Heavy Snow in Washington, December 8 to 10, 2022

Area Forecast Discussion
National Weather Service Portland OR
334 PM PST Wed Dec 7 2022

.SYNOPSIS...The wet weather will resume tonight, as the first
of a series of fronts will push across the region. Another front
arrives later Friday into Saturday. Expect plenty of rain along
the coast and into the interior, with heavy snow for the
Cascades, and even on the higher coastal mountains. Will be
breezy along the coast as well, mainly tonight, and again
Friday into Friday night. Along with coastal winds, will see
somewhat heavy seas as well, mainly on Friday. Bit of a break
later in the weekend, but the cool, unsettled weather continues
next week.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...A broad low pressure
system with associated front will move over the forecast area
tonight into Thursday. This low is wrapped in cold air from the
Gulf of Alaska which will cause snow levels to drop slightly as
it pushes inland. 850 mb temperatures are around -3 to -5 C
through late Thursday so snow levels will reach around 2000 ft
by Thursday morning. Precipitation with this front is more
robust than the last front which will cause precipitation in
increase significantly. There is widespread consensus in
precipitation location, but there is still some challenges in
regards to precipitation amounts. Rainfall accumulations will be
around 1 to 1.5 inches from late tonight into Thursday
afternoon around the Coast with around a quarter to a half inch
inland. With cooling temperatures aloft and a slight east wind
leading into the frontal passage, will see enough cold air to
make snowfall increase through this storm. Accumulations will
range from 6 to 10 inches above 2000 ft in the central and
northern Oregon Cascades and foothills, all the way to near 22
inches along Mt St Helens. Have advisories in place for all of
the Cascades above 2000 ft aside from the south Washington
Cascades where a warning is in place for elevations above 4000
ft to account for higher accumulations.

In addition to snowfall concerns, freezing rain is a bit of a
challenge overnight into Thursday morning. Taking a little
closer look at the Gorge for tonight into Thursday, increasing
offshore gradients will advect colder subfreezing temperatures
found near The Dalles and east into the Gorge overnight. Ahead
of the front, temperatures will be warming in the 950-875mb
(1500-4000ft) layer above freezing, as evidenced on the forecast
soundings at Troutdale. Its likely that some of this warmer air
aloft will push its way into the western and potentially into
the central Gorge, bringing a chance of freezing rain with it as
it falls into the subzero temperatures. If the warm layer stays
in tact once the precipitation starts falling, likely around
the midnight, light accumulations (up to a few hundredths) of
freezing rain could occur before the air column cools enough
from what little evaporation will occur for it to change over to
snow. While NBM 4.0 has 45-55 percent chance of 0.1 inch of ice
accumulation, NBM 4.1 shows less than 5 percent. Believe the
actual chance is between these two solutions, and having the
HREF guidance showing small areas of freezing rain to a mix
increased our confidence of seeing this precip type, if only for
a small window. Best chance for this will be between Bonneville
Dam and Viento State Park, affecting both highway 14 and I-84.
Any freezing rain should switch over the snow relatively fast,
with an area of convergence likely occurring with the deeper
cold air over the central Gorge. This will likely end up with
snowfall amounts near a couple inches at the convergence with
around 0.5 to 1 inch elsewhere along river level. A Winter
Weather Advisory has been issued to handle both the snow amounts
with some mixed precipitation. This will likely make for
slippery roads early Thursday morning.

