642
FXUS66 KPQR 212158
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
258 PM PDT Fri Jun 21 2024

..SYNOPSIS...Well begin our cooling trend on Saturday, albeit
slowly at first, as high pressure maintains above normal
temperatures for inland areas. More seasonable temperatures
return Sunday into Monday as onshore flow strengthens and
chances for morning cloud cover increase. Conditions will remain
mostly dry through Tuesday aside from areas of light rain and
drizzle Saturday night into Sunday morning for the coast, west
slopes of the Coast Range, and Cascade foothills, and potentially
the eastern Portland metro. Our next precipitation chances
return to northwest OR and southwest WA Wednesday into Thursday
as an upper level trough moves inland, which will also bring
relatively cooler temperatures yet again.

&&

.SHORT TERM...TONIGHT THROUGH TUESDAY...Through the weekend
into next week the large scale upper-level pattern stays fairly
progressive overall beginning with the gradual departure of a
high pressure ridge tonight into Saturday. As a result
conditions will start to change by Saturday afternoon as
westerly flow near the surface becomes more prominent and high
temperatures decrease several degrees compared to
today(Friday), although the mid to upper 80s projected inland is
still roughly ~10 degrees above normal for mid-June.

Come Saturday night into Sunday morning low-level onshore flow
strengthens further and a weak upper level shortwave passing
into western Washington deepens the marine layer to 1-1.5 km
according to deterministic NAM/GFS soundings. This will help
spread cloud cover inland by Sunday morning, which should limit
high temps to the 70s despite afternoon sunshine - a welcome
change. Still cannot completely rule out highs reaching into the
lower 80s Sunday afternoon if the westerly push is weaker than
expected and morning cloud cover rapidly scatters. There`s a 5-20%
chance to meet or exceed 80 degrees over portions of the the
Willamette Valley and Portland metro according to the NBM.

Although conditions should remain mostly dry over the interior
lowlands, it still appears areas of drizzle and/or light rain will
impact the coast, western slopes of the Coast Range, Willapa
Hills, and south WA/north OR Cascade foothills Saturday night
into Sunday morning due to the deepening marine layer, moist
westerly upslope flow at low levels, and lift associated with
the upper level shortwave. We`ll also need to watch for light
precipitation extending towards I-5 in western Clackamas County
early Sunday morning as well due to models like the GFS, NAM,
and HREF indicating an area of converging surface-925mb winds
which would locally enhance upward lift and thus provide better
forcing for drizzle/light rain. The latest NBM PoPs still
appear a bit too low across portions of western OR/WA given the
aforementioned set-up, especially when guidance like the EPS
and HREF are significantly more bullish for light QPF by
comparison. Decided to increase PoPs to better match the model
consensus during this Saturday night through Sunday morning time
period.

Dry and seasonable on Monday before temps trend a warmer again
on Tuesday. This is in response to another transitory ridge of
high pressure amplifying and moving overhead. Tuesday afternoon
temperatures likely push into the mid 80s across the inland
valleys although there is an outside chance(15-35%) Salem up to
the Portland/Vancouver metro could touch 90 degrees again. At
least these probabilities are a little lower than previous runs
on the NBM and the overall spread of temperatures for Tuesday
afternoon has tightened indicating increased confidence in the
current forecast. However, it still appears likely we`ll see a
cooler and wetter pattern return later next week bringing a
swift end to any early to mid week warm-up. -Schuldt


.LONG TERM...WEDNESDAY THROUGH FRIDAY...00z Fri WPC Cluster analysis
reveals high confidence in a shortwave trough moving in near the end
of the workweek, but uncertainty regarding the exact timing still
exists. 80%+ confidence in the shortwave trough arriving by Thursday
at the latest. Rainfall rates look fairly mild; around a 30% chance
of rainfall exceeding 0.25" each day from Wednesday onwards (24-hr
periods), decreasing to 10% in the Willamette Valley. Temperatures
show a 75% chance of being below 75 degrees on Thursday and Friday,
though Wednesday sees slightly more uncertainty due to uncertainty in
the timing of the trough arriving. /JLiu


&&

.AVIATION...VFR prevailing except along the coast where IFR/LIFR
marine stratus lingers just offshore. Northwesterly winds persist
along the coast with gusts up to 20 kt. Generally north winds
across the airspace for inland locations. 35%-45% probability of
IFR/LIFR marine stratus returning to the coast starting around
03Z Saturday, which will likely develop into a mixture of
MVFR/IFR conditions around 12Z Saturday.

PDX AND APPROACHES...VFR with mostly clear skies. Northerly winds
around 8-10 kt. /42

&&

.MARINE...High pressure offshore remains anchored over the region
through early Saturday. A weak front is expected to approach the
coastal waters Saturday afternoon, which will weaken the
aforementioned high pressure. This will result in onshore flow
developing with northwesterly winds around 10-15 kt with gusts up
to 20 kt. General seas through Saturday around 4 to 6 ft, but the
weak front will also bring an increasing westerly swell. As a
result seas are only likely to build toward 5 to 8 ft ft on
Sunday. High pressure gradually return for the stat of next week.
/42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.
&&

$$

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