Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 061831 AAA

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1030 AM PST Tue Dec 6 2022

Updated aviation discussion

.SYNOPSIS...Damp, dreary, and seasonably cool weather will persist
through Wednesday as low level moisture lingers. Weak weather
disturbances in northwest flow aloft will occasionally act on this
moisture to wring out some drizzle or light rain through Wednesday.
Most of the lowlands should remain just above freezing, but a few
pockets of freezing drizzle will be possible in outlying areas and
within the Columbia Gorge. A stronger frontal system will likely
bring rain and mountain snow Thursday, with low snow levels in the
Gorge. Active weather looks to continue through the weekend, while
cool temperatures keep snow levels on the low side, especially in and
near the Columbia Gorge.


.SHORT TERM...Today through Thursday...Latest radar imagery from KRTX
shows drizzle and light rain blossoming over the Portland metro area
and points north. Meanwhile, inland temperatures remain almost
uniformly in the mid 30s with minimal dewpoint depressions. Therefore
we are expecting precipitation to remain mostly in liquid form as it
falls and accumulates, and most areas below 2000 feet in elevation
should not see issues with the drizzle freezing to surfaces. Above
2000 feet, as well as for some lower elevations presently holding
snow on the ground, there is some potential of temps 32 deg F or
colder. Subfreezing temps are also fairly widespread in the Gorge,
with temps well below freezing east of Stevenson. A few Willamette
Valley locations adjacent to Cascade drainages may also dip just
below freezing due to cold air drainage from higher, snow covered
elevations. The difference should only be a degree or two, but with
temps so close to freezing, that cold air drainage may tip the scales
toward the drizzle freezing along our highways that tend to follow
the river drainages up into the Cascades. Thus far the precipitation
has done a fairly good job of staying away from our subfreezing
areas. Am not anticipating any advisories for the time being, but
will need to watch temps closely over the coming hours. Temps will
likely remain in the 30s for most of our inland areas for much of the
day, as weak flow and low sun angles keep clouds stubbornly in place.

The mechanism responsible for this morning`s drizzle is likely very
weak isentropic upglide associated with weak shortwave energy
rippling through the region on northwest flow aloft. Latest GFS
theta-e diagnostics show this weak isentropic lift between 925-700 mb
this morning, then pausing for a period later this afternoon and
evening, then resuming later tonight into Wed morning. Another pause
comes later Wednesday before much stronger isentropic lift takes hold
Wednesday night with rain increasing as a frontal system approaches
the coast. Suffice it to say that most of the forecast area north of
Salem will see occasional drizzle and slightly below normal temps
through Wednesday, with some freezing drizzle at times in the Gorge
and above-mentioned cooler areas.

Models show good agreement that a band of light to moderate
precipitation will sweep across the Pac NW early Thursday. The system
responsible for this is just beginning to take shape over the
Aleutians and western Gulf of Alaska. Shortwave energy and west Pac
moisture presently entering the western Gulf of Alaska will approach
the coast Wednesday night, pushed along by strong W-NW jet energy
over the north-central Pacific. However this jet max remains well
west of 130W through Thursday, so the initial frontal system will
likely be in the process of slowing and weakening while moving
onshore. This is not a good thing for any hopes that this system will
scour out cold air east of the Cascades and in the Columbia Gorge; in
fact strengthening easterly gradients Wed night may actually pull
cold air back westward through the Gorge. At this point, it appears
the cold air will have moderated sufficiently to keep temps above
freezing at Troutdale Wed night/Thu morning as east winds redevelop.
The chances of remaining above freezing Wed night/Thu decrease as one
heads farther east into the Gorge. Latest model soundings suggest
this will mostly be a snow event for the Gorge at least as far west
as Stevenson, with a period of freezing rain in the Gorge perhaps as
far west as Rooster Rock. Otherwise, valleys will see rain, with snow
levels starting out 2500-3500 feet ahead of the front and falling as
low as 1000-1500 feet behind the front. Snow amounts will largely
depend on how much moisture remains with this system by the time it
arrives, but at this point it appears this will be an advisory level
event with 1-3 inches in the Gorge and up to 4-7 inches for the
higher Cascade elevations.

