985
FXUS66 KPQR 200501 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1001 PM PDT Sun May 19 2024

Updated aviation discussion...

.SYNOPSIS...Isolated to scattered showers into this evening, mainly
over the higher terrain. Clearing after sunset, with a cool night
expect as temperatures drop into mid to upper 30s for many areas.
High pressure will bring a sunny warm day for Monday. But, another
front arrives on Tuesday, with clouds and rain. Showery with a
thunderstorms possible on Wednesday. Will maintain unsettled with
near average temperatures for rest of the week.
&&

.SHORT TERM...(Sunday afternoon through Wednesday)...Still have
somewhat onshore flow across the region. Air mass still a bit
unstable with the cooler air aloft. As such, will maintain isolated
to scattered showers across the region, though bulk of those will be
over the higher terrain.

Once the sunsets, air mass will gradually stabilize, allowing showers
decrease and come to an end later this evening. Mid/low level flow
will turn more northerly overnight, allowing for clearing skies.
Models continue to show bit of thermal trough building along the
south Oregon coast. As it does, will see low level flow turn bit more
northeasterly, or offshore. As such, will see tad enhancement of
downslope flow, with cool drainage flow into the lowlands. Many
valleys likely to drop back into upper 30s to lower 40s. Current
forecasts look on track, but may have to watch. If gets a few deg
cooler, could have to worry about patchy frost in mountain valleys
and even in the Willamette and Cowlitz Valleys, But, given the
clearing expected and dry cooler air mass, highest potential of frost
looks to be south of a Corvallis to Albany line. So, will put up a
Frost Advisory for the south Willamette Valley and adjoining
lowlands. In addition, will include Hood River Valley under the Frost
Advisory as well, where likely see areas of frost, mainly to west and
south of Middle Mountain.

After a cool start to the day, will see much more sunshine for
Monday. Sunny skies and temperatures climbing back to that expected
for mid-May, 60s on the coast and 70 throughout the I-5 corridor.

Enjoy the warmer and dryer day on Monday because cooler and
wetter conditions return Tuesday. Ensemble guidance indicates
good confidence in a closed upper low pushing south along the
British Columbia coast Tuesday with a shortwave ahead of this
parent low moving through the PacNW. The shortwave will have
more of a west to east progression with an associated surface
frontal system, which will bring a period of stratiform
precipitation to the region from the northwest to southeast. The
Willamette Valley will likely be rain shadowed due to the
direction of the flow with higher precipitation amounts in SW
Washington (including lowlands), the Coast Range, and Cascades.
NBM indicates only a 20-40% probability of 0.25 inch or higher
rain amounts in 24 hours in the central and southern Willamette
Valley ending 5am Wednesday. These probabilities increase to
50-60% in the Portland/Vancouver metro area, and 75-90% in the
lowlands north of Clark County. The Coast Range and Cascades
north of a line from Tillamook to Government Camp are forecast
to receive the highest QPF, anywhere from 0.75-1.25 inch. NBM
indicates 50-70% probability of QPF greater than 0.75 inches for
these areas through Tuesday night. Snow levels will remain above
5500-6000 feet through Tuesday, so snow at or below pass level
is not expected.          /HEC/Rockey

.LONG TERM...Wednesday through Friday...Ensemble guidance has
been in better agreement of the track of the closed low as it
moves south from British Columbia into the PacNW on Wednesday.
85% of the 00Z Sunday WPC 500mb clusters, along with ensemble
means, indicate the low will move through NW Washington to the
central/east central Washington/Oregon border, then southeast
through eastern Oregon. This pattern will continue onshore flow
with cooler and more moist conditions. The other 15% of the
clusters track the low much farther east through eastern
Washington into Idaho in a more "inside slider" pattern. This
would bring more of a breezier northerly wind pattern and dryer
conditions. NBM is hedging towards the first pattern, as well,
with showers continuing into Wednesday with thunderstorm chances
as colder air aloft moves over the region.

Ridging builds again over the eastern Pacific Thursday with dry
conditions returning. Beyond Thursday, ensemble guidance turns
more uncertain. WPC clusters indicate general troughing moving
back over the region sometime Friday into Saturday, but timing,
location, and strength of the trough are all uncertain. NBM
forecast indicates daytime temperatures remaining fairly steady
in the 60s across the lowlands and 50s across the coast through
the weekend with precipitation chances of 15-30% across the
lowlands Friday and Saturday and up to 40-50% across the higher
elevations. -HEC

&&

.AVIATION...Satellite imagery as of 0430z Mon show low clouds
forming over the Oregon Coast Range and Cascades due to upsloping
flow. These low clouds over the high terrain will likely backbuild
tonight, with guidance showing high confidence (70-90% chance) of
MVFR cigs over the north OR coast (KAST) and Portland Metro Area
(KPDX/KTTD). Expect cigs to fall to high-end MVFR by around 09z Mon
along the coast and 12z Mon in the Portland Metro Area, burning off
by 18-21z Mon. Elsewhere, expect to maintain VFR thresholds tonight.
VFR conditions continue tomorrow (Monday) with scattered cloud cover
as the region maintains north/northwest flow aloft. Winds will be
mostly calm tonight, then pick up to 05-10 kt tomorrow. Occasional
gusts to 20-22 kt are possible Monday afternoon along the coast and
Willamette Valley.

PDX AND APPROACHES...Guidance suggests a 70-80% chance of MVFR cigs
between 12-18z Mon due to clouds backbuilding against the Cascades.
Otherwise, expect predominately VFR thresholds after 18z Mon with
scattered cloud cover. Winds will be northwesterly and around 5-10
kt.     -Alviz

&&

.MARINE...Increasing winds over the coastal waters will bring back
wind-driven Small Craft Advisory conditions to all areas Sunday
afternoon, with seas around 8 feet at 8 seconds. NW winds up to 25
kt will turn N over Sunday night, beginning to weaken after
midnight. By the end of Sunday night, the inner waters will have
decreased down to 15 kt or so, while outer waters could see a few
gusts above 20 kt until mid-morning Monday.

The next frontal passage of any significance is set to arrive late
Monday night into Tuesday morning, bringing rain and a shift to
westerly winds. However, wind gusts look to generally stay below 20
kt as the pressure gradient associated with this frontal passage
appears rather weak. The northern outer waters will see slightly
stronger gusts, with the possibility for Small Craft Advisory-level
gusts up to 23-25 kt. Gusts there likely persist through Wednesday
morning. -TK/JLiu

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Frost Advisory from 1 AM to 8 AM PDT Monday for ORZ116>118-121-
    124-125.

WA...None.
PZ...Small Craft Advisory until 5 AM PDT Monday for PZZ251>253.

     Small Craft Advisory until 11 AM PDT Monday for PZZ271>273.
&&


$$

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