670
FXUS66 KPQR 232257
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
257 PM PST Sat Nov 23 2024

.SYNOPSIS...Coastal/valley rain showers and mountain snow
showers persist the second half of the weekend into early next
week with the two main impacts being breezy winds along the
coast Sunday and additional snow accumulation at pass-level
(3000-4000ft+). Once we get towards the middle of next week the
trend is towards calmer and drier weather, albeit slightly
cooler than normal for this time of year. Confidence in the
current forecast and overall pattern progression is moderate to
high the next 5 to 7 days.


.SHORT TERM...Tonight through Tuesday night...Conditions
through Monday and Tuesday will continue to be dominated by a
broad upper-level low pressure sitting off the coast of the
Pacific Northwest. Current satellite and radar observations this
afternoon show scattered shower activity over western Oregon
and Washington although most of this activity is confined to the
Cascades and along the coast. A couple smaller-scale wave
features rotating around the aforementioned upper-level low will
help to modulate shower frequency going forward. The first and
most pronounced of these weak impulses arrives on Sunday,
leading to an increase in showers, while the broad low pressure
off the coast begins to meander southward. High resolution
guidance like the HREF shows the bulk of the activity holding
near the coast and coast range Sunday morning before shifting
further inland Sunday afternoon into the evening. Snow levels
hold around 3500-4000ft through this period so the Cascade
passes and most local ski areas likely receive 2-4 inches of
new snow accumulation Sunday through Sunday night; nothing
advisory worthy but still notable. HREF probabilities to exceed
4 inches of snowfall using 10:1 snow ratios during this period
are around 20-35% at the Cascades passes adding some confidence
to only limited winter weather impacts. Well also likely see
southerly winds pick-up along coast Sunday morning with gusts of
30-40 mph for beaches, headlands, and exposed ridge-tops in the
coast range, decreasing Sunday afternoon and evening.

On Monday the center of the weakening upper-level low pressure
off the Oregon coast wanders to the WSW and finally progresses
inland into far southern Oregon and northern California
Tuesday/Tuesday night. Through this period showers likely
continue, especially for the coast, coast range, and
Cascade/Cascade foothills where the best lift is available to
initiate activity. Thats not to say showers become absent over
the inland valleys, the I-5 corridor will see some action as
well, but showers will trend less frequent/intense with time
This will be most apparent on Tuesday as the bulks of the
energy/moisture associated with the upper-level low undercuts us
to the south. Snow levels stay near 3000-4000ft with continued
light snow accumulation across the Cascade passes.


.LONG TERM...Wednesday through Saturday...For the middle to end
of the week, WPC Cluster Analysis shows relatively good
agreement in the upper-level low rapidly ejecting into the
Midwest with broad ridging gradually building into the northeast
Pacific. This should result in drier, calmer, and stable
conditions Wednesday through Saturday, however, lingering moist
NW flow on the former day may still facilitate light pop-up
showers across the higher terrain features. Its all dependent
on how quickly the ridge is able to establish itself. Still,
there is a subset of roughly 30% of the total ensemble members
between the EPS/GEFS/Canadian which either the keep the ridge
less amplified and/or shunted further west in which case light
precipitation chances would linger into the end of the week.
Conversely the other 70% of ensemble members, and the most
likely solution going forward, move the ridge overhead. Well
need to keep an eye on overnight temperatures Wednesday night
through Friday night because if the more likely scenario plays
out, clearer skies and longer nights will lead to better chances
for overnight freezes/freezing fog for the inland valleys.
Currently, the NBM has a 50-80% chance for sub-freezing
overnight low temperatures in spots like the central/southern
Willamette Valley, Tualatin Valley, portions of SW Washington,
and coast range/Cascade valleys for the end of the week into
the first half of the weekend. Forecast confidence should
continue to increase in regards to the upper-level ridge
amplitude and placement in the next few days. -Schuldt

&&

.AVIATION...Satellite and radar imagery from early Saturday
afternoon depicted isolated light rain showers over the area with
convective activity decreasing a bit. Going through the TAF
period expect mainly VFR conditions although showers may
temporarily drop CIGs/VIS to high-end MVFR. The board low pressure
over the Pacific spins another weak disturbance into the region
late overnight into Sunday morning mainly resulting in an increase
in rainfall for the coast towards 12z Sunday. Rain will then
spread inland thereafter. Expect southerly/southeasterly winds
generally under 15 kt across the airspace. However, could see some
gusts up to 20 kt around the KONP starting around 00Z Sunday.

PDX APPROACHES...Generally VFR, with brief periods of high-end
MVFR under shower activity. Expect showers and southerly winds
under 10 kt throughout the TAF period. -Schuldt/TK

&&

.MARINE...Expect a benign weather pattern for this time of year
over the next week with no signs of significant winds or
hazardous seas. The only exception to that is Saturday
night/Sunday when a weak small-scale surface low will move northward
over the coastal waters. This low will bring a period of stronger
winds and elevated seas for a brief period of time. There is
still some uncertainty regarding exactly how strong winds will
get, but confidence in gale force gusts of 35-40 kt has increased
enough to upgrade the Gale Watch to a Gale Warning. Given the
short duration of these stronger winds, seas are not expected to
have enough time to build much beyond 10 to 13 ft, with a peak
likely Sunday afternoon. Beyond Sunday, benign conditions are
expected to return with winds and seas subsiding below 20 kt and
10 ft respectfully. -TK/42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory until 4 PM PST Sunday for PZZ210.

     Small Craft Advisory until 9 PM PST this evening for PZZ251>253-
     271>273.

     Gale Warning from 9 PM this evening to 4 PM PST Sunday for
     PZZ251>253-271>273.
&&

$$

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