499
FXUS66 KPQR 042132
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
131 PM PST Wed Dec 4 2024

.SYNOPSIS...High pressure over the region with air stagnation
issues in sheltered interior valleys. A weak disturbance will bring
an end to these issues on Thursday, though rain now looks to hold
off until late Friday for areas away from the coast. Rain chances
increase across the area this weekend, with accumulating snow
possibly returning to the Cascades Saturday night into Sunday.

&&

.DISCUSSION...High pressure persists over the region, but water
vapor satellite imagery shows an upper shortwave trough sliding east
across toward the Pacific NW. Another cool, mostly sunny day, except
stratus lingering in the Hood River Valley and the south Willamette
Valley, beneath a strong subsidence inversion. The upper level
disturbance and associated front will weaken as it approaches the
region later tonight into Thursday. As such, will maintain a 20-30%
chance for light rain along the coast, while inland expected to
remain dry through at least Friday afternoon. The low level flow
should generally remain offshore, but do expect increasing clouds
into Thursday morning. This would help reduce radiational cooling at
the surface, but still anticipate another chilly night tonight with
most interior lowlands falling below freezing again. The upper level
ridge builds over the region again on Friday, but this will be brief
as a deeper upper level trough moves into the NE Pacific.

East winds continue to spill out of the Columbia River Gorge, while
the pressure gradient between KTTD-KDLS appears to have peaked
around -9.0 mb this morning, with maximum wind gusts of around 75
mph at Crown Point and Corbett, and around 45 mph in the Troutdale
area. The gradient is forecast to gradually decrease to around -4.0
to -6.0 mb by Thursday afternoon, likely resulting in east winds
only gusting up to 20 to 30 mph in the eastern Portland metro
through Friday.

Precipitation becomes more likely across the area this weekend
as the ridge axis finally shifts east of the Cascades and opens
the door to the next upper level trough approaching the region.
In terms of potential amounts, NBM mean QPF guidance currently
depicts around a inch of rain at the coast and closer to a half
inch inland for the 48 hour period ending early Monday morning.
So, while rain is likely to return to the region, not
anticipating anything that would lead to any hydro concerns at
this time. Snow levels will initially be well above the Cascade
passes, but will lower to 3000-4000 feet Saturday night into
Sunday, bringing the potential for the next round of
accumulating snow in the High Cascades. Global ensembles then
hint at a return of upper level ridging and drier conditions
during the early part of next week. DH/CB

&&

.AVIATION...High pressure continues to bring generally clear skies
over the airspace with the exception being areas within the
Southern Willamette Valley. For locations south of KCVO, expect
persistent LIFR/IFR conditions through the TAF period, with a
5%-10% probability of some brief VFR/MVFR conditions starting
around 22Z Wednesday. Areas near KSLE, KUAO and KHIO have a
55%-65% probability of IFR/LIFR conditions return starting around
12Z-14Z Thursday. Otherwise, predominately VFR conditions are
expected.

With temperatures below freezing for much of the central and
southern Valley, freezing fog is expected to be a concern for any
terminals that will see fog. Rime icing will be possible in the
coolest areas.

A pattern change is on the horizon as a weak frontal system will
start to bring light precipitation to the coast around 12Z-14Z
Thursday. This precipitation will spread eastwards, but will
likely not bring any precipitation to inland locations given they
very dry conditions.

PDX AND APPROACHES...VFR conditions with clear skies. Easterly
winds below 10 kt, increasing to 10 kt at times due to strong
winds from the Columbia River Gorge. /42

&&

.MARINE...High pressure persists resulting in mild conditions
across all waters through at least tonight. Winds will remain
under 10 kts and seas around 4 to 6 feet at 13 seconds. Expect
strong ebbs to return to the Columbia River Bar around 6PM
Wednesday with seas around 7 feet, therefore have issued a Small
Craft Advisory from 3PM to 9PM Wednesday.

Early Thursday morning, a weak front passing through the area
will briefly elevate southerly winds, giving a 20%-40% probability
of Small Craft Advisory conditions. Another, stronger front on
Friday/Saturday will bring a 50%-60% probability of Small Craft
Advisory conditions top all waters with seas building towards 10
feet on Saturday and continuing to build into towards 16 to 19 ft
(60%-75% probability) by late Saturday/early Sunday and will
likely remain in the low to mid teens through Monday morning and
are expected to continue to subside through the start of next
week. 42~Hall

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Air Stagnation Advisory until 4 AM PST Thursday for ORZ104-105-
     108-109-113>118-123>125.

WA...Air Stagnation Advisory until 4 AM PST Thursday for WAZ202-
     204>206-208.

PZ...Small Craft Advisory until 9 PM PST this evening for PZZ210.
&&

$$

www.weather.gov/portland

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