627
FXUS66 KPQR 171037
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
337 AM PDT Tue Sep 17 2024

.SYNOPSIS...An incoming frontal system will bring increasing clouds,
cooler temps, and increasing chances for rain on Tuesday. Mainly dry
Wednesday through Friday with onshore flow and slightly below
average temps. Rain potential returns this weekend, but confidence is
low in the overall pattern.

&&

.SHORT TERM...Now through Wednesday Night...The high pressure
ridge that brought sunny and warm weather to the region will
shift rather abruptly on Tuesday as the next upper level trough
digs south over the NE Pacific and sends a frontal boundary
through the area, bringing the next round of rain. However, forecast
rain amounts continue to decrease and the timing has shifted a bit
with rain now expected to begin later in the day compared to previous
forecast updates. Now expecting rain to arrive along the coast during
the late morning to early afternoon hours before spreading into the
Willamette Valley in the mid to late afternoon. In fact, the HREF
suggests rain will most likely hold off until 3-5 pm for Clark and
Cowlitz Counties as well as the Portland/Vancouver metro and
Willamette Valley. The latest possible start time in these locations
is 7-10pm, suggesting there is a possibility for nearly the entire
day to stay dry (albeit <10% of HREF members suggest a start time
this late and 3-5pm remains the most likely start time).

As mentioned earlier, there has also been a notable downward trend in
rain amounts with the latest model guidance, especially with hi-res
model guidance and the HREF. This downward trend seems to make sense
from a conceptual standpoint with the trough remaining offshore the
Oregon Coast on Tuesday and evolving into another closed low over
northern California by Wednesday morning. As such, rain amounts east
of the Coast Range remain around 0.10-0.20 inches from Kelso to
Portland to Salem. The probability for rain amounts over 0.25 inches
have fallen from around 20 percent down to 10 percent in these areas
according to the 00z iteration of the HREF, with the best chances
(40-60%) found in the south valley around Eugene and also in
orographically favored parts of the Cascades. Probabilities remain
higher along the coast to the south of Astoria and along the west
slopes of the Coast Range (generally above 70%). That said, forecast
rain amounts have decreased even in these areas, down from 0.50-0.75
inches to 0.25-0.50 inches.

A few showers may linger across the area Tuesday night into Wednesday
morning as the low resides over northern California, but expect this
activity to diminish fairly quickly as a positively tilted ridge
noses into the region from the Pacific. This will yield
seasonable conditions with temperatures in the low to mid 70s
from Wednesday into the latter half of the week. Areas of fog are
likely to develop early Wednesday morning for locations that
experience some clearing given the wet ground conditions and calm
winds. That said, it appears cloud cover will be too expansive for
widespread fog to develop. Locations in the Willamette Valley to the
west of I-5 and the Coast Range lowlands have the best chance for
some clearing and subsequent fog development (25-50% chance for
surface visibilities below 1/2 mile due to fog according to the
HREF).-TK/CB


.LONG TERM...Thursday through Monday...Thursday and Friday will
feature benign, pleasant September weather as the ridge axis
shifts slowly southward across the region, opening the door to
increased onshore flow as we head into the end of the week.
Expect inland high temperatures generally in the low to mid 70s
each day. Forecast confidence remains low headed into the
weekend as ensemble clusters remain fairly evenly split between
ridging or troughing developing over the region Sunday into
Monday. Meanwhile, most deterministic solutions depict
a somewhat zonal flow pattern Saturday into Monday. This
uncertainty is reflected by the continuing high spread between
the 25th and 75th percentile NBM temperature guidance, which
places highs for next Sunday and Monday anywhere between the mid
60s and lower 80s across the interior lowlands. As such,
the deterministic forecast continues to stick closely to the NBM mean
which depicts seasonable temperatures and broad brushed chance to
slight chance PoPs across the area this weekend into early next week.
 -CB/TK

&&

.AVIATION...Widespread VFR, except at the coast where marine
stratus with low-end MVFR CIGs around 1000-1500 ft. Guidance
suggests there is a 40-60% chance that conditions deteriorate to
IFR this morning, then 60-80% chance this afternoon into the
evening hours. Stratus likely pushing into the Willamette Valley
12-15Z Tuesday, then filling in for the remainder of inland areas
15-19Z Tuesday. Cigs most likely in the 2500-3500 ft range, but
there is a 20% chance for cigs 1500-2000 ft. Light rain is likely
to begin at the coast around 16z Tuesday morning, and gradually
spreading inland by 20-22z Tuesday afternoon.

PDX APPROACHES...VFR with variable high clouds expected through at
least 12z Tuesday. Then, stratus begins building across the area
with chances for MVFR CIGs increasing to 40-60% after 14z Tue
morning. Chances for light rain increases through Tuesday
afternoon. Northwest winds become light and variable after 08Z
Tuesday. /mh /DH

&&

.MARINE...Later today into Wednesday a weather system drops down
from British Columbia. However by the time it reaches the waters,
it will elongate and weaken. Will see winds back to the south
today as a front approaches. Peak wind gusts, up to 20 kt, over
the coastal waters looks to be this afternoon as the front nudges
against the coastline, increasing southerly winds. Winds should
then 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. /mh

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

www.weather.gov/portland

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