Heavy Snow in Washington, December 14 to 15, 2016

Area Forecast Discussion
National Weather Service Portland Oregon
1007 AM PST Wed Dec 14 2016

.SYNOPSIS...Ample moisture will spread north across northwest Oregon
and into southwest Washington today over a cold air mass especially
over the interior areas fed by cold east winds that will lead to
mixed winter precipitation down to the valley floors. The
precipitation could be heavy at times. The precipitation will
decrease from the west later tonight. The wintry pattern will remain
through the weekend with the coldest air of the season and some snow
chances each day.
&&

.SHORT TERM...Today through Saturday...Only minor changes to the
forecast this morning to slow down the timing for areas near Salem
and north and to add chance of freezing rain as far north as
Woodburn.

Mix of sleet and snow being observed on the leading edge of the
precipitation falling near Corvallis. Expect precipitation to
transition to all snow as precipitation rates increase. There is a
weak warm layer aloft on the 12Z Salem sounding which will easily be
overcome with wetbulbing and dynamic cooling. The greater freezing
rain concern will arrive this afternoon once a stronger warm layer
surge arrives ahead of the surface low. HRRR is quite aggressive with
this warm air surge and attempts to bring freezing rain as far north
as the PDX metro. Do not think the warm nose will get this far north,
but areas near Salem and as far north as Woodburn may have a period
of freezing rain this afternoon and early evening before
transitioning back to all snow.

There is a lot of dry air in the north Willamette Valley which will
need to be overcome with surface dewpoints in the teens. Expect it
might take the deeper moisture arriving later this afternoon, to
overcome this low level dry air and begin precipitation in the
Portland metro. Therefore, do not expect snow to begin until the 2pm
to 5pm timeframe for the Portland Metro.

While precipitation type remains the most difficult part of the
forecast for the south and central Willamette Valley, the northward
push of the best moisture remains the concern for the PDX metro. Even
as of this morning, model QPF ranges between 0.15" (GFS) and 0.6"
(NAM) which is the difference in low end advisory snow and warning
level snow. GFS has been very consistent as the outlier with the
drier solution and we are approaching the moment of truth where the
GFS will either look like a genius, or a fool.

Trended snowfall amounts up very slightly for the PDX metro as the
gut feeling says we will be more towards the wetter solutions. Part
of the support for this feeling is the 15Z Water Vapor representation
of the upper wave which indicates a negatively tilted shortwave has
ejected from the main surface low. If the wave is indeed on the
stronger side and has a bit more negative tilt, there will be more
opportunity for precipitation to get further northwest. In addition,
the surface low seems to be trending northward with the best pressure
falls located near Eugene at 16Z. The HRRR is also showing this
further north surface low track which is part of the reason for the
increased chance of freezing rain for the Salem to Woodburn area.

In this last minutes, while writing this discussion, the reflectivity
has really begun to blossom across the south half of the area. This
has been corroborated by surface reports of increasing precipitation
rates in Eugene and Corvallis. Expect this trend to continue over the
next few hours with a slow northward push through the afternoon.
/Bentley

.LONG TERM...Saturday night through Tuesday...No Changes. Previous
Discussion Follows. The extended forecast period will see slow
moderation in temperatures, as the modified arctic air mass over the
interior slowly gets chipped away by increasingly warm air aloft. The
00z GFS hopped on board with the 12z/00z GEM Sat night/Sun in
bringing a fairly vigorous shortwave trough down from the BC coast
and southward along the spine of the Cascades. However, further
analysis of the GFS ensembles shows that
the operational run is quite an outlier amongst its ensemble
members. While we favor the 00z ECMWF solution versus the GFS/GEM,
we did increase PoPs slightly to accommodate the 00z GFS input. If
this system did run down the coast or southward across Western
WA/OR, there is a chance it could bring another chance of lowland
snow to the inland valleys. However, the more likely scenario
appears to be for a mainly dry clipper-like system, or an "inside
slider" like the 00z ECMWF depicts, especially considering the
increasingly progressive flow developing upstream across the North
Pacific as the blocking ridge deteriorates over the Gulf of Alaska.

