614
FXUS66 KPQR 010441 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
941 PM PDT Fri May 31 2024

Updated aviation discussion...

.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...Tonight through Sunday night...Not much change in the
forecast thinking through this weekend. GOES satellite imagery, and
looking outside our window, shows mostly sunny skies along with some
passing cirrus. Our 12 at 12 rule of thumb would produce a high
temperature in the upper 70s late this afternoon. High pressure
aloft will shift east of the Cascades this evening, allowing a weak
shortwave system to move onshore. 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. Temperature on Saturday are expected to be near
normals for early June, lower 70s for inland valleys.

Whatever rain falls on Saturday will likely pale in comparison to
the rain expected with the next, stronger system Sunday and Monday.
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. Deep moisture under the
right circumstances can lead to unusually high rainfall rates. As
such, Sunday afternoon and evening look quite wet for SW Washington
and NW Oregon, with NBM probabilistic QPF suggesting the vast
majority of the CWA has a better than 70-80% chance of receiving 1
inch or more of QPF. 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.  -DH/Weagle

.LONG TERM...Monday through Friday...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 10-20% 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 12z 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 over
WA/OR (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...Satellite imagery as of 0430z Sat shows increasing high
clouds over the region as a weak front approaches. Expect VFR
conditions to continue across all terminals through late tonight.
After 12z Saturday, the weak front will move closer to land and
potentially drop cigs to MVFR along the coast. Guidance suggests a
50-70% chance of MVFR cigs developing along the coast after 12z Sat
and persisting through the end of the TAF period. Meanwhile, inland
terminals on Saturday will likely maintain VFR cigs. Could see some
light, non-impactful shower activity near KAST and the Portland
Metro with this weak front. Winds will be light (under 5 kt) and
variable tonight, then increase and turn more westerly/southwesterly
through the day Saturday around 5-10 kt.

PDX AND APPROACHES...VFR conditions through the entire TAF period.
Expect increasing cloud cover tonight, with cigs lowering to low-end
VFR throughout the day Saturday. Winds turn more westerly tomorrow,
remaining under 10 kt.      -Alviz

&&

.MARINE...Small Craft Advisory wind gusts up to 25 kt south of
Cape Falcon (marine zones PZZ 272,273,252,253) will continue through
the evening. A weakening Pacific front approaches the area tonight
and will bring lighter west/southwest winds 10-12 knots on Saturday.

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
is also a 10-20% chance of a brief period of Gale Force winds of 35
kts for the latter half of Sunday. Seas will build to 8-10 feet at
10-12 seconds Sunday afternoon, persisting into Monday. Another
westerly swell will enter the waters toward the middle of next week
with seas of 10-12 feet at 14-16 seconds.
-Batz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

$$

www.weather.gov/portland

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