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fxus66 kpqr 212236 

Area forecast discussion
National Weather Service Portland or
231 PM PST Thu Nov 21 2019

Synopsis...high pressure aloft over the region will gradually weaken
over the next couple of days. A very weak weather system moves
through the area early Friday and will have minimal impact. Another
frontal system drops southeast through the area Saturday night and
Sunday. A more active pattern is expected much of next week. Holiday
travelers should be prepared for snow across the Cascades at the very
least through mid to late next week.


Short term...tonight through Sunday...early afternoon water vapor
imagery revealed a deep upper low centered over the Desert Southwest
and an elongated, weakening frontal zone along 135w. High pressure
was sandwiched between these two features. The fog that developed
along the Columbia River between kpdx and just east of kast and also
the Cowlitz River Valley had nearly completely dissipated by 21z.

Little change in forecast thinking for the short term period. The
extended period is more complicated and challenging (see long term
discussion below). Offshore low-level flow continues through the
Columbia Gorge this afternoon. At 21z the kttd-kdls gradient was
around -4 mb, down from the -6 mb this morning. The latest model
runs continue to show this gradient weakening through tonight as a
dying frontal system approaches the coast. The NAM and GFS show the
upper level front reaching SW Washington and far northwest Oregon around
12z Fri. The current offshore gradient becomes neutral late tonight
and then slightly onshore Fri morning. The combination of weakening
surface gradients, somewhat higher dew points and minimal cloud
cover will lead to better fog and low cloud potential, especially
for the interior valleys.

The dying front pushes southeast through the area Fri afternoon. NAM
time-height section and sounding for kpdx shows substantial dry air
from around 925 mb to 400 mb Fri afternoon. Thus, expect another nice
afternoon after any morning fog and low clouds. Models agree another
shortwave trough, currently over the western Aleutian Islands, will
race east and drag a front into the Pacific northwest Saturday night
into Sunday. This front reaches SW Washington and far northwest Oregon
between 06z and 12z sun and then slides southeast sun. This system
opens the door for additional weather systems to impact the area. One
other issue for the Fri-sun period will be an elevated risk for
sneaker waves. Model guidance suggests wave heights will be 12 to 14
ft Fri with periods around 16 sec. These values are favorable for
sneaker waves. Wave heights ease a little over the weekend, but
periods remain near 15 seconds. Weishaar

Long term...Sunday night through Thanksgiving day...the
unseasonable doldrums which have dominated much of November across
SW Washington and northwest Oregon appear to give way to a significantly
more active pattern as we head into early next week. Beyond the
first front expected to move through the forecast area Sunday,
additional shortwave energy will move through for more precipitation
through Monday. Meanwhile, snow levels will be lowering as flow
aloft turns more northwesterly, likely lowering below the passes
Sunday night then remaining there through much, if not all, of
Thanksgiving week. Therefore by Monday one can expect to encounter
wintry travel across the Cascades, with snow levels lowering into
the foothills Monday and Monday night as a large upper level trough
carves out over the pac northwest.

Attention then turns to the low pressure system shown by many
models/ensembles rounding the base of said upper trough and moving
into the West Coast, mainly Tuesday. There are still enormous
differences between members regarding the evolution of this system,
with the range of landfall solutions extending from Vancouver Island
to south of San Francisco and solutions for the low's intensity
ranging from sub-960 mb to around 1000 mb as it moves onshore. Given
the cold air pushing down with the upper trough, and the potential
for the low to come onshore to the south of all or part of the
forecast area, this system will need to be watched for the potential
for low elevation snow north of the low. That said, no single
solution can be trusted (nor discounted) at this point, especially
given the variance between guidance. For now, we continued to trend
our forecast colder, with snow levels generally around 2000 feet
Tuesday then down to 1000-1500 feet by Wednesday. Longer range
guidance appears to be in decent agreement that the upper trough
Will Park over the pac northwest through Thanksgiving, with scattered
showers and possibly even a few flurries reaching down to the lowest
elevations. Weagle

Aviation...VFR sky clear across the area as of 21z. The last of the
IFR fog/stratus that was along the Cowlitz river in SW Washington
finally dissipated. VFR to prevail through at least 08z Fri.
Offshore low-level flow will continue through this evening, but
weaken. Surface gradients go nearly neutral between 09z and 12z
Fri and this will likely to lead to an increasing potential for
IFR or lower conditions late tonight into Fri morning for the
interior valleys. High-resolution guidance suggests MVFR cigs
likely to develop along the South Washington and north Oregon
coast late tonight and hold through Fri morning and possibly into
early afternoon. Elsewhere, VFR will prevail Fri afternoon, after
fog and low clouds that occur dissipate.

Kpdx and approaches...VFR conditions through at least 10z Fri.
Offshore surface gradient through The Gorge will continue to
weaken through the evening, primarily impacting east approaches.
There is the potential for IFR or lower conditions to develop at
the terminal and vicinity late tonight, but VFR to prevail after
18z. Weishaar


Marine...significant long period northwesterly swell expected to
come in fast tonight with seas increasing quickly to around 12 ft at
17 seconds by sunrise Friday and peaking in the 13-14 foot range
Friday aternoon. That swell decays slowly through the weekend with
periods remaining long and heights remaining above 10 ft.

There is low confidence in the forecast for next week as models have
been inconsistent both between each other and between runs. The high
end of the models would bring in seas close to 20 ft early next week
with gale winds while the low end shows seas remaining just above 10
feet with winds below 20 kt. Have kept the forecast closer to the
low end for now, but anyone making plans to be out on the water next
week should keep a close eye on the forecast as it may change
significantly over the coming days. Bowen


Pqr watches/warnings/advisories...
Pz...Small Craft Advisory from 2 am Friday to 4 PM PST Saturday for
coastal waters from Cape Shoalwater Washington to Florence or out
60 nm.

Small Craft Advisory from 11 PM this evening to 4 PM PST Friday
for Columbia River bar.



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