As a strengthening and increasingly zonal jet pushes into the Gulf
of Alaska and north-central Pacific, the north end of the blocking
ridge will likely wear down early next week. This would open the
door for a mild Pacific system to bring some rain to our district by
midweek. It is likely the medium-range models are underestimating
the low-level cold pool over the Columbia Basin, so it would not be
a surprise if this event started off as sleet or freezing rain in
and near the Columbia Gorge. Added a chance of freezing rain in the
Gorge, as well as the Hood River and Wind River Valleys to cover
this Mon/Tue...gradually eroding from west to east. Since this
system appears to be coming in from the W-NW, there should be decent
southerly flow in the Willamette Valley ahead of it. This would
likely push temps into the 40s for the Willamette Valley, mitigating
the chance for freezing or frozen precip in the Eastern PDX Metro
area...however a brief mix near Troutdale is not out of the question
depending on how this system evolves. Plenty of time between now
and then to sort out the details.  Weagle
&&

.AVIATION...VFR conditions across most of the area this morning.
Only exception is returning MVFR cigs down at KEUG with -FZRA.
Precipitation north of Lane County will likely fall mostly as snow
for terminals in the Willamette Valley, though can't rule out of a
period of sleet at KSLE. Along the coast, should see mostly rain
with possibly a period of snow after 00Z on the north coast, but
with little accumulation. Expect MVFR to IFR conditions with
precipitation today into tonight.

KPDX AND APPROACHES...East winds gusting to 35 kt this morning
will increase with gusts to 40 kt likely from KPDX to KTTD through
this afternoon. Winds will start to weaken after 03Z. Snow
expected to reach the terminal around 22Z and continue until
around 06Z Thursday. Expect a mix of MVFR and IFR flight
conditions after snow begins and through at least 06Z.
Bowen/McCoy

&&

.MARINE...Offshore winds through the day today, with gusts to 20
to 30 kt north of Cascade Head. Small craft advisory for winds
continues through this afternoon. Have seen a couple of gusts to
just above 20 kt at buoy 50, but doesn't appear frequent enough to
warrant an extension of the advisory to that area. Will start to
see winds back off this evening as a weak surface low lifts north
across the waters this evening. This will result in backing wind
from E to NNE as winds begin to ease.

In the longer term, wind speeds remain near or below 15 kt
through the weekend. Models diverge early next week, but there is
reasonable confidence that small craft advisory wind speeds
develop late Sun night or Mon morning, with some potential for
low-end gales.

Seas are forecast to remain well below 10 feet for the next
several days. Sea conditions will become more wind wave driven
and possibly quite choppy at times, however. Seas may build up
into the middle teens early next week. Bowen/McCoy

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning from noon today to 2 AM PST Thursday for
     Central Columbia River Gorge-Upper Hood River Valley-
     Western Columbia River Gorge.

     Winter Storm Warning until 4 AM PST Thursday for Cascades in
     Lane County-Northern Oregon Cascade Foothills-Northern
     Oregon Cascades.

     Winter Storm Warning until 2 AM PST Thursday for Cascade
     Foothills in Lane County.

     Winter Weather Advisory until midnight PST tonight for Greater
     Portland Metro Area.

     Wind Advisory until 8 PM PST this evening for Greater Portland
     Metro Area.

     Winter Storm Warning until midnight PST tonight for Central
     Coast Range of Western Oregon-Central Willamette Valley-
     South Willamette Valley.

     Winter Weather Advisory until 9 PM PST this evening for Coast
     Range of Northwest Oregon.

WA...Winter Storm Warning from noon today to 2 AM PST Thursday for
     Central Columbia River Gorge-Western Columbia River Gorge.

     Winter Weather Advisory until midnight PST tonight for Greater
     Vancouver Area.

     Wind Advisory until 8 PM PST this evening for Greater Vancouver
     Area.

PZ...Small Craft Advisory for winds until 4 PM PST this afternoon
     for Coastal Waters from Cape Shoalwater WA to Cascade Head
     OR out 60 nm.

&&

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.