Heavy Snow in Oregon, December 25, 2020

Area Forecast Discussion
National Weather Service Portland OR
337 AM PST Fri Dec 25 2020

.SYNOPSIS...High pressure aloft will continue to shift east today as
a weather system approaches the area. This system is expected to
reach the coast a little before sunrise and then move inland during
the day. Offshore low-level flow will continue through the Columbia
Gorge through Friday. Cold air will remain trapped in the Upper Hood
River Valley today and tonight, resulting in a mix of freezing and
frozen precipitation. A return to a drier pattern is to begin
Saturday night and continue through early next week. Forecast
confidence lowers for the latter half of next week, with the best
scenario trending toward climatology.

&&

.SHORT TERM...Today through Monday...Early morning GOES-17 water
vapor imagery showed an upper level ridge pushing east into the
Northern Rockies. An elongated frontal system extended from the Gulf
of Alaska to the offshore waters well off the south California coast.
The parent low was located neasr 53N 138W. However, another surface
low appeared to be developing near 48N 130W with possibly a third
surface low at 38N 129W.

Persistent fog remains in the central and south Willamette Valley
early this morning. However, it appears visibilities have shown some
improvement since 08Z. At 10Z one-quarter mile visibility was noted
at K77S and KUAO. Temps are at or slightly below freezing, resulting
in freezing fog potential. The increasing mid and high level clouds
should help to erode the valley fog during the morning.

Meanwhile, attention turns to the approaching frontal system and
potential highlights. The frontal will slow down as it nears the
coast due to surface low development along the boundary. Expect the
offshore low-level flow through the Gorge to strengthen as the front
approaches. The 06Z NAM shows the KTTD-KDLS gradient peaking around
-8 mb at 15Z. However, at 09Z the gradient was already -8 mb. Thus,
would not be surprised to see it peak closer to -9 mb or so. Gusts
70-75 mph continue at Crown Point. Peak gusts were close to 90 mph a
little before midnight. The offshore flow has also resulted in temps
in the mid to upper 20s in the Upper Hood River Valley and far east
portion of the Central Gorge. Do not see this cold air mixing out or
going anywhere soon and will help set the stage for some mixed winter
precipitation.

Expect timing for rain onset at the coast looks to be between 12Z and
15Z and then spreading into the Willamette Valley and Clark County
around 18Z and finally to the Upper Hood River Valley and Central
Gorge between 18Z and 21Z. The NBM 1D viewer for Parkdale shows a 60
to 70 percent chance of -FZRA mid-afternoon through early Sat
morning. The snow probablility peaks during the late afternoon and
evening and then diminishes tonight. Thus, it appears freezing rain
will be more prevalent than snow. In fact, the NBM viewer forecasts
up to a tenth of an inch of ice accumlation late this afternoon
through early Sat morning, with about inch or two of snow before
becoming primarily freezing rain. The 00Z HREF indicates a 60 to 70
percent chance of greater than 0.06 inches ice accumulation for the
Upper Hood Valley and also some valleys in south Washington Cascades
south of Mt. Adams. With all this said, will go with a winter weather
advisory for the Upper Hood River Valley starting at noon today and
running through 15Z Sat.

The other area of concern is the South Washington Cascades. Forecast
QPF and lower snow levels due to the offshore flow results in
potential snow advisory criteria. The most favorable orographic
enhancement looks to occur Fri evening through Sat morning. Per
coordination with KPDT, will go with an advisory for the South
Washington Cascades starting at noon and going through 3 pm Sat.
Cannot rule out some localized -FZRA in and around K4S2 late this
afternoon through tonight. However, the NBM 1D viewer for K4S2 shows
very minimal ice accumulation and only a 20-30 percent chance of
-FZRA late this afternoon through mid-evening. Rain becomes the
highest probability precip type Friday night.

The deterministic ECMWF and GFS have come into better agreement
regarding a surface low that is forecst to reach Vancouver Island by
12Z Sat. The ECMWF ensembles are in very good agreement showing the
surface low near the south end of Vancouver Island 12Z Sat, with a
central pres around 995 mb. Thus, the south Washington and north
Oregon coastal areas will likely see breezy to windy conditions Fri
afternoon into Saturday morning.

