989 
FXUS66 KPQR 282247
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
243 PM PST Thu Nov 28 2019

.SYNOPSIS...High pressure and offshore flow will lead to mainly dry
but chilly weather today and Friday, aside from a few flurries or
light snow showers near the Cascade crest this morning. Next low
pressure system will approach from the southwest this weekend,
bringing another chance for mainly light precipitation while snow
levels remain low Saturday night and Sunday. 

&&

.SHORT TERM...Today through Friday night...Visible satellite imagery
continues to reveal low clouds in the Columbia Basin. Low level
easterly to northeasterly flow continues to advect drier air into the
lower Columbia Basin, which is slowly eroding cloud cover as
suggested by the NBM and HREF guidance, but there remains some
uncertainty if it will completely dissipate by Friday morning. 

Otherwise, offshore flow across the region tonight through Friday 
night should lead to mostly clear skies and cool temperatures for 
late November. Slightly higher dewpoints in the southern Willamette 
Valley this afternoon will once again make the I-5 corridor between 
Cottage Grove and Harrisburg susceptible to fog late tonight and 
Friday morning. Expect low temperatures to once again drop down well 
into the 20s in locations that decouple tonight and again on Friday 
night. While high temperatures should generally top out in the low 
to mid 40s for most valley locations Friday. It should be noted that 
the aforementioned cloud cover uncertainty around Hood River does 
make the low temperature forecast in the upper Hood River valley a 
particularly low confidence forecast tonight given the fresh snow 
cover on the ground. Temperatures could realistically end up in the 
single digits in a clearer scenario vs only dropping into the 20s in 
a cloudier scenario. For now have gone with a blend of model 
guidance to account for the uncertainty. /Neuman

Saturday through Sunday...While NBM still shows some level of
differences in precipitation chances in the Sat to Sun time frame,
operational models seem to be fairly locked in with lifting a
shortwave north through the forecast area mainly in the Sat night to
Sun morning time frame, so will lean pops a bit towards the
operational solutions for pops and timing. Thermal fields continue to
suggest air mass marginally cold enough to support snow initially in
much of the Willamette Valley, north Coast Range, Gorge and I-5
corridor in sw WA. Gradually though some warmer air is depicted
pushing north west of the Cascades overnight and Sunday morning,
switching the most likely precipitation type over to rain at lower
elevations form south to north. NBM favors light precipitation
potential, with any snowfall likely to be under an inch at low
elevations, and likely limited to under two inches in the Coast Range
and Cascades. 


.LONG TERM...Sunday night through Tuesday...Operational models all
continue to indicate the low/trough off the coast cutting off and
moving south Sun night through Tue. This quickly results in
precipitation chances dropping off Sun night night and Mon, with the
remaining chances shifting south. Ensembles however are not quite as
definitive, so will need to keep some pops in for Sun night into Mon.
This is a little problematic though, as the air cools quickly Sun
night with the continuation of a cool offshore flow and decrease in
cloud cover, but if some threat of precipitation remains, cooling
will be slowed. For now will keep a small mention of friezing rain
chances initially in the Gorge, but expanded out into the north half
of the Willamette Valley late Sun night. At this time though, it
would appear the likelihood of a freezing rain scenario is rather
small, just not quite small enough to completely ignore yet.

As we head into the Wed and Thu time frame, model differences become 
more apparent with operational runs as the east-west positioning of 
a trough offshore becomes more critical. For now will trend towards 
middle of the road pops and near seasonal temperatures. 


&&


.AVIATION...Offshore flow will bring predominantly VFR conditions
areawide through 00z Saturday. The main exception to this will be
in the southern Willamette Valley between Cottage Grove and 
Eugene where LIFR fog may once again redevelop tonight between 
~12-18z Friday.

KPDX AND APPROACHES...Offshore flow will bring VFR conditions
through 00z Saturday. Expect easterly wind gusts of 25 to 30 kt 
this evening to decrease to less than 15 kt Friday. /Neuman


&&


.MARINE...Easterly winds will continue to weaken throughout the 
day. On Saturday, a low pressure system will move towards the 
waters, switching winds to the south and may bring winds gusting 
to 25 kt primarily over the outer waters. General seas will be
between 3 to 5 ft through Saturday morning and reach 10 ft by
Saturday night. Stronger wind waves from the southeast will 
interact with a northwesterly swell which could create steep, choppy
conditions Saturday night into Sunday. -BPhillips


&&


.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.

&&


$$


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This discussion is for Northwest Oregon and Southwest Washington 
from the Cascade crest to 60 nautical miles offshore. This area is 
commonly referred to as the forecast area.