Winter Storm and Heavy Snow in Oregon, December 8 to 9, 2022

Area Forecast Discussion
National Weather Service Portland OR
323 AM PST Thu Dec 8 2022

.SYNOPSIS...First front will push across the region today, with rain
and mountain snow. Bit of a break later tonight into Fri am, but next
front will push more rain and mountain snow into the region Fri
afternoon into Sat. Afterwards, expect cool, showery weather for Sun
and Mon. Lots of uncertainty for next weeks weather, but does appear
that will be turning cooler as the week progresses.

&&

.SHORT TERM (Today through Saturday)...

Key Points...
 *  Front moves across region today with valley rain/mtn snow.

 *  Next potent front arrives later Fri/Sat, with valley rain and
      mountain snow, with heavy snow for the Cascades.

 *  Breezy southerly winds along the coast Fri night/Sat.

Weather is dominated by rather large upper trough offshore, with
surface low pressure moving inland over western British Columbia.
From that low, a front extends southward over far western Washington
to just off the north Oregon coast (as of 2 am) with rain along and
ahead of the front. This front will push further inland today, with
rain and mountain snow. As the front shifts inland will see rain
transition to showers along the coast early, and inland north this
morning.  But, there is plenty of upper level energy digging in the
bottom of the trough off the north Calif coast. As such, this will
contribute to slowing the inland progression of the front over
southwest Oregon, as well as in east Lane and Linn Counties. This
will result in the transition of steady precipitation to showery
precipitation to occur later, mostly likely mid-afternoon.

But, there is bit of concern for precipitation type in the Columbia
River Gorge and the Hood River Valley. Air mass is cool, with
temperatures only in the upper 20s to lower 30s. Modeled soundings
still indicated that precipitation will begin as snow, with
transition to showers later in the afternoon. But, there will be a
time this am as the air mass aloft warms slightly and cold air will
still be in place closer to the ground. As such, will see a
transition to wintry mix (sleet, freezing rain, snow) this am, mainly
for areas from east of Multnomah Falls to just east of Cascade Locks,
then change to rain as bit of warm air filters into the Gorge. But,
areas closer to Hood River, as especially Hood River Valley, will
stay as steady snow into the afternoon, then snow becoming more
showery with only a minor bump of snow level up to around 800 to 1000
ft. Will maintain current Winter Weather Advisories for all the
Columbia Gorge and Hood River Valley. Expect minor accumulations of
ice (less than a tenth of an inch) from any freezing rain this am,
with 1 to 4 inches of snow in the Gorge, and 3 to 8 inches of snow
for the Hood River Valley.

Snow will continue to pile up over the Cascades and foothills.
Various winter weather advisories remain in effect through tonight,
with 2 to 8 inches for areas 2000 ft to 3500 ft, with 6 to 12 inches
above 3500 ft. Would not be surprised to see locally heavier amounts
over the higher peaks, such as Mt St Helens and Mt Hood. Again,
steady precipitation will come to an end later this am into the
afternoon, with snow showers for rest of the day and tonight.

As the upper trough shifts inland tonight and early Friday, will see
showers decreasing. Will trend PoPs downward for late tonight and
early Fri. But, next system will push out of the north Pac across the
southeastern Gulf of Alaska on Fri. Models in fairly good agreement
that this system will push toward the Pac NW. As it does, will see a
strong occluded front extending from the low over the southeast Gulf
of Alaska down to a new developing surface low around 42N 135W Thu
night. This front will push towards the Pac NW, with rain increasing
along the coast Fri afternoon, and rain spreading inland by Fri
evening. But, the developing low on the south side of the frontal
boundary will be driven towards northern Calif by rather potent jet.
As such, seems heaviest rainfall will be farther to our south, over
far sw Oregon into northern Calif.

On Saturday, models still showing the northern low dropping
southeastward for a bit,  then pushing towards the north Oregon coast
by later Sat afternoon.  As such, will see rain increasing again on
Sat. Rather tight pressure gradient around the low, with breezy
southerly winds for the coast, with 40 to 50 mph gusts. Still some
question about the low and where it will track, but models suggest
that the low will essential stall and wobble about just offshore for
late Sat through Sun, before heading on the south. Will generally
remain wet on Sat, with potential of 1 to 2 inches of rain along the
coast, 0.50 to 1.00 inch for interior lowlands. Snow only expected
for the Cascades, as snow levels stay at 2000 to 3000 feet. Good bet
will see 10 to 18 inches of snow for the Cascades. Will likely need
another Winter Weather Advisory for the Cascades Friday night through
Sat, perhaps even Winter Storm Warning. For now, will defer to later
shifts, as have current advisories up.	  /Rockey

.LONG TERM...SATURDAY THROUGH TUESDAY...There are a few traits of
this system though that will likely occur no matter the track. First,
there will be more of a south to southeasterly wind at 850 mb and at
the surface which will usher in warmer air and thus, raise snow
levels. This will also impact snow ratios dropping them from the
around 10:1 of the system expected Thursday, to around 7:1. This
means we would see more of a 'wet snow' with more compacting.
Precipitation will still be higher along the coastline and coast
range no matter the solution. NBM 4.1 50th percentile shows around
1.5 to 2 inches of rain along the coast from 00Z Saturday to 00Z
Sunday, and around 2 inches of QPF along the northern Cascades. The
90th percentile is around 2 to 3 inches of rain along the coast, and
the 10th percentile is around 1 inch. Will say that the 4.1 is
trending on the wetter side for QPF but WPC has trended towards
the consistency with NBM 4.1. Will continue to watch QPF amounts
through the next several runs.

