Debris Flow in Washington, May 14, 2023

Area Forecast Discussion
National Weather Service Portland OR
310 PM PDT Sun May 14 2023

.SYNOPSIS...Summerlike weather will continue for much of the
upcoming week as a blocking ridge of high pressure persists
over western Canada. Low pressure aloft will move westward
across the Great Basin, eventually enhancing shower and
thunderstorm chances across much of Washington and Oregon Monday
and Tuesday. The associated cloud cover will take the edge off
the heat beginning Monday, but there will be an increase in
humidity that will likely persist for much of the week.

&&

.DISCUSSION...Another afternoon of mostly clear skies west of
the Cascades today as strong high pressure remains anchored over
western Canada. Strong offshore flow aided by a well defined
thermal trough along the coast has allowed temperatures to once
again climb into the upper 80s to low 90s in most locations,
including many coastal communities. Will see temperatures settle
in the low to mid 90s across most of the area this afternoon,
with breezy easterly winds continuing through the evening hours.
Visible satellite reveals an extensive cumulus field with
scattered high clouds across much of eastern Oregon this
afternoon, associated with a cutoff upper low presently centered
over far northwest Nevada. Will see these clouds start to
spread east of the Casacdes later this evening as the low
continues to drift westward and undercut the ridge. Expect
another night of very warm lows in the low to mid 60s, likely
putting all time May high minimum temperature records in
jeopardy for the second consecutive day in many locations and
contributing to a moderate heat risk again on Monday.

Offshore flow will begin to wane tonight as surface low
pressure along the coast pushes offshore and induces a
southerly wind reversal and a return of marine layer cloudiness
in the coastal waters. This will give coastal locations a
reprieve from the heat on Monday with temperatures moderating
closer to seasonal norms even as the marine layer fails to make
much inland progress. Meanwhile, 850 mb temps around +15 C will
still allow interior locations to warm well into the 80s and
perhaps crack 90 for a third consecutive day in many locations.
Heat advisories remain on track with previous thinking, with
limited seepage through the coastal gaps moderating the south
and central Willamette Valley enough to drop the advisory, while
the Portland area and points north stay a few degrees warmer
and increasing moisture associated with the arriving cutoff low
leads to slightly more humid conditions.

Speaking of the cutoff low, that feature will move directly
overhead on Monday, bringing an influx of mid level moisture
which will push precipitable water values above one inch by
Monday afternoon. The added moisture will also enhance
instability, with some models depicting MUCAPE values over 1000
J/kg across a wide swath of the area. The moisture and
instability coupled with lift provided by the upper low will
lead to a favorable environment for showers and thunderstorms,
mainly along the Cascades but also potentially drifting
westward over the valley Monday afternoon/evening.

The low lingers overhead as it continues to weaken on Tuesday,
potentially allowing for another round of showers along the
Cascades Tuesday afternoon. Otherwise, WPC cluster analysis and
other available medium/long term guidance continue to depict
high pressure dominating the western CONUS through the end coming
week, with 850 mb temps remaining in the mid teens and allowing
inland locations to reach the 80s each day and possibly push 90
at times. The coast should remain milder in the coming week
given the absence of any strong signal for offshore flow. /CB

With this unseasonable heat wave occurring over the weekend, there
is a heightened concern that people will be lured into dangerously
cold water, whether fresh water in rivers and lakes, or the cold
waters of the Pacific. Either way, temperatures are mainly in the
50s, plenty cold enough to cause cold water shock and hypothermia
for those without a proper wetsuit. Rivers and streams are running
especially fast and cold in the Cascades; many of them are still
showing water temperatures in the 40s. These waters are dangerously
cold and best enjoyed from a distance.

&&

.AVIATION ...00z TAFs preliminary thoughts: VFR thresholds with
clear skies prevail at all terminals through this evening. A
thermal trough remains along the coastline as of 2130z, which has
induced easterly winds across the region with gusts between 20-30
kt. Expect gusty winds to die down this evening after 03z Mon. The
thermal trough will lift northward tonight into Monday, with
winds shifting southerly after 06z Mon. Areas of fog and low IFR
stratus push northward along the coast overnight. This could reach
KONP sometime after 12z Mon. Current hi-res guidance suggest a
15-30% chance of IFR cigs at KONP between 13-16z Mon.

An upper low moving into our region will also promote at a 15-25%
chance of thunderstorms in the Cascades and Willamette Valley
beginning 18z Mon through the end of the TAF period. Expect more
mid to high level clouds.

KPDX AND APPROACHES...VFR under clear skies through this evening
with easterly winds gusting up to 30 kt. Expect gusts to diminish
after 03z Mon. VFR thresholds prevail Monday. Slight chance for
thunderstorms mid-morning onward in the Cascades/Valley, which
could bring periods of locally heavy rain. -Alviz

&&

.MARINE...Winds have generally subsided below 20 kt across the
coastal waters per afternoon buoy observations and ship reports,
as winds transition from northerly to more of an offshore
component. Will still likely see a few gusts to 25 kt this
afternoon in the nearshore waters, but these should be localized
enough to coastal terrain gaps as to not warrant any additional
headlines. Thermal trough currently hugging the coast and centered
between Cape Blanco and Cape Mendocino will transition offshore
as a surface low tonight and Monday, triggering a southerly wind
reversal over the waters through Tuesday morning. This will allow
marine layer cloudiness and fog to spread back over the waters
tonight. Winds will then be generally light from mid to late week
as weak surface high pressure resides along the coast. Seas look
to generally remain at 5 ft or less through the week with a
modest westerly swell as the dominant wave group. /CB

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Heat Advisory until 11 PM PDT this evening for Cascade Foothills
     in Lane County-Central Coast Range of Western Oregon-
     Central Oregon Coast-Central Willamette Valley-Coast Range
     of Northwest Oregon-North Oregon Coast-Northern Oregon
     Cascade Foothills-South Willamette Valley.

     Heat Advisory until 8 PM PDT Monday for Greater Portland Metro
     Area-Lower Columbia.

WA...Heat Advisory until 11 PM PDT this evening for South Washington
     Coast-Willapa Hills.

     Heat Advisory until 8 PM PDT Monday for Greater Vancouver Area-I-
     5 Corridor in Cowlitz County-South Washington Cascade
     Foothills.

PZ...None.

&&

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