Rip Current in Oregon, May 12, 2023

Area Forecast Discussion
National Weather Service Portland OR
923 AM PDT Fri May 12 2023

.SYNOPSIS...An early season spell of warm to hot weather begins today
as a strong ridge of high pressure builds over the Pac NW and western
Canada. Inland temperatures should warm 5 to 10 degrees above
Thursday's highs. Increasingly offshore flow will bring plenty of
sunshine and well-above normal temperatures all the way to the coast
this weekend. Higher clouds may increase as early as Sunday as low
pressure from the Great Basin moves west and undercuts the ridge.
That said, a warm and increasingly subtropical air mass will keep
temperatures well above May normals next week, especially inland.
Expect it to feel a bit more humid next week, and the additional
humidity will result in a better chance of showers and thunderstorms.

&&

.DISCUSSION...An upper level ridge is amplifying across the Pac NW
and into western Canada this morning. The high pressure is allowing
much of SW Washington and NW Oregon to start off the day clear, with
the notable exception of marine stratus along the coast south of
roughly Pacific City, extending locally into some Coast Range valleys
early this morning. With 850 mb temps warming to around +14 deg C
today per most models (versus +8 to +10 deg C Thursday), inland areas
should be a good 5-10 deg F warmer than yesterday. This will likely
yield widespread highs in the 80s today for our inland valleys. Sea
breezes will keep the coast cooler today, but that will change as we
head into the upcoming weekend.

Models show impressive agreement that rather disorganized shortwave
energy presently over Alberta will consolidate into a more organized
upper low while sliding southward along the Rockies tonight into
Saturday. The resulting system then turns west, crossing the Great
Basin while undercutting a blocking ridge of high pressure over
western Canada. Assuming this scenario verifies, this will serve to
enhance easterly flow across the Cascades, enhancing downsloping
winds and thus heating up areas west of the Cascades.

While temperatures will almost certainly be well above normal this
weekend, the big wild card appears to be the impact of any moisture
aloft brought westward by the low pressure system. Latest guidance
seems to show enough moisture to bring considerable mid to high cloud
cover to the forecast area by Monday morning...perhaps enough to hold
temperatures down Monday afternoon. In fact, most models show some
showers and possible thunderstorms developing with the upper low by
Monday afternoon. Either way, it appears likely offshore MSLP
gradients will weaken sufficiently for sea breezes to return to the
coast by Monday, especially along the Oregon coast. This will likely
bring the return of marine stratus to the coast Monday, though it may
struggle to get north of the Columbia.

Given reasonable model agreement, there is a high level of confidence
(90 percent or higher) that temperatures will be some 10 to 20
degrees above seasonal normals this weekend, and possibly a bit
warmer. That said, the hotter ensemble members appear to have fallen
off the board for the most part, so confidence is also high (greater
than 90 percent) that our main airport stations will remain below 100
deg F this weekend. Given this "hot, but not dangerously hot for
most" range of temperatures, we decided to upgrade the existing
Excessive Heat Watches to Heat Advisories, with most of the lowlands
maxing out in the 85 to 95 deg F range Saturday and/or Sunday.
Temperatures will be slow to cool off Sat night for urban areas and
the lower hills, but most other areas should cool down into the mid
50s to lower 60s, which should provide at least some relief
overnight. Significant cooling will likely occur along the coast
Monday as the sea breeze returns, but it will take longer - perhaps
much longer - for inland areas to benefit from the cooling.

For Tuesday and there on out, have stuck with the NBM forecast
package. By design, it will slowly adapt to the overall daily
variations from the various model inputs. This is a very reasonable
course of action given the complex subtleties presented with cut off
low pressure very much separated from any mid-latitude steering
flow. The result is temperatures remaining 5-15 degrees above normal
along with a threat of showers and thunderstorms mainly centered
over the Cascades on each day. While the intensity of this "heat
wave" appears to be backing off a bit, this one may be remembered
more by its duration for inland areas.	Weagle/JBonk

&&

.AVIATION...Westerly flow aloft along with high pressure over the
region will maintain VFR under mostly clear skies. Still have
rather solid MVFR marine stratus along the coast and over the
Pacific from near Lincoln City southward. As day progresses, will
see this stratus break up and dissipate from north to south.

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

KPDX AND APPROACHES...Dry westerly flow aloft continues, with VFR
under mostly clear skies. -BMuhlestein

MARINE..Generally under influence of high pressure, with thermal
low pressure over far southwest Oregon into northwest CA. Seas
mostly 3 to 5 ft, with mix of predominately westerly and smaller,
but longer period (16-17 sec) south swell.

As high pressure rebuilds tonight and Fri, and with thermal
trough building over southwest Oregon, should see return more
typical early summer winds, with northerly winds gusting 20 to 25
kt in the afternoons/evenings for Fri into Sat, with more of an
northeast component later Sat into Mon. Seas will come up a bit
as well with 5 to 7 ft, thanks to the extra wind chop. /mh

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Heat Advisory from 2 PM Saturday to 11 PM PDT Sunday for Central

     Coast Range of Western Oregon-Central Oregon Coast-North
     Oregon Coast.

     Heat Advisory from 2 PM Saturday to 8 PM PDT Monday for Cascade
     Foothills in Lane County-Central Willamette Valley-Coast
     Range of Northwest Oregon-Greater Portland Metro Area-Lower
     Columbia-Northern Oregon Cascade Foothills-South Willamette
     Valley.

WA...Heat Advisory from 2 PM Saturday to 8 PM PDT Monday for Greater
     Vancouver Area-I-5 Corridor in Cowlitz County-South
     Washington Cascade Foothills-South Washington Coast-Willapa
     Hills.

PZ...None.

&&

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