486
FXUS66 KPQR 242144
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
243 PM PDT Tue Sep 24 2024

.SYNOPSIS...High pressure rest of Tuesday afternoon with temperatures
around 10 degrees F above normal. Cooler wet front moves in on
Wednesday bringing widespread rain. Showers persist through Friday
morning, then ridging persists through early next week.

&&

.SHORT TERM...Now through Friday...Warm and dry conditions for the
rest of today, with temperatures currently in the mid-80s for most of
the Willamette Valley. Peak temperatures will reach right about up to
90 for lowland areas. Generally northerly flow under 10 mph, shifting
to become westerly as the next front begins to move in Tue night.

The next low pushes in late Tue night/Wed morning, bringing rain
throughout daytime hours Wednesday. Rain looks to bring around 0.25"
of accumulation for most areas, though northwestern areas (Willapa
Hills, Washington coast) will see closer to 0.75" of accumulation.
The jet stream lies slightly further north, allowing for better
synoptic forcing. In addition, around a 15% chance of isolated
thunderstorms in the south Washington Cascades only, due to
orographic lift combined with this synoptic forcing. Much cooler
temperatures are expected as well, with a 60-70% chance of Willamette
Valley temperatures not exceeding 70 degrees. This increases to near
100% at the coast and in the Lower Columbia River Valley. Westerly
flow between 10-15mph is expected throughout the region while this
precipitation is ongoing.

Rain ends for the most part by Thursday morning, with any residual
showers ending by midday Thursday. Conditions remain cool and
unsettled as a few lows rotating around each other (a process known
as the Fujiwhara effect) continue to impact the region. Accumulations
are more uncertain for any precipitation following Wednesday, with
confidence of 24-hr accumulations over 0.10% being 20% for areas
south of Portland on Thursday and Friday. This chance will be higher
further north as jet stream continues to create synoptic forcing that
allows for better accumulations. /JLiu


.LONG TERM...Friday Night through Monday...Little to say in the
long-term forecast as yet another ridge builds in. The jet
stream will become zonal on Friday night and shift well to the
north of the region. This will allow for the ridge to build, and
conditions to dry and clear. Temperatures will stay seasonable
with northerly winds in the afternoon. Now something to note
though is that on Saturday night into Sunday morning, there is a
shortwave trough that is moving through at the mid-levels. At
this point, not quite sure at what the impact will be, but it`s
not uncommon to see breezier winds and a more unstable
atmosphere. This trough is most likely associated with a weak
tropical storm that is over the California coast. Dry and
seasonable conditions persist through Monday. -Muessle

&&

.AVIATION...VFR conditions prevail everywhere except the coast
where IFR fog/stratus is redeveloping this afternoon. An upper
trough and associated front, now well offshore will move to the
area late tonight into Wednesday. As flow turns more onshore this
evening, expect (60-80% chance) for conditions to deteriorate at
KONP and KAST to IFR or LIFR. Prevailing VFR inland with variable
high clouds through this evening then with increasing onshore
flow, expect stratus to push inland with high chances (80-90%) for
MVFR CIGs between 10-18Z Wed, then followed by decreasing chances
for MVFR. Rain reaching the coast 09-12Z Wed and inland after 15Z
Wed.

PDX APPROACHES...VFR with variable high clouds expected through
this evening, then high chances (80-90%) for MVFR cigs after
11Z-18Z Wed. Winds expected to remain light. /mh

&&

.MARINE...Thermal trough along the Oregon coast will maintain
breezy north winds with gusts up to 20 kt across the coastal
waters through into early evening then weakens as the thermal
trough shifts further inland. An approaching front will turn the
winds southerly tonight into Wednesday. Guidance continues to
show a 60-70% chance that wind gusts to 20 kt briefly ahead of
the front, with highest probabilities across the inner waters
between 8 AM and Noon on Wednesday. Winds are expected to
abruptly shift to the northwest with the frontal passage by Wed
afternoon.

Winds turn southerly again on Thursday ahead of another frontal
system. Still expecting a low pressure system to rapidly
intensify across the far NE Pacific and move toward Haida Gwaii on
Thursday. The trailing cold front associated with this low will
likely approach the coastal waters later Thursday. While the
majority of models suggest Small Craft Advisory winds, there is
a 15-30% chance of Gale Force southerly wind gusts to 35 kt
across the northern coastal waters Thursday afternoon.

Seas continue build to around 8 to 9 ft on Tuesday as a fetch of
northwest swell moves into the coastal waters. Seas likely linger
around 7 to 9 ft through Thursday. Another west to northwest
swell is expected to move in behind the front on Friday with seas
likely building up to around 12 to 14 ft. Seas settle to near 10
ft over the weekend.  /mh /DH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

www.weather.gov/portland

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