Will say that models have decreased QPF even more and this
front is moving through a bit quicker than previously
noted. Ultimately this would impact the precipitation timing and
whether there is enough of a period of strong accumulations. After
this first front moves in, a secondary shortwave trough follows
behind it within the overall flow late Thursday into Friday.
This trough will enhance the already convective like post-
frontal showers. Instability will increase and the chances for
thunderstorms follows suit over the waters. Chances are around
15% for these thunderstorms, but with warmer air starting to
filter in behind the cold air pool, forcing could be great
enough to initiate lightning. Showers will persist through
Friday with limited accumulation and some sun breaks. This pause
in precipitation will be short-lived as yet another strong low
pressure system drops down from the Gulf of Alaska overnight.
											-Muessle/Kriederman

.LONG TERM...SATURDAY THROUGH TUESDAY...Late Friday into
Saturday will see yet another low pressure system advance
towards the coastline. This low is well realized by models, but
there still are some differences between them that will impact
precipitation type and accumulations. The track taken in this
forecast is that of the GFS and the Canadian. These two models
are in decent agreement in regards to the low position as it
makes landfall over western Washington and shifts southeast over
the inner mountain west. This system has the potential to bring
in another round of significant precipitation. NBM forecast QPF
did run higher in recent runs which may or may not come to
fruition. The accumulation will be greatly dependent on the
track of this storm. If it were to trend with the current
solution of more of a northerly position, then precipitation will
manifest everywhere over our forecast area. Snowfall likely will
be greatest along the south Washington Cascades. However, if it
follows the ECMWF solution, it would move much further south
leaving the northern areas less wet, and the Lane County
Cascades under more of a rain band. As previously stated,
cluster analysis is consistent with this system happening, but
there is enough uncertainty that feeds into the forecast.

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.

One concern that is rising is that of snow melt and added rain.
With rising snow levels, added rain and possible melting snow,
could see some slight river rises. After discussion with the
River Forecast Center, have deemed rivers capable of holding
water. However, if we trend on the higher side of the forecast,
will have to monitor river levels.

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...00Z TAFs: Low stratus has been stubborn to erode in
the Willamette Valley today, with varying degrees of improvement
at the inland terminals ranging from IFR at KHIO and KUAO to VFR
at KPDX and KEUG. Expect flight categories to generally range
from low end MVFR to VFR across most of the area ahead of the
front crossing the region later this evening. Timing remains on
track with regards to the front. Expect rain to arrive at the
coast between 02-05z Thu and then spread into the valley 05-08z
Thu, with conditions gradually trending back down 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
10z Thu.

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...Timing of headlines remains on track as a moderately
strong front enters the coastal waters early this evening, with
Gale Warnings beginning at 4 PM in the outer waters and spreading
to the nearshore waters at 10 PM. The front will quickly cross the
waters and push inland late tonight, with gales ending rapidly
from west to east overnight. Seas will be primarily driven by
short period wind-driven waves, allowing seas to settle
relatively quickly and leading to a brief lull in conditions with
light to moderate westerly breezes through the first half of the
day on Thursday. A building westerly swell will push seas back
above 10 ft by Thursday afternoon, promoting the next round of
small craft headlines across the waters.

Models have come into somewhat better agreement regarding the
next system to impact the region on Friday, with a surface low
bottoming out around 983 mb well off the Washington coast and
sending a stronger front into the coastal waters Friday afternoon
and evening. Expect at least another solid round of gales with
this system, with low end storm force gusts also a possibility.
Will continue to assess the next round of headlines associated
with this system in coming forecast packages. 				-CB

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 4 AM Thursday to midnight PST
     Thursday night for ORZ010>013.

     Winter Weather Advisory from midnight tonight to noon PST
     Thursday for ORZ014>016.

WA...Winter Weather Advisory from midnight tonight to noon PST
     Thursday for WAZ045-046.

     Winter Storm Warning from midnight tonight to 4 AM PST Friday
     for WAZ019.

     Winter Weather Advisory from midnight tonight to midnight PST
     Thursday night for WAZ040.

PZ...Gale Warning from 10 PM this evening to 4 AM PST Thursday for
     PZZ210-251>253.

     Small Craft Advisory until 10 PM PST this evening for
     PZZ251>253.

     Small Craft Advisory until 4 PM PST this afternoon for
     PZZ271>273.

     Gale Warning until 4 AM PST Thursday for PZZ271>273.
&&

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