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922
FXUS66 KPQR 172130
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
230 PM PDT Tue Sep 17 2024

.SYNOPSIS...
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A weak frontal system will continue to bring clouds,
cool temps, and light rain showers through this evening. Dry
weather returns Wednesday through Friday with onshore flow and
slightly below normal temps. Rain potential returns this
weekend, but confidence is low in the overall pattern.
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&&

.SHORT TERM...
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Now through Thursday Night...Radar imagery as of
230 PM PDT depicts shower activity beginning along the coast as
an upper level trough pushes a weak frontal boundary into the
area. Expect light rain showers to gradually move inland through
the afternoon, reaching portions of the Willamette Valley by 5
PM. Precipitation amounts are forecast to be very light as
models suggest the track of the upper trough will be slightly
offshore and moving southward, minimizing rainfall amounts
across northwest Oregon and southwest Washington. The highest
forecast amounts are are around 0.10-0.40 inch, generally south
of a line from Tillamook to Oakridge. Northward, expect less
than 0.10 inch of rain. Winds will also be from the south/southwest,
with the strongest gusts (25-30 mph) through the central Columbia
River Gorge and over the Cascade crest.

Tonight into tomorrow (Wednesday), the upper level trough will
exit our area and continue moving into California. High pressure
will re-build as a positively-tilted ridge noses into the region
from the Pacific. Winds will gradually weaken and shift back to
a north/northwest direction. Expect decreasing PoPs and a return
to dry conditions. Will also see decreasing cloud cover overnight,
so some locations could see patchy fog development given the
moist conditions and calm winds. HREF currently suggests spotty
chances (10-30%) of visibility falling below 1/2 mile for
locations east of the Cascades. Otherwise, expect a mostly
sunny day tomorrow with high temperatures rebounding to the low
to mid 70s for interior lowland valleys and mid 60s along the
coast. Thursday remains benign as high pressure lingers, with
continued onshore flow and similar high temperatures as
Wednesday.      -Alviz
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&&

.LONG TERM...
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Friday through Tuesday...Friday looks to be another
warm and pleasant September day characterized by increasing
onshore flow as a positively tilted ridge axis slowly shifts
south of the region. Expect temperatures slightly below seasonal
norms with inland highs in the low 70s. Forecast confidence
remains below normal this weekend into the early part of next
week as model solutions continue to diverge significantly with
regard to how the upper level pattern will evolve beyond Friday.
Ensemble clusters remain just about evenly split in depicting
ridging, troughing, and zonal flow regimes over the Pacific
Northwest from Saturday through Monday. This obviously will have
large implications for the forecast as anything ranging from
warm and dry weather to a cooler and wetter pattern remains on
the table for days 5-7. This continues to be reflected in the
NBM interquartile ranges for temperatures which range anywhere
from the upper 60s to the upper 70s between the 25th and 75th
percentile guidance. As such, and until models begin to converge
on a solution, have opted to stick with the NBM mean which
splits the difference between these solutions and depicts broad-
brushed chance to slight chance PoPs over the area late this
weekend through early next week.   /CB
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&&

.AVIATION...
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Currently, widespread MVFR CIGs around 2000 to 3000ft
inland and 1000 to 2000ft along the coast as a frontal system
fills the airspace with stratus. Expect coastal terminals
to bounce between MVFR and IFR, with a higher recurrence at KONP.

Expect most MVFR CIGs inland through the TAF period, and some
showers beginning around 00Z Wednesday. Probability of MVFR CIGs
will remain around 40-60% through most of the TAF period. There
is a decrease to a 20-30% chance of MVFR between 00Z Wednesday
and 06Z Wednesday suggesting the possibility of brief VFR
conditions.

As for the coast, expect a mix of MVFR/IFR conditions until 06-08Z
Wednesday, then becoming prominently IFR with a 20-30% chance of
LIFR until 18-19Z Wednesday. One exception is KONP, with a steady
20-30% chance of LIFR conditions through the TAF period.

PDX APPROACHES...Current stratus will keep mostly MVFR conditions
through all of the TAF period. There is a 20-30% chance of MVFR
CIGs between 01-06Z Wednesday, offering a possibility of brief
VFR conditions. Chances for light rain between Tuesday afternoon
and night. Light and variable winds through the period. General
southerly flow will shift to northwesterly by 11-12Z Wednesday.
                                                    -Hall
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&&

.MARINE...
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A weather system drops down from British Columbia
tonight into Wednesday. However by the time it reaches the
waters, it will elongate and weaken. Winds have shifted southerly
as the front nears. Peak wind gusts up to 15 kt along the coastal
waters will relax tonight as the system falls apart. There is a
slight chance for thunderstorms tonight primarily over the outer
zones.

Northerly winds return Wednesday as high pressure rebuilds offshore.
Winds may gust 20 to 25 kt at times, primarily over the outer
zones. Seas around 4 to 6 ft today and tonight, increasing to
around 8 ft later Wednesday into Thursday, then settling to around
5 ft late in the week.      -Hall/mh
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&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...None.

&&

$$

www.weather.gov/portland

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