801
FXUS66 KPQR 111031
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
331 AM PDT Wed Sep 11 2024

.SYNOPSIS...A strong low pressure system crossing the region
today will bring wetting rain to most locations through this
evening, along with a chance for a few embedded thunderstorms. A
few showers linger into Thursday, with dry weather returning by
Friday. Another trough arrives Saturday and maintains showery
and cooler weather over the region through early next week.

&&

.SHORT TERM...Now through Friday Night...Water vapor satellite
imagery reveals a potent upper level trough located offshore of
the Pacific Northwest early this morning. An attendant surface
cold front is analyzed over the coastal waters as of 2 AM
Wednesday, with numerous rain showers spreading inland across
northwest Oregon and southwest Washington. A few embedded
thunderstorms are also noted over the coastal waters. Widespread
showers will continue through the day as the upper trough moves
inland across the region, with the heaviest rain likely to fall
through later this morning along the coast and into the
Willamette Valley, shifting to the Cascades this afternoon and
evening. Rainfall amounts so far range from a few hundredths to
as much as a quarter inch in a few spots. Expected rainfall
totals remain more or less on track from previous forecast
packages, with guidance depicting 0.50-0.75 inches in the
interior valleys and 0.75-1.00 with locally as much as 1.25" for
the Coast, Coast Range, and Cascades through late this evening.
A few embedded thunderstorms will also remain possible into
this evening with a 15-25% chance across the area. May see a bit
of an uptick in thunderstorm potential in the afternoon if any
clearing materializes behind the front to generate some surface
based instability. Overall, thunderstorm concerns remain
minimal as locally heavy rain will accompany any storms that do
develop.

Showers and thunderstorms diminish tonight as the trough shifts
east of the Cascades, though northwest flow in the wake of the
trough will keep the threat of a few showers over the area into
Thursday afternoon. Expect these showers to mostly favor the
higher terrain, with model QPF amounts still limited to around
a tenth of an inch across most of the area and perhaps slightly
higher to around a quarter inch for orographically favored
spots in the Cascades. Expect a general clearing trend from
west to east through the afternoon with highs inching back
upwards into the low 70s. Temperatures will continue to
incrementally increase into the low to mid 70s on Friday as
shortwave ridging moves overhead and brings an end to any
lingering shower activity. /CB

.LONG TERM...Saturday through Tuesday...Medium range guidance
has slightly sped up the arrival of the next trough into the
region on Saturday, which will bring about another period of
showery conditions and cooler temperatures this weekend into the
early part of next week. Model suites then show some notable
differences on how the trough evolves from Sunday into Monday
which will have implications on the extent of shower coverage
and potential additional rainfall amounts during the period.
European guidance is favoring keeping the trough offshore and
digging towards northern California, whereas the GFS tends to
move the trough directly overhead and Canadian guidance splits
the difference between the two. The farther south solution would
lead to generally less shower coverage and thus lower
precipitation amounts, with the farther north solution
representing the higher end solution for both shower coverage
and precipitation amounts. For now, favored a broad brushed,
ensemble based approach keeping a chance of showers over the
region through much of the period until guidance comes into
better agreement. Regardless, the presence of upper level
troughing will favor cooler temperatures into the long term,
with guidance depicting highs in the 60s in the inland valleys
for the start of next week. /CB

&&

.AVIATION...A front is currently in the process of passing through
the region, producing generally a mix of VFR/MVFR cigs. Rain
showers as well as a few isolated thunderstorms near the coast
will continue while the frontal passage continues. Possibility of
thunderstorms at coastal terminals is around 30%, though any
thunderstorms will end by 12z Wed. Southerly winds will generally
remain under 10 kt, though any passing thunderstorms or heavier
showers could produce isolated gusts up to 20 kt. While frontal
passage is ongoing, confidence in inland TAFs deteriorating to IFR
or lower is 0-20%.

The front finishes moving through the area around 20z Wed, after
which VFR conditions will return to inland terminals with 75%
confidence. This will be accompanied by a shift in winds to more
westerly, remaining under 10 kt. Coastal terminals will still see
around a 40% chance of remaining MVFR, increasing to 80% by 06z
Thu. Of note, with post-frontal instability, currently seeing
around a 10-20% chance of very isolated thunderstorms at all
terminals Wednesday during daytime hours, from around 20z Wed to
03z Thu.

PDX APPROACHES...A mix of VFR and MVFR conditions are currently in
place as a front continues to move through the area. Rain showers
and southerly winds up to 10 kt are possible while this front is
passing through. Chance of MVFR cigs essentially ends once the
front finishes moving through the area around 20z Wed, and winds
will shift westerly around this time. Around a 20% chance of an
isolated thunderstorm Wednesday afternoon, between 20z Wed to 03z
Thu. /JLiu

&&

.MARINE...A front is currently in the process of moving through
the area. Any elevated southerly winds over 21 kt caused by the
leading edge of the front should end by 12z Wed, after which winds
will remain below SCA for the foreseeable future. Before 12z Wed,
a stray thunderstorm or two could be possible in the southern and
central waters, but by 12z this ends. Winds will thereafter slowly
shift westerly, with max gusts remaining below 15 kt.

High pressure returns to the northeast Pacific late in the week
and brings a brief period of northerly winds Friday. Currently
around 20% chance that winds of greater than 21 kt work their way
northward into the waters off the central coast of Oregon Friday
afternoon and evening while winds will be even weaker farther
north towards the mouth of the Columbia River. Uncertainty in the
forecast details and timing of storm systems grows thereafter, but
another low pressure system and attendant front or two will drop
southeastward across the waters during this time. Given the
relatively weak winds across the waters over the next week, expect
seas to generally hover between 4-7 ft through early next week.
/Neuman/JLiu

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.

&&

$$

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