782
FXUS66 KPQR 170346 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
846 PM PDT Mon Sep 16 2024

.SYNOPSIS...Cool and wet weather on Tuesday as a frontal system
brings widespread light rain to the region. Mainly dry
Wednesday through Friday with onshore flow and slightly below
average temps. Rain potential 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. Still expect
rain to arrive along the coast around mid morning and then push
inland to the Willamette Valley by early afternoon, lingering
through Tuesday evening before tapering off from northwest to
southeast during the overnight hours. There has been a notable
downward trend in precipitation amounts with the 12z model runs
as more hi-res model guidance enters the picture, and this seem
to make sense from a conceptual standpoint with the trough
remaining offshore of the Oregon Coast on Tuesday and evolving
into another closed low over northern California by Wednesday
morning. As such, have lowered amounts east of the Coast Range
to around a tenth or two from Portland down to Salem. The
probability for a true wetting rain of quarter inch has fallen
to around 20 percent in these area, with the best chances
(50-60%) found in the south valley around Eugene and also in
orographically favored parts of the Cascades. Probabilities
remain higher along the Coast, where most areas are still
expected to see roughly a half to three quarters of an inch of
rain through Tuesday evening.

A few showers may linger across the area into Wednesday morning
as the low resides over northern California, but expect this
activity to diminish by the afternoon 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. -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 by Sunday into
early next week. Meanwhile, most deterministic solutions depict
a somewhat zonal flow pattern through next 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 upper 70s across the interior lowlands. As such,
continues to stick closely to NBM mean which depicts seasonable
temperatures and broad brushed chance to slight chance PoPs
across the area this weekend into early next week. -CB


&&

.AVIATION...Northerly flow aloft tonight as high pressure ridge
moves across the region. Widespread VFR this evening, except at
the coast where marine stratus is developing with low-end MVFR
CIGs around 1000-1500 ft. Guidance suggests there is a 40-60%
chance that conditions deteriorate to IFR through Tuesday morning,
especially at KONP. A southerly wind reversal begins around 12Z
Tuesday ahead of an approaching low pressure system, with stratus
likely pushing into the southern Willamette Valley. Stratus is
expected to become more widespread inland between 12-14Z Tuesday,
with chances for MVFR CIGs around 40-60%. 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. -DH

&&

.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 waters where
winds are stronger. Winds are expected to decrease into tonight,
therefore will end the Small Craft Advisory at 11PM tonight.
Seas will remain between 5 to 8 ft today and tomorrow.

Late tomorrow into Wednesday, we will see yet another pattern
shift as a strengthening weather system drops down from British
Columbia. By the time it reaches the waters, it will elongate and
weaken. Considering the expected speed of this weakening system
as it will move south along the West Coast, impacts look to be
minimal. This storm will elevate seas up to 8 ft and winds up to
20 kt.  -JH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 11 PM PDT this evening for
     PZZ271>273.
&&

$$

www.weather.gov/portland

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