610
FXUS66 KPQR 311036
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
336 AM PDT Fri May 31 2024

.SYNOPSIS...Warm and dry weather will prevail today as high pressure
persists over SW Washington and NW Oregon. The high pressure weakens
enough Saturday to allow a weak frontal system to clip the region
with clouds and a few showers, mainly along the coast and over the
higher terrain. A stronger system will spread rain throughout the
forecast area Sunday; this rain may be locally heavy as a moderate to
strong atmospheric river of moisture will be involved. Unsettled
weather will linger through the first half of next week, then a turn
toward warmer and drier weather appears likely by the end of the
week.

&&

.SHORT TERM...Today through Sunday...High pressure will lead to a
pleasantly warm and dry day to close out the month of May, with
temperatures a few degrees above normal inland. With 850 mb temps
climbing to around +10 deg C over much of the Willamette Valley and
little cloud cover to speak of, inland highs will push well into the
70s with a couple locations potentially touching 80 degrees this
afternoon. In fact, latest NBM probabilistic guidance shows 60-80%
probs of reaching 80 degrees this afternoon across much of the PDX
metro area. These chances decrease to the south of Salem, with the
central/south Willamette Valley in the 20-50% chance range.
Regardless of whether or not the inland valleys reach 80 degrees,
this afternoon looks warm with plenty of sunshine inland. As is often
the case, coastal temps will be a few degrees cooler due to afternoon
sea breezes.

The high pressure responsible for the warm and sunny weather today
will weaken tonight into Saturday, allowing a weak shortwave system
to move onshore near the WA/BC border. This system will deepen the
marine layer, potentially bringing some drizzle or light rain to
north coastal areas Saturday morning. Just enough moisture and
low-level instability could linger into Saturday afternoon for a few
showers, with the best chances being along our north coastal zones
and across the higher terrain.

Whatever rain falls on Saturday will likely pale in comparison to the
rain expected with the next, stronger system Sunday. Models and their
ensembles continue to suggest a deep tap to subtropical moisture,
somewhat associated with a tropical system which emerged from the
western Pacific over the past 24-48 hours. GEFS members suggest a
high likelihood (>90%) of integrated vapor transport (IVT) exceeding
500 kg/m/s, with about a 50/50 chance of IVT exceeding 750 kg/m/s
along the Oregon coast Sunday evening. Meanwhile the forecast area
will be in or near the left-exit region of an unseasonably strong jet
streak; this will enhance lift as the deepest moisture moves across
the region. As such, Sunday afternoon and evening look quite wet for
SW Washington/NW Oregon, with NBM probabilistic QPF suggesting the
vast majority of the CWA has a better than 50% chance of receiving 1
inch or more of QPF, with these chances at around 70% for the Salem
and Portland metro areas. The same guidance suggests 1 inch or more
of rain is a near certainty in the Coast Range and Cascades north of
Highway 20, with NBM means suggesting 1.5-3.0 inches of QPF for the
same area Sunday night into Monday. High-end (95th percentile) QPF
suggest a couple locations could reach 4 inches of QPF if everything
were to line up properly. With this in mind, it is not out of the
question that there could be rapid rises or even minor flooding on a
couple of our flashier creeks and drainages (the Grays River comes to
mind), but based on latest guidance it appears the chances of this
occurring at any given location are 10% or less.  Weagle

.LONG TERM...Monday through Thursday...The unseasonably strong jet
stream over the NE Pacific remains aimed toward the Pac NW through
Tuesday, keeping the weather unsettled across SW Washington and NW
Oregon. Latest NBM thunder probs are in the 15-20% range for much of
the CWA Monday afternoon, which seems reasonable given the pocket of
cool air aloft swinging through the region and the strong early June
sun. Depending on how much sunshine can break through the clouds
Monday, a couple of the thunderstorms could become strong enough to
produce small hail and/or gusty winds. Either way, Monday will likely
be somewhat blustery given the 30-40 kt SW winds at 850 mb shown by
the 06z NAM. Guidance suggests another system potentially moving into
the Pac NW Tuesday, though there is a lot of variability on the
strength of this system.

Starting midweek, models are suggesting a high likelihood of a strong
upper level ridge of high pressure developing somewhere over the
Western United States. Looking at the 00z WPC cluster analyses, there
is still a lot of variability in where models and their ensembles are
placing the upper ridge axis...which will be key in determining just
how warm it will be toward the end of next week. Based on the 00z WPC
clusters, it appears there is only about a 20-30% chance of a pattern
that would avoid significant positive 500 mb height anomalies (i.e.
significantly above-normal temperatures), with the remainder of
guidance suggesting temps several degrees above normal (at least
inland) by the end of the week. By next Friday, NBM probabilistic
guidance suggests a 30-50% chance of highs reaching the 90s in the
Willamette Valley, with a 5-15% chance of highs exceeding 100
degrees. With this in mind, those sensitive to heat should pay close
attention to the forecast the next few days, and perhaps consider
where they may go to stay cool late next week should the hotter
forecast guidance prevail.  Weagle

&&

.AVIATION...VFR conditions under mostly clear skies prevail
through most of the TAF period as high pressure continues over the
region. The high pressure begins to breakdown later today as a
front approaches off the northeast Pacific. HREF guidance
indicating 40-60% chance for MVFR or lower conditions at the coast
towards 12Z Sat. Winds generally north to northwest less than 8
kt in the morning, then picking up in the afternoon 8 to 12 kt
with gusts to 25 kt along the coast and 15 to 20 kt for inland
locations.


PDX AND APPROACHES...VFR conditions through the entire TAF period
with mostly clear skies. Winds diurnally driven with northwest
wind to 5 kt, then 8 to 12 kt from 00Z Sat.  /mh


&&

.MARINE...Surface high pressure continues offshore today with a
thermal trough along the south Oregon coast. This will drive
northerly winds across the waters. Expect Small Craft Advisory
winds gusting to 25 kt south of Cape Falcon (marine zones PZZ
272,273,252,253). The high pressure breaks down later this
afternoon and evening as a weakening Pacific front approaches.

The weak front is expected to move through the waters on
Saturday, shifting winds westerly to southwesterly. A more
organized and stronger storm system will move into the northeast
Pacific Sunday shifting winds southerly. Small Craft Wind gusts
are likely with 90+% chance of gusts exceeding 21 kts. There`s
also a 20-30% chance of a brief period of Gale Force winds of 35
kts Sunday afternoon or evening. /mh

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory until 11 PM PDT this evening for
     PZZ252-253-272-273.

&&

$$

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