Transitory high pressure moves into the area Sat evening into Sat
night as a broad 500 mb low takes shape in the eastern Pac. This will
induce offshore low-level flow Sun. Some lingering light
precipitation impacts the south Washington and north Oregon coast Sun
morning. This upper low eventually drops south and then heads into
northern California Sun night. This should leave the forecast area
dry, with valley inversions developing Mon. Will need to watch for
wrap-around moisture into the far south portion of the Sun night and
Mon. Weishaar

.LONG TERM...Monday night through Friday...Monday night into early
Tuesday should remain dry, but the deterministic GFS tries to bring a
splitting short-wave into the area late Mon into Mon night. Not very
confident this will materialize. Ridging starts to break down Tue as
models show a warm front moving toward the area. The deterministic
ECMWF shows over-running precp spreading into the area Tue evening.
Will need to watch this as offshore flow will be in place, which will
result in fairly low snow levels. There is fairly low forecast
confidence Wed and beyond. The operational ECMWF shows a fairly wet
system moving into the Pac NW late Wed. The GFS holds on to some
degree of 500 mb ridging with a baroclinic boundary sitting over the
area. In fact, some GEFS and EPS Integrated Vertical Transport (IVT)
Plumes suggest a possible weak AR. Not jumping on this just yet, but
will be something to keep an eye on over the next few days. The total
mean 500 mb cluster valid Wed maintains ridging over the Pac NW.
Forecast confidence becomes even lower Thu and Fri with fairly large
model variances and large differences in the 500 mb cluster analyses.
Weishaar.

&&

.AVIATION...12Z TAFS: A front will continue to move eastward
through today. This will result in a mixture of low end VFR/high
end MVFR conditions through the TAF period. Rain is already
present along the coast and will move inland through the morning,
it will then become more showery in nature through this
afternoon. A strong pressure gradient will continue to sustain
breezy east winds into KTTD with gusts up to 35 kt possible this
morning. Parts of the central and southern Willamette Valley will
remain under IFR stratus and fog and are likely to persist
through the majority of today. Conditions could briefly improve
to MVFR for the central and south Willamette Valley as winds help
scour out the cold pool. However, if this happens it will be
short lived as IFR/LIFR conditions are expected to return during
the evening.

As the front approaches the Oregon coast and moves inland, there
is a chance that local LLWS sets up as modest offshore flow gets
over topped by increasing southerly winds.

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

KPDX AND APPROACHES...VFR through the morning with increasing
high to mid level clouds. Chance of rain increases late this
morning as a front approaches from the west. Gusty east-
southeast winds with gusts 20-25 kts will likely develop by this
time too. Cigs below 4000 arrive around 21Z Friday then MVFR
cigs 2500 to 3000 near 01Z Saturday. Cold front crosses the
field 26/05-07Z. /42

&&

.MARINE...A frontal system located inside of 130W will continue
to move northeast towards Vancouver Island today. In addition to
this frontal system, models are in good agreement that a new 995
mb low will spin up and track through the outer waters early this
afternoon and quickly follow a similar path to the early morning
frontal system. Southerly winds will continue with gusts up to
45 kt for zones PZZ270 and PZZ275,with the strongest winds
beyond 30 NM. These winds will persist through the remainder of
today. Zones PZZ250 and PZZ255 will hold the current small craft
conditions as well, with winds out of the south around 15 to 25
kt with gusts more near 30 kt. Though cannot rule out isolated
gusts up to 40 kt. Therefore will continue the current Gale
Warning and Small Craft Advisory through at least the evening.
There is a good chance that the Small Craft Advisory for the
inner waters will be continued in addition to the outer waters
having their own small craft advisory through Saturday.

Saturday models continue to remain in good agreement as to the
overall pattern. They show a distended upper level trough and
the extended attending front, which will likely maintain small
craft winds but, there is some discrepancies regarding strength
between the model families which is cause enough to not want to
extend the current Small Craft Advisory. Through the start of
next week the overall pattern looks to be far less active.

Seas start around 11 to 13 ft at 10 to 13 seconds and will build
towards 15 to 19 ft at 12 to 15 seconds through the morning.
Seas will likely be very chaotic as a southerly fresh swell, a
westerly main swell and higher wind waves will all be impacting
the sea state. There is a small chance seas could reach near 20
ft through the outer waters south of Cascade Head briefly this
morning. As this front weakens expect seas to subside slightly
to near 12 to 15 ft by Saturday morning where they will remain
through early next week. /42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Dense Fog Advisory until 9 AM PST this morning for Central
     Willamette Valley-South Willamette Valley.

     Winter Weather Advisory from noon today to 7 AM PST Saturday
     for Upper Hood River Valley.

WA...Winter Weather Advisory from noon today to 3 PM PST Saturday
     for South Washington Cascades.

PZ...Gale Warning until 7 PM PST this evening for coastal waters
     from Cape Shoalwater WA to Florence OR from 10 to 60 NM.

     Small Craft Advisory until 7 PM PST this evening for coastal
     waters from Cape Shoalwater WA to Florence OR out 10 NM.

     Small Craft Advisory until 6 AM PST Saturday for Columbia River
     Bar.

&&

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