Steepening lapse rates behind the cold front will turn precipitation
more convective Thursday afternoon; the NBM shows a slight chance of
thunder for our north coastal zones Thu afternoon into Thu night, so
we incorporated this into our forecast.  Weagle

.LONG TERM...FRIDAY THROUGH MONDAY...There is general agreement
moving into the long term with most of the WPC clusters leaning
towards an upper level trough across the coast of British Columbia
Friday and persisting through the weekend. Although the general
pattern is there amongst the clusters the magnitude and placement of
the trough is not being agreed up and this will have different
outcomes near the surface. Comparing the deterministic runs the EC is
leaning towards a closed low moving onshore of Vancouver Island
Friday, the GFS is near the OR/CA border and the CMC is completely
disorganized and further south. The mslp locations between all
ensemble members are spread up and down the Canadian coastline and
down to OR/CA border.

Where the low level front comes ashore will determine where the
heaviest rainfall will be but almost all ensemble guidance shows rain
continuing across SW Washington, NW Oregon through the weekend. The
NBM shows the heaviest period of precipitation occurring during the
first half of the day Friday. Snow levels will be trending around
2500 ft through the weekend so expect good snow accumulations across
the Cascades. Depending on how our late week system evolves, cold air
may remain trapped in the Columbia Gorge and Hood River Valley,
leading to lower or potentially much lower snow levels in and near
the Columbia River Gorge.  Weagle/BMuhlestein


.AVIATION...18Z TAFs: At 1820Z Thursday, a low stratus deck was
being observed across the Willamette Valley with cigs around 300
to 700 ft, lowest over the southern Willamette Valley where areas
of dense fog were also occurring. High pressure and calm winds
over the area will keep this stratus deck in place through the
remainder of today, except cigs should lift to MVFR thresholds
after 21-22Z Tuesday. Fog should also lift at KEUG around that
time. At the coast, VFR cigs were being observed and should
generally remain above 3000 ft through 18Z Wednesday.

Areas of light drizzle or light rain will also continue through
Tuesday evening as a weak upper level shortwave trough provides
lift within the stratus layer. Expect periodic visibility
reductions with drizzle, dropping to 3-4 SM or lower at times.
Off-and-on drizzle is expected at all TAF sites, except at KSLE
and KEUG.

For detailed Pac NW aviation weather information, go online to:

KPDX AND APPROACHES...IFR cigs will continue through the morning
arrivals to at least 20-21Z today. After 21Z, cigs should lift to
1000-1500 ft and then potentially to VFR towards 03-04Z Wednesday.
However, would not be surprised to see cigs fall below 1000-1500
ft once again tonight given the calm winds and lingering low-level
moisture. However, the NBM is only showing a 20% chance of IFR
cigs at the KPDX terminal tonight and a 50% chance of MVFR cigs.


.MARINE...Dare we say unusually pleasant conditions for the
coastal waters for Tuesday through early Wednesday as high
pressure transits the area. Winds become southerly Wednesday
midday and begin increasing as low pressure off the B.C. coast
swings a cold front across the waters Wednesday night. Gale gusts
around 40 kts ahead of the front seem like a solid bet and have
hoisted a Gale Watch to include wind driven seas of 10-12 feet.
Expect these frontal winds to push from west to east and cover
all of the waters, including the Columbia Bar. The front pushes
onshore 0400-0600 AM Thursday morning with the seas subsiding back
down to 6-8 feet for a short time. However, a trailing swell will
drive seas back to 10-14 feet by later Thursday morning.

The forecast is far from a certainty thereafter as models show
notable differences with the surface pressure pattern with nearly
every run. As such, have not latched onto any single model
solution but will stick with the NBM and its slower evolution
tendencies. Can say that Friday/Friday night appears to bring a
developing surface low somewhere to the PacNW coastal waters with
some models showing storm force gust potential. Then, another
weaker and dissipating low to the waters Saturday night. In most
cases, combined seas will remain elevated with high probability
of remaining 10-15 feet. /JBonk



PZ...Gale Watch from Wednesday evening through late Wednesday night
     for coastal waters from Cape Shoalwater WA to Florence OR
     out 10 nm.Columbia River Bar.

     Gale Watch from Wednesday afternoon through late Wednesday night

     for coastal waters from Cape Shoalwater WA to Florence OR
     from 10 to 60 nm.



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