705
FXUS66 KPQR 162108
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
208 PM PDT Mon Sep 16 2024

.SYNOPSIS...Pleasant and warm conditions across the area today
will rapidly change to 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.

&&

.SHORT TERM...Now through Wednesday Night...Visible satellite
reveals mostly sunny skies across northwest Oregon and
southwest Washington this afternoon, with any lingering mid
level cloud cover mainly restricted to the Cascades as low
pressure over northern California pulls away from the region.
This will temporarily leave the area under the influence of
transient shortwave ridging this evening, which will likely
make today the warmest day of the coming week as afternoon
highs climb into the upper 70s in the interior lowlands.

The pattern 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...Currently, widespread VFR conditions throughout the
airspace, with clear skies and northwesterly flow aloft. These
conditions are expected to persist along the coast until 00-03Z
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 further, resulting in a 40-60% chance of IFR and a
20-40% chance of LIFR (highest chances at KONP). Inland, stratus
from the coast will funnel in and lower CIGs to MVFR by 12-14Z
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...VFR through most of the TAF period. There is a
30-50% chance of MVFR CIGs beginning 12-14Z 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 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.
&&

$$

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