Heavy Snow in Washington, November 4 to 5, 2017

Area Forecast Discussion
National Weather Service Seattle WA
300 PM PDT Sat Nov 4 2017

.SYNOPSIS...Low pressure developing offshore tonight will
track across inland southern Washington Sunday morning. Rain and
snow will increase over the area tonight with snow levels falling to
500 feet or less. A couple inches of wet snow may accumulate on
hilltops around Puget Sound, except down to the surface along hood
canal, the Kitsap, and the Strait. Showers will taper off Sunday
afternoon and dry air will arrive later Sunday into Monday. A
weather system will stall off the Coast on Tuesday before moving
into the area later on Wednesday through the end of the week.

&&

.SHORT TERM...IR/WV Satellite imagery shows the low pressure
circulation center off Vancouver Island moving southeast toward our
area. MSAS surface analysis indicates a 1007 mb low which is in line
with the various models which agree well on the depth and track of
the system. Rain and even some spotty snow down to sea level is
spreading into Western Washington this afternoon ahead of the
surface low. Models are all in agreement in depicting increasing
precipitation rates along the thermal gradient which sets up just
north of the surface low track. The UW WRF and other models show
this axis setting up from around Mason county eastward across
south/central Puget Sound. If strong northerly flow develops from
the surface up to 800 mb as some models suggest, an isothermal
profile should drag snow levels down to 500 feet or lower. Hood
Canal and the Kitsap are most susceptible to set up and heaviest
snowfall will probably occur there overnight into early Sunday.
Upslope flow along the Strait/Olympics should also result in some of
the heavier snowfall, especially just inland from the water with a
little elevation. Up to 3 or 4 inches of wet snow is certainly a
possibility for these areas and the winter weather advisory was
expanded to the Strait and also with an earlier starting time this
evening. For the rest of Puget Sound, including the I-5 metro
corridor from Everett south to Olympia, snow may not quit reach sea-
level or at least accumulate at sea-level. But above 200 to 500 feet
a couple inches of wet snow on grass or colder surfaces is a good
possibility Sunday morning. No changes were made to the advisory for
there areas. Will need to watch areas west of Puget Sound as warning
amounts as high as 4-6 inches are not out of the question. But most
models including the WRF are showing no more than a few inches and
will stick with that forecast.

Snow may linger into the midday or early afternoon hours Sunday.
Some models hold back a weakening surface low from the coast to the
south interior with QPF still over the I-5 corridor east of Puget
Sound if the GFS/ECWMF are correct. However, temperatures will not
likely support much if any accumulation after midday. The area
should start to dry out by Sunday evening with cold breezy north
winds Sunday night. Dry weather is expected Monday and Tuesday next
week with cold morning lows, reaching the 20s in cold pockets.

.LONG TERM...The next broad upper low approaches the Pacific
Northwest on Wednesday. Latest model runs are slower and more n/s
oriented with the low. This could delay precipitation until late
Wednesday. A showery and unsettled pattern lasts through at least
Friday as the low will be slow to shift east. It will not be as cold
as the system this weekend but it will be wet at times with highs
possibly reaching the low 50s. Mercer

&&

.AVIATION...West flow aloft over Western Washington will briefly
shift to southwesterly tonight as an upper trough drives southeast
across the area. Flow aloft will become northerly Sunday
afternoon as the trough moves east. At the surface, northeast and
north flow will increase tonight into Sunday as a low pressure
system off the northwest tip of Vancouver Island drops southeast
onto the Central Washington Coast late tonight then moves east
through Southwestern Washington on Sunday. The air mass is moist
and stable.

Radars and observations show some weak warm advection from north
of Seattle northward producing some light snow (with rain below
200-500 ft MSL) this afternoon. This activity is producing MVFR to
local IFR conditions across the northern half of the area and
there is little if any accumulation occurring.

After perhaps a brief decrease in light precipitation late this
afternoon, expect steadier and heavier precipitation to spread
inland after 03Z then to gradually decrease midday Sunday.
Moderate precipitation rates are expected tonight for the Olympic
Peninsula and from Whidbey Island southward in the interior later
tonight and Sunday morning. Precipitation most areas is expected
to begin as rain, with a rain snow mix on higher hills near the
Strait of Juan de Fuca and from the Kitsap Peninsula to Everett
area this evening. Precipitation will likely change to all snow
after 12Z then end from the west early in the afternoon. MVFR
conditions are expected with the mixed precipitation or rainfall,
then IFR to LIFR conditions during the period of snow. KCLM and
higher terminals from KPAE southward will likely see 1-3 inch
slushy accumulations, and KPWT and KPAE may see locally higher
amounts. KBFI and possibly KOLM will likely see brief trace to
less than an inch accumulations while KHQM remains snow-free. KBLI
will be near the source of drier air coming out of the Fraser and
will likely see trace amounts through the event. Temperatures
will generally remain near or just above freezing during the
event where accumulations occur. Albrecht

KSEA...Light east to southeast winds will become east 6-9 kt this
evening then back to north to northeast 10-14g20kt late tonight
into Sunday morning. Expect ceilings 030-035 at times with a
higher cig around 050 though this evening, then cigs falling to
025 with -ra this evening. The main period of concern is after
about 10Z until about 20Z with vis and cigs fall to 3/4-2SM -SN
BR CIGS 004-009 with the northerly flow. Expect 1-3 inch snow
accumulation at the terminal between 12Z and 18Z with temperatures
33-34 deg F. Albrecht

&&

.MARINE...A 1007 mb low the north tip of Vancouver Island will
deepen to 1002 mb as it moves inland around Grays Harbor early
Sunday morning. The low will then travel eastward across
southwestern Washington on Sunday. The low will produce small
craft advisory conditions all waters as it passes tonight through
Sunday. Strongest winds are likely with renewed Fraser Valley
Outflow in the Northern Inland Waters and around the West Entrance
to the Strait of Juan de Fuca late tonight into Sunday morning.

High pressure will build over the region late Sunday through
Monday.

A frontal system will approach the region slowly from the west
Tuesday and Wednesday; gale force southeast winds are possible
over the coastal waters, especially over the outer portions of the
waters. Albrecht

&&

.HYDROLOGY...River flooding is not expected over the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from midnight PDT tonight to 10 AM PST
     Sunday for Bellevue and Vicinity-East Puget Sound Lowlands-
     Everett and Vicinity-Seattle and Vicinity-Southwest
     Interior-Tacoma Area.

     Winter Weather Advisory from 6 PM PDT this evening to 4 PM PST
     Sunday for Cascades of Pierce and Lewis Counties-Olympics.

     Winter Weather Advisory from 9 PM PDT this evening to 10 AM PST
     Sunday for Admiralty Inlet Area-Bremerton and Vicinity-
     Eastern Strait of Juan de Fuca-Hood Canal Area.

PZ...Small Craft Advisory until 9 PM PST Sunday 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 from midnight PDT tonight to 6 PM PST
     Sunday for Admiralty Inlet-East Entrance U.S. Waters Strait
     Of Juan De Fuca-Puget Sound and Hood Canal.

     Small Craft Advisory until 3 AM PST Monday for Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory until 11 AM PST Sunday for West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 11 AM PST Sunday for Central U.S.
     Waters Strait Of Juan De Fuca.

&&

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html