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000
FXUS66 KPQR 241031
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
330 AM PDT Wed Apr 24 2024

.SYNOPSIS...High pressure will weaken and shift eastward through
Wednesday; meanwhile a weak Pacific frontal system will approach the
Pac NW coast. Drizzle will be possible along the coast by Wed
morning, but inland areas will likely see rain hold off until Wed
night or Thu morning as the next, better organized frontal system
approaches the coast. This frontal system will mark the beginning of
a prolonged period of cool and unsettled weather, with snow possible
down to the Cascade passes as soon as Thu night/Fri morning.

&&

.SHORT TERM...Wednesday through Friday night...Satellite
observations from 2-3:30am Wednesday showed high clouds beginning to
increase across southwest WA and northwest OR in response to a weak
cool front approaching the coast. In addition to the high clouds, a
low marine stratus deck was beginning to fill in at the coast. This
stratus deck will deepen between 4-10am Wednesday morning as the
aforementioned cool front approaches. Model soundings continue
showing the marine layer rapidly deepening to over 1-1.5 km at that
time, which will be deep enough to support some light drizzle and/or
light rain. A broken marine stratus deck will briefly attempt
filling in over the far south Willamette Valley/Lane County Cascade
foothills Wednesday morning given the southwest-northeast oriented
marine push in place. This low cloud layer will be shallow and
broken enough to clear out for the afternoon according to the
HREF/model soundings, however high clouds will be on the increase
once low clouds lift. As such, the high temperature forecast has
continued to trend cooler. The deterministic NBM is no longer
showing highs near 70 degrees over the inland valleys like it did 24
hours ago; the latest run is showing highs in the low to mid 60s
(50s at the coast). The probability for highs of 70F or warmer have
decreased from 5-25% to 1-5%.

Clouds lower/thicken in earnest Wednesday night as a fairly well
organized (but weak) low pressure system approaches the Pac NW coast,
along with its attendant warm front. Rain becomes likely throughout
the forecast area late Wed night/early Thu morning, with PoPs
increasing to 100% across the entire area by Thursday afternoon. With
total precipitable water values in the 0.8-1.0" range, precipitation
will probably be modest despite decent forcing. The NBM mean is
suggesting the lowlands will receive 0.60-0.75" of rain by 5am
Friday, with QPF in the mountains ranging from 0.75-1.5" south of
Highway 20 and 1-2" to the north. QPF amounts are also near 2" along
the west slopes of the north Coast Range. So, after about two weeks
of relatively dry weather, this system will be a beneficial rain
maker for the valleys. After this warm front moves through, the
associated upper trough and low pressure will settle in near the Pac
NW, pushing a weak cold front through and allowing for some cold
advection later Thursday into Friday. NBM probabilistic guidance
shows a 50/50 chance of snow levels lowering to the Cascade passes by
5 AM Friday, so any snow for the passes themselves will probably be
wet with a very low snow-to-liquid ratio. The higher ski resort
elevations above 5000 ft should see a few inches of snow, but
advisory-level accumulations of 6 inches or more will likely be
relegated to elevations above 6000 ft. For the lowlands, the upper
trough will maintain slow moving rain showers, with about a 10-15%
chance of enough instability to support a thunderstorm or two (best
chance in south Willamette Valley/Lane County Cascade foothills
according to the latest NBM thunder probabilities). Temperatures will
be considerably cooler Thursday and Friday with widespread highs in
the 50s across the lowlands. The probability for highs above 60F is
at 1-5% on Thursday and 10-30% on Friday (except around 1% at the
coast). -TK/Weagle


.LONG TERM...Saturday through Tuesday night...The long term forecast
is highlighted by a prolonged cool and wet weather pattern as a
series of Pacific fronts send several rounds of showers over the area
this weekend and well into next week.

The majority of 00z ECMWF/GFS ensemble members bring in another upper
low/trough with a reinforcing shot of cool air, maintaining the
possibility of late season snow for the Cascade passes and above into
early next week. For the lowlands, expect cool and showery weather
more reminiscent of early April rather than the end of April. The
deterministic NBM suggests high temps will be in the upper 50s to
lower 60s across inland valleys each day and mid 50s along and near
the coast. Depending on the amount of clearing each night, the air
mass may become cold enough to support patchy frost formation.
However, chances appear quite slim for frost most nights. The
probability for low temps of 35F or colder (which would support frost
formation) is generally <5%, except Tuesday morning when
probabilities increase to 10-25% from Longview to Castle Rock, Salem
to Eugene, and in Hillsboro and Battle Ground. Probabilities are
highest over the Upper Hood River Valley at 25-40%. Overall
confidence is low regarding the frost potential as the outcome will
be highly dependent on exact cloud cover conditions. -TK/Weagle

&&

.AVIATION...Satellite imagery and terminal observations as of 09z
Wed show MVFR marine stratus along the coast and high clouds
moving in from an approaching weak front. Weak lift in the marine
stratus may bring areas of drizzle along the coast through 13-14z
Wed. Expect mostly MVFR cigs along the coast today as the marine
layer holds on. Light rain increases along the north Oregon coast
throughout the day, but is not expected to impact visibility. VFR
thresholds prevail for Willamette Valley terminals, with cigs
gradually falling to low-end VFR throughout the day as the weak
front moves inland. Winds across all terminals will generally be
west/southwest and under 10 kt.

After 00z Thu, guidance suggests a 40-60% chance of IFR cigs or
lower along the coast as the next frontal system approaches.
Meanwhile, probabilities for MVFR cigs or lower for Willamette
Valley terminals increase to 20-40% after 06z Thu.

Notes: The ASOS at KTTD is only available via the dial in option.
The AWOS at KMMV is still out of commission. As it is not
maintained locally by the National Weather Service, there is no
estimated time of repair.

PDX AND APPROACHES...VFR thresholds throughout the TAF period with
cigs gradually falling to low-end VFR thresholds throughout the
day. 20-30% chance of MVFR cigs or lower between 06-12z Thu as the
next frontal system approaches. Winds today will generally be
westerly and under 10 kt. -Alviz

&&

.MARINE...Relatively quiet conditions prevail across the waters
today (Wednesday) as the region remains in a lull before the next
frontal system arrives. Models show a weak front pushing through
the waters this morning, which will result in winds backing to a
more westerly/southwesterly direction. Because this passing front
is so weak, winds are generally not expected to exceed 10-15 kt
today. Seas will also remain around 4-5 ft.

Wednesday night to Thursday, the next frontal system arrives over
the waters, resulting in widespread southwesterly gusts to 25-30
kt. Therefore, a Small Craft Advisory has been issued for all
waters including the Columbia River Bar beginning early Thursday
morning and lasting through at least early Friday morning. High
resolution guidance also suggests a 30-50% chance of occasional
gale force gusts of 34 kt or greater during the peak frontal
passage between 12-4 PM PDT Thursday. Due to the sporadic nature
and low confidence for gale force gusts, a Gale Warning would not
be warranted. Expect seas to build slightly on Thursday to around
6-8 ft.

Looking ahead into the weekend and early next week, marine conditions
will be characterized by a series of relatively weak frontal systems.
NBM guidance suggests a less than 5% chance of wind gusts exceeding
34 kt across the waters in the long term forecast. -Alviz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory from 5 AM Thursday to 5 AM PDT Friday for
     PZZ210-251>253-271>273.
&&

$$

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