Moving beyond the weekend, a sharp rise in heights aloft between
systems will likely bring dry conditions Sunday night into Monday.
The next system is quick to move in Tuesday bring another round of
rain and mountain snow to the area.
  -Muessle

&&

.AVIATION...06Z TAFs: Lows stratus lifted early this evening for
most of the area, but is now increasing again at the coast in
advance of a front to move across the region tonight.
Timing remains on track with regards to the front. Expect rain to
arrive at the coast by 06z Thu and then spread into the valley
08-10Z Thu, with conditions gradually trending towards MVFR
overnight into Thu morning. Will also see some gusty south winds
at the coast with passage of the front, with southeast winds
picking up into the 10-15 kt range ahead of the front at some of
the inland terminals. Expect a modest Gorge wind to impact KTTD
later in the period as offshore flow strengthens, which could
introduce some marginal LLWS concerns with frontal passage after
09z-13Z Thu. Expect a transition to more showery conditions and
variable VFR/MVFR cigs behind the front on Thu, while some
guidance shows MVFR continuing

For detailed Pac NW aviation weather information, go online to:
http://weather.gov/zse

KPDX AND APPROACHES...VFR expected through this evening.
Approaching front will bring rain and MVFR conditions back to the
terminal after 08z Thu, with southeast winds also increasing above
10kt with frontal passage. Expect a transition to more showery
conditions and variable VFR/MVFR cigs behind the front on Thu.
/CB

&&

.MARINE...The cold front is about to clear the central Oregon
waters as it continues pushing inland today. It should fully clear
the waters by 0400 with gusty SW winds trailing for a few hours.
More importantly, a trailing west swell will quickly build
reaching 10-13 feet after a brief drop in combined seas from the
loss of the wind wave and fresh swell. Despite gusts at buoy 50
remaining around 34 kts, decided to cancel the gale warning a bit
early in deference to the upcoming headlines spelled out below.
Did, however, advance the start time of the SCA to cover this
transition period as the fresh swell is producing rather steep
seas for the time being.

Models continue to show strong agreement on a deepening 981 mb
low approaching the waters from the west Friday through Saturday
and stall about 80 nm offshore. The low will send a strong front
across the waters late Friday afternoon and night with storm force
gusts possible, mainly off the central Oregon coast, and gales
elsewhere. Seas will also build to 14-17 feet with this system. As
alluded to prior, have decided to issue a Gale watch now for our
northern zones and a Storm watch for the central Oregon waters. In
both cases, winds across the inner waters reaching gale and storm
force is a little in doubt given the strong offshore component to
the pressure gradient, but also owed some of that to the coarser
models used in producing the wind forecast for this package.
Expect using higher resolution model data moving forward will
bring enough coverage closer to shore to justify the watch
conditions. Winds weaken Saturday morning behind the front with
SCA, possibly hazardous seas, conditions remaining thereafter.

The surface low will then weaken over the outer waters and drift
south Sunday and Monday with winds easing and seas subsiding below
SCA criteria. /JBonk

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Weather Advisory until midnight PST tonight for Cascade
     Foothills in Lane County-Cascades in Lane County-Northern
     Oregon Cascade Foothills-Northern Oregon Cascades.

     Winter Weather Advisory until noon PST today for Central
     Columbia River Gorge-Upper Hood River Valley-Western
     Columbia River Gorge.

WA...Winter Weather Advisory until noon PST today for Central
     Columbia River Gorge-Western Columbia River Gorge.

     Winter Storm Warning until 4 AM PST Friday for South Washington
     Cascades.

     Winter Weather Advisory until midnight PST tonight for South
     Washington Cascade Foothills.

PZ...Small Craft Advisory from 1 PM this afternoon to 10 AM PST
     Friday for Columbia River Bar.

     Gale Watch from Friday morning through Saturday morning for
     coastal waters from Cape Shoalwater WA to Cape Foulweather
     OR out 10 nm.Columbia River Bar-Coastal waters from Cape
     Shoalwater WA to Cape Foulweather OR from 10 to 60 nm.

     Small Craft Advisory until 10 AM PST Friday for coastal waters
     from Cape Shoalwater WA to Florence OR out 60 nm.

     Storm Watch from Friday morning through Saturday morning for
     coastal waters from Cape Foulweather OR to Florence OR out
     60 nm.
&&

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