Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 302231 AAA

Area Forecast Discussion
National Weather Service Portland OR
230 PM PST Mon Jan 30 2023

.SYNOPSIS...Strong high pressure will keep SW Washington and NW
Oregon dry through Thursday morning. A weak front passing to
our north may bring some drizzle to the coast Tuesday night.
Temperatures will gradually warm through the week with a return to
more active pattern by Friday.


Oregon, SW Washington as high pressure pushes east towards the coast.
Clear skies under a subsidence inversion with relaxed winds are
allowing daytime temperatures across the high terrain to warm 10-20
degrees warmer than this same time [2 PM] yesterday.

GOES-West imagery shows upper level moisture associate with a weak
shortwave moving across the northern periphery of the ridge. This
shortwave will provide just enough lift to bring some light rain to
the northern coast Tuesday night into Wednesday. Inland areas are
very likely to stay dry through Wednesday as a continued offshore
component to the low level flow should keep most of this moisture
from making it east of the Coast Range.

Temperature inversions will build through Wednesday as the air mass
aloft moderates. The air mass is still looking too dry for fog
development tonight through Wednesday night. Moderation and a little
more sunshine should allow valleys to warm a bit closer to normal
Wednesday. Air stagnation may also result in some deterioration in
air quality tonight through Wednesday, but the period of stagnant
conditions still remains too short (less than 48 hours) to warrant an
advisory.  -BMuhlestein/Weagle

.LONG TERM...THURSDAY THROUGH SUNDAY...The upper-level ridge over our
region is expected to break down  and/or move eastward on Thursday,
based on ensemble guidance and WPC`s 00Z cluster analysis. Following
this exiting ridge, an upper- level short-wave trough is expected to
move in and bring our next round of precipitation beginning Thursday
night into Friday. NBM PoPs show a 40-60% chance of precipitation
across our region Thursday afternoon, increasing to a 60-80% chance
Friday morning. The majority of WPC clusters agree with this timing;
however, 34% of Euro ensemble members suggest that our region will
still be under the exiting portion of the ridge. If that were the
case, we would likely see precipitation arrive later than currently
expected. As we enter this wet pattern, ensemble guidance shows on-
shore winds from the southwest/west moving in, which will moderate
temperatures across the region to near seasonal values.

By Saturday afternoon into Sunday, all cluster analyses agree that a
deeper, upper-level trough will begin to move in to our region and
bring another round of precipitation. Totals from this system are
expected to be higher than the initial Thursday-Friday system. This
system will also bring breezy south winds to the area, but forecast
wind gusts are less than 25-30 mph. By Sunday afternoon, 80-90% of
all members from the GFS, Euro, and Canadian ensembles agree that
total precipitation will exceed half an inch. At this time, forecast
rain amounts are not high enough to prompt flood concerns.  -NAlviz


.AVIATION...00Z TAFs: VFR flight conditions will continue through
00Z Wednesday. High clouds will move into the region from the
northwest tonight into tomorrow, but cigs should be above 15-20
kft. Expect light and variable winds around 4-5 kt or less
tonight through tomorrow.

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

KPDX AND APPROACHES...VFR flight conditions will continue through
00Z Wednesday. High clouds will begin to move in from the
northwest after sunset, but cigs will be above 15-20 kft. -TK


.MARINE...Buoy observations from 2 PM PST Monday showed relatively
light northeast winds around 5 to 10 kt with seas around 5 ft,
primarily due to a long period westerly swell. The ongoing long
period westerly swell is expected to continue through Wednesday
at around 4-5 ft along with sub-small craft criteria winds under
20 kt. The exception will be Wednesday afternoon when wind gusts
should increase close to 20-25 kt. For now, will hold off on
issuing a small craft advisory on Wednesday given how marginal
forecast wind speeds currently are.

The main concern in regards to winds and seas is Thursday into
Friday when the next Pacific frontal system is set to impact the
coastal waters. Although there is still considerable uncertainty
regarding exact timing and exact wind speeds with this system,
confidence is increasing that gale force wind gusts will occur. In
fact, the NBM is showing a 65-75% chance of southerly wind gusts
in excess of 34 kt with this system. Even storm force wind gusts
cannot be completely ruled out yet, as the NBM suggests there is a
10-20% chance of wind gusts in excess of 48 kt over the outer
waters. For now, will continue to forecast wind gusts in the 30-40
kt range, strongest over the outer waters beyond 10 nm offshore. A
fresh swell will also build with the increasing winds, likely
pushing significant wave heights up to at least 13-16 ft. Seas
will likely reach 20 ft or higher if wind gusts end up stronger
than 40 kt. -TK






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