 3 to 6
inches. Went ahead and extended the winter storm warning through 7
PM PST with the transition to showers occuring later than expected.
The rest of the Cascades have seen more of a mix of rain and snow,
even at higher spots like Paradise, holding down snow amounts. As
colder air finally does filter in this evening, a couple inches of
snow could fall in the passes but that should be the extent of it.
The advisory was dropped earlier. Precipitation over the area will
become showery later this evening and mostly dissipate by morning.

A few showers may be around Sunday morning and a weak convergence
zone could affect Snohomish county. The next vigorous frontal system
will approach the area and spread rain to the coast by late morning
and inland during the afternoon. Snow levels will start off fairly
low, around 3000 to 3500 feet but should fall further to 2500 feet
as precipitation intensity increases Sunday evening. While the
system is progressive, the combination of strong onshore flow and
strong NW flow at 850 mb near 40-50 kt along/behind the front will
provide excellent orographic effects for W/NW facing mountain slopes
and some passes. Mount Baker, Hurricane Ridge, and even Stevens pass
could exceed a foot Sunday night. Some lower passes like Snoqualmie
will also see heavy snow, but probably a bit less in the 5 to 10
inch range. A winter storm watch was posted to address this next
system. Travel through passes is likely to be affected. The strong
onshore flow may also cause some blowing and drifting of snow, even
in the passes. Those planning travel late Sunday through Monday
morning should check road conditions ahead of time by calling 5-1-1
and be prepared for delays or even pass closure. The heaviest period
of snow looks to be Sunday evening. For the lowlands, expect breezy
to locally windy conditions with rain changing to showers by Monday
morning.

Showers will taper off across the area by Monday afternoon. There is
a chance the Puget Sound convergence zone could become active Monday
and Monday night. This could produce rain bands around Puget Sound
from Everett south to Tacoma and possibly localized snow accumulation
from around Stevens pass to Snoqualmie pass. The flow turns more
northerly by evening so this activity should dissipate with drier
weather developing. Tuesday looks to be mostly dry with a ridge of
high pressure traversing the area.

.LONG TERM...Models are in good agreement showing yet another system
arriving in cold northwest flow. This system looks weaker than the
Sunday night storm, but it should still bring some rain and
accumulating mountain snow to the area Wednesday and Wednesday
night.

Global models all seem to agree on a more amplified flow pattern
toward the end of the week. A lull or break in precipitation appears
possibly Thursday and Friday. If the flow is light enough, fog could
be around and colder low temperatures, especially in the prone cold
pockets like Olympia. There is still some uncertainty on timing
systems toward the weekend. Decided not to undercut the MOS min
temps too much as any quicker timing could bring clouds in faster.

Long range models are hinting at retrogression of the long wave ridge
starting around next Saturday. If this shift were to occur, colder
systems could arrive from the north with even lower snow levels than
we have seen so far this year. Just something to watch at this
point.  Mercer

&&

.HYDROLOGY...Heavy rain ended around noon, but the Skokomish River
rose again today. It appears to be cresting now around 17.5 ft, half
a foot above flood stage, and is now forecast to remain above flood
stage through Monday morning. The Chehalis River crested earlier at
Grand Mound and is near its crest at Porter. Only minor flooding is
occurring. The flood warning will remain in effect tonight as the
flood crest works its way downstream into Grays Harbor. The Chehalis
surge plain may experience minor flooding around the time of high
tide at 11 PM tonight.

Landslides remain a concern. USGS landslide indices show a continued
threat from the recent heavy rain. A minor landslide was reported
near Quilcene today. A special weather statement addresses this.

The systems Sunday night and Wednesday appear to be fast moving.
Though more rain will fall, flooding amounts are not forecast. A
cooler and drier period is forecast later in the week. After the
current flooding episode has ended, more flooding is unlikley over
the next seven days. Burke

&&

.AVIATION...An upper trough will move through the area tonight. Flow
aloft will be northwesterly on Sunday. At the surface, light flow
tonight will become moderate to strong onshore by Sunday night.
The air mass is moist and stable. Rain over the region will taper
tonight.

Ceilings are generally VFR except for pockets of MVFR under rain in
the northwest and southwest interiors. All areas will become MVFR
later tonight and remain MVFR through Sunday.

KSEA...Discussion above applies. Light wind will become northerly 4-
8 KT this evening, then become southerly 6-12 KT Sunday. CHB

&&

.MARINE...Winds are currently less than 20 KT except for small craft
advisory strength northwest winds over the outer coastal waters.
Winds will remain light tonight and Sunday as high pressure briefly
builds over the area. Then another front will arrive Sunday night.
Post frontal winds are forecast to be strongly onshore, with gales
possible on the coast and down the strait Sunday night into Monday.
A gale watch is in effect.

In the wake of the front Sunday night, westerly swells of 15 to 20
feet are possible on the coast Monday night. Another front will
arrive around Wednesday. This one has been weaker in recent model
runs and does not show gales now. CHB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...

WA...Winter storm warning through 7 PM PST this evening for the
Cascades      of Whatcom and Skagit counties.

     Winter storm watch Sunday afternoon through early Monday morning
     for the Olympics and the Cascades of Whatcom, Skagit, Snohomish,
     King, Pierce, and Lewis counties.

     Various flood watch/warnings in affect for portions of Western
     Washington. See latest flood statements for details.

PZ...Small craft advisory outer coast tonight.

     Gale watch coast and strait Sunday night.


&&

$$

www.weather.gov/seattle

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