868
FXUS66 KPQR 161704 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1004 AM PDT Mon Sep 16 2024

Aviation Discussion Updated...

.SYNOPSIS...High pressure will bring plenty of sunshine and the
warmest temps of the week on Monday. Rapid change to cool and wet
weather on Tuesday as a frontal system brings widespread rain to the
region. Mainly dry Wednesday through Friday with onshore flow and
slightly below average temps.

&&

.SHORT TERM...Now through Tuesday night...The short term forecast is
highlighted by a quick transition from warm and dry weather to cool
and wet weather. High pressure will remain over the area through
Monday, bringing mostly sunny conditions and warm afternoon temps in
the mid to upper 70s, except upper 60s at the coast. While most of
the day should be sunny, cannot rule out some shallow patchy fog in
the lowlands during the early morning hours. NBM probabilities for
high temps of 80 degrees or warmer are around 15-25% across the
Willamette Valley, Portland/Vancouver metro and Columbia River Gorge,
except 40% in the Salem area. Monday will be a great day for outdoor
activities.

Conditions will quickly change to cool and wet on Tuesday as a
stronger frontal system spreads widespread stratiform rain over
western WA/OR. The HREF mean suggests rain will most likely begin at
the coast during between 7-10am before spreading inland between
1-3pm, which aligns well with NBM PoPs and simulated reflectivity
guidance from the latest suite of hi-res model guidance. The earliest
possible start time according to the HREF is 3-6am for the coast and
9-11am inland. The latest possible start time is noon-2pm at the
coast and around 5pm inland. Regardless of the exact start time,
expect stratiform rain to transition to post-frontal showers Tuesday
evening or Tuesday night.

How much rain will fall Tuesday through Tuesday night? Likely over
0.25 inch for all lowland locations west of the Cascade crest (80-90%
chance). Higher rain amounts are likely for the coast, mountains and
southern Willamette Valley; these areas have a 75-90% chance for rain
amounts over 0.5 inches and a 40-60% for more than one inch.  -TK


.LONG TERM...Wednesday through Sunday...Ensemble guidance
suggests that the upper level trough associated with Tuesday`s
rain will exit southward toward California on Wednesday. Expect
some post-frontal showers early Wednesday morning, however showers
will quickly diminish by the afternoon as weak high pressure re-
builds. Model soundings also suggest decreasing clouds Wednesday
afternoon, allowing temps to rebound into the upper 60s and lower
70s. Expect similar temps Thursday through Saturday. Most ensemble
guidance depicts weak shortwave ridging Thursday into Friday,
resulting in dry conditions.

The forecast becomes more uncertain over the weekend. This is
highlighted well by the latest ensemble clustering analysis as
roughly half of the total ensemble space from the GEFS/ENS/GEPS
suggests an upper level trough will reside over the Pacific Northwest
while the other half depicts ridging. If troughing occurs, cooler and
wetter weather would return. If ridging occurs, warmer and drier
weather would prevail. Given this uncertainty have maintained NBM
PoPs which show a 20-40% chance of rain going into Saturday and
15-20% chance on Sunday. The NBM 1D Viewer highlights uncertainty in
the temperature forecast over the weekend as the 10th-90th percentile
for high temps range from the lower 60s to mid 70s on Saturday and
upper 50s to lower 80s on Sunday. With no clear regime favored over
the other, have also left the deterministic NBM for the official
temperature forecast, which suggests high temps in the low to mid 70s
inland and upper 60s for the coast.  -TK/Alviz

&&

.AVIATION...Currently, mostly widespread VFR conditions throughout
the airspace, with clear skies and northwesterly flow aloft.
These conditions are expected to persist along the coast until
00-3Z Tuesday and inland until 12Z Tuesday, with stratus building
in thereafter. The coast will begin to see CIGs degrading to low-
end MVFR around 00-03Z Tuesday as stratus begins to push in, with
a 30-40% chance of lowering to IFR. By 06-09Z Tuesday, conditions
will degrade, resulting in chances of IFR around 40-60% and LIFR
around 20-40% (highest chances around KONP). Inland, stratus from
the coast will funnel in and lower CIGs to MVFR by 12Z Tuesday,
with chances of MVFR around 30-50%. Otherwise, expect
northwesterly winds to increase up to 10-15 kt this afternoon
(highest around KONP). Winds will decrease and shift southerly by
09-12Z Tuesday as a low pressure system approaches the airspace.


PDX APPROACHES...General VFR through most of the TAF period. There
is a 10-20% chance of MVFR CIGs beginning 12Z Tuesday. Winds may
become breezy in the late afternoon, but gusts not expected to
exceed 15 kt.
-Hall

&&

.MARINE...High pressure over the outer waters continue today with
northerly pressure gradients bringing gusty north wind gusting 20
to 25 kt. With the persistent northerly winds seas will be
somewhat choppy, especially over the outer water where winds are
stronger. Overall, seas stay in 6 to 9 ft through Monday, with
highest seas over the outer waters.

Later Tuesday into Wednesday we will see yet another pattern
shift as a strengthening low pressure system drops down from
British Columbia. By time it reaches the waters, it will
elongate/weaken. Models still vary some on the timing, strength,
and position of this system which will have an impact on the
forecast. So will have to see how things shape up as get closer
in time to Tuesday.
/mh

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory until 11 AM PDT this morning for PZZ252-253.

     Small Craft Advisory until 11 PM PDT this evening for PZZ271>273.
&&

$$

www.weather.gov/portland

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