167
FXUS66 KPQR 150503
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1002 PM PDT Sun Jul 14 2024

Updated aviation discussion

.SYNOPSIS...Above normal temperatures will persist for inland
portions of SW