828
FXUS66 KPQR 171803 AAA
AFDPQR

Area Forecast Discussion...Updated
National Weather Service Portland OR
1103 AM PDT Tue Jun 17 2025

Updated Aviation...

.SYNOPSIS...A weak cool front will bring a chance of light rain
showers to much of the area Tuesday night into Wednesday, mainly to
the north of Eugene. High pressure makes a brief return on Thursday,
bringing dry and mild conditions. Transitioning to a cool and showery
weather pattern Friday through Sunday as a closed upper level low
settles over the region and brings more substantial rain amounts to
the area. There is also a 15-20% chance of thunderstorms Friday and
Saturday. Stronger showers or thunderstorms that develop Friday into
the weekend will produce locally heavy downpours.

&&

.SHORT TERM...Now through Thursday night...High pressure will
maintain mostly clear skies and calm winds over western WA/OR this
morning. With plenty of sunshine expected through the day, temps
should easily be able to warm to around 80 degrees over the interior
lowlands and mid 60s at the coast. Will note cloud cover will begin
to increase at the coast Tuesday evening ahead of a cool front that
will be decaying over the coastal waters before moving inland. As
such, precipitation is unlikely to occur with this front.

A secondary cool front on the heels of the first front is set to move
inland Tuesday night through Wednesday morning. With this front being
slightly stronger than the first and a relatively higher amount of
mid-level moisture in place, at least some light rain will likely
occur along/near the coast, southwest WA, the Portland/Vancouver
metro, northern Willamette Valley, Columbia River Gorge and Cascade
foothills. Locations in the central and southern Willamette Valley
are more likely to be missed by precipitation, and this is also where
PoPs and QPF are lowest. Locations that do observe rain should expect
anywhere from a trace to 0.1". With more cloud cover around
on Wednesday in combination with a chance of showers, temperatures
will be cooler with highs in the mid 60s to lower 70s, coolest over
southwest WA and at the coast.

Thursday is still shaping up to be a dry and mild day with seasonable
temperatures as high pressure makes a brief return and onshore flow
continues. Given the wetter weather pattern that is set to arrive
Friday into Saturday, Thursday will be one of the best days of
the week for outdoor work/activities. The shift to a cooler and
wetter weather pattern beyond Thursday is discussed below in the long
term discussion.  -TK


.LONG TERM...Friday through Monday...The long term forecast remains
highlighted by a cool and showery weather pattern Friday through
Sunday with more significant rain amounts. Probabilities for 48-hr
rain amounts over 0.25" from 5 AM Friday to 5 AM Sunday have
increased once again with the latest NBM, now reaching 70-90% for all
locations in southwest WA and northwest OR west of the Cascade crest.

This is in response to excellent model agreement for a cool upper
level low that will be settling directly overhead Friday into
Saturday. With this low will come cooler air aloft, helping to
steepen mid-level lapse rates. Abundant moisture will accompany this
low as well, with PWAT values likely reaching 0.8-1.0 inches by
Saturday afternoon according to the LREF grand ensemble mean. Given
enough breaks in cloud cover and subsequent surface heating, do
expect enough surface-based instability to warrant heavy downpours
with any stronger showers or thunderstorms that develop Friday and
Saturday afternoon/evening. 6-hr NBM thunder probabilities have
increased to around 20%, which seems reasonable for now.

Given the showery nature of precipitation this weekend, some
locations will significantly overperform for total rain amounts while
other locations underperform. That said, forecast rain amounts are
fairly substantial for all locations. WPC QPF amounts Friday through
Saturday now exceed 0.40-0.45" for all locations. QPF amounts are
highest over the Cascades and foothills at 1-2 inches. Regardless of
uncertainty with exact rain amounts, confidence is high this system
will bring the highest rain totals the area has observed thus far
this month. Note the entire month of June has been dry so far, aside
from June 4-5 when light rain fell over most of southwest WA and far
northwest OR, albeit missing Salem and Eugene where no measurable
rain has been observed this month. Beyond this system, most ensemble
guidance trends towards warmer and drier weather for early next week.
 -TK

&&

.AVIATION...Visible satellite shows clear skies across most of the
the area as of 18z Tue, with the lone exception being a patch of
broken cloud cover over northern coastal areas bringing cigs
around FL050 to KAST. Expect VFR to persist at all sites through
the afternoon, with stratus returning to coastal terminals with
MVFR cigs 01-03z Wed. A weak front will approach the region
06-12z Wed, bringing the chance for light rain showers to coastal
sites starting 07-09z and inland terminals 10-12z Wed. Rainfall
amounts will be very minimal with this feature, but do expect the
front to bring MVFR cigs to inland sites through around 16-18z
Wed. Conditions will then improve back to VFR with winds shifting
light westerly behind the front towards the end of the TAF period.

*KTTD ASOS is currently INOP except for altimeter and will be AMD
 NOT SKED.

PDX AND APPROACHES...VFR under mostly clear skies through at least
03-06z Wed. Winds increasing out of the northwest to around 8 kt
00-06z. Light rain and lowering ceilings to MVFR expected with a
weak front 10-12z. Conditions improving back to VFR with any
showers tapering off 16-18z Wed. /CB

&&

.MARINE...Winds have shifted southerly behind a weak front that
moved through the waters Monday evening. Winds remain southerly
and increase to 10-15 kts with gusts up to 20 kts Tuesday morning
and afternoon as another weak front moves through the waters.
Winds shift westerly on Wednesday behind the front, then shift
northerly Thursday as weak high pressure builds over the waters.
An increasing westerly swell has caused significant wave heights
to increase to 5-6 feet at 10 seconds, remaining through Tuesday.
Waves decrease to 3-5 feet overnight Tuesday into Wednesday.
Another westerly swell arrives Thursday, increasing significant
wave heights to 5-8 feet at 11-12 seconds. More active weather
returns Friday into the weekend as multiple frontal systems are
forecast to move through the region. -HEC

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

www.weather.gov/portland

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