Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 272154
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
254 PM PDT Tue Sep 27 2022

.SYNOPSIS...Sunny skies continue through this evening as high
pressure shifts east of the region. An approaching front will
bring measurable rain to much of the area on Wednesday, with
cool and showery weather continuing through Thursday as an upper
level trough crosses the region. High pressure returns on
Friday, ushering in another warm and dry stretch through the
weekend.

&&

.SHORT TERM...THROUGH THURSDAY NIGHT...Mostly sunny skies
prevail again this afternoon in southwest flow aloft between
broad upper level ridging over the Rockies and a sharp and
deepening trough over the northeast Pacific. Visible satellite
imagery reveals a well defined low pressure system associated
with the upper level trough near 46N 135W. An occluded front
extends well out ahead of the low center, approximately 250
miles offshore of the Oregon Coast. Expect rain to develop along
the coast late tonight as the front approaches and then spread
inland to the Willamette Valley and the Cascades Wednesday
morning. Virtually all of the area will see at least some
measurable rain by Wednesday evening, but amounts should remain
rather modest with valley locations picking up less than a
quarter of an inch while coastal and north OR Cascade locations
could see a quarter to a half inch of precipitation. Rain will
will become more showery in nature Wednesday night as the low
crosses the area, but inland valleys could see up to another
tenth of an inch through Thursday morning while the north OR and
south WA Cascades pick up as much as another quarter or an
inch. The low will have weakened considerably by the time it
reaches the coast, helping to minimize the wind potential
although coastal areas will still likely see some gusts in the
30-40 mph Wednesday afternoon and evening. Showers will linger
on and off through much of Thursday on the back side of the low,
but should diminish relative quickly Thursday evening as the
upper trough departs to the east and is replaced by another
ridge for the end of the week. /CB

.LONG TERM...FRIDAY THROUGH TUESDAY...WPC`s cluster analysis
for 500 mb heights depict good agreement amongst models and
their ensembles in regards to the synoptic-scale pattern Friday
through the weekend, suggesting upper level ridging will result
in above normal 500 mb heights. The NBM keeps the area dry
during that time and suggests highs will rise back into the low
to mid 80s for the interior lowlands. This is a good 10 to 15
degrees above normal for the beginning of October. It appears
Saturday will be the warmest day; this is when the NBM 25th to
75th percentile guidance is generally in the 80s. We leaned on
NBM 75th percentile numbers for our high temperatures in the
extended. TK/Weagle

&&

.AVIATION...00Z TAFs: A low pressure system offshore will
approach the region through Wed morning. This will maintain
generally southwest flow aloft with some mid to high clouds moving
across the area though tonight. At the coast, strong onshore flow
will maintain IFR to low MVFR through this evening. Elsewhere,
VFR conditions are expected to persist inland. Precipitation is
expected to begin ahead of the initial front along the coast after
09Z Wednesday and gradually spread inland around 14-16Z Wed
morning. Increasing clouds are expected overnight with ceilings
lowering, but conditions inland should remain VFR. The HREF is
suggesting around a 30-50% chance that cigs do fall to MVFR late
in the period around 16-18Z Wednesday.

For detailed Pac NW aviation weather information, go online to:
https://weather.gov/zse

KPDX AND APPROACHES...VFR through at least 15Z Wednesday, then
around a 30% chance of MVFR cigs and light rain likely. Light
onshore wind through this evening, becoming light southeast after
midnight. /DH

&&

.MARINE...A low pressure system approaching the area will bring
increasing southerlies and rain across the coastal waters tonight
through Wednesday. South winds will quickly ramp up as the front
ahead of the low moves over the waters, further intensifying
through Wednesday afternoon. Looking at widespread gusts up to 25
kt throughout all of the waters, with chances for gusts up to 30
kt in the outer waters. Issued a Small Craft Advisory for the
outer waters beginning late tonight, and one for the inner waters
beginning 8 AM Wednesday morning. SCA conditions are expected to
persist into early Thu morning.

Initially seas will remain in the 4 to 5 ft range through
this evening but will begin to build in response to the
approaching weather system on Wednesday. Seas will be primarily
driven by a 10 ft westerly swell at around 11 seconds. But
southerly winds waves will make the seas steep and choppy, with
seas peaking around 12 ft by Wednesday evening. Seas should
gradually subside to below 10 ft late Thu morning or afternoon.
There is also a slight chance for thunderstorms in the outer
waters along the backside of the front Wed afternoon and early
evening.

The new moon will produce stronger tidal currents during the
evening ebb cycles Tuesday through Friday. Those moving in and out
of harbors and crossing coastal bars should use caution and be
aware of any bar restrictions that may be in place. /DH

For information about recent marine zone changes, go online to:
https://www.weather.gov/pqr/marinezone

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory from 8 AM Wednesday to 5 AM PDT Thursday
     for coastal waters from Cape Shoalwater WA to Florence OR
     out 10 nm.

     Small Craft Advisory from 11 PM this evening to 5 AM PDT
     Thursday for coastal waters from Cape Shoalwater WA to
     Florence OR from 10 to 60 nm.

&&

$$

www.weather.gov/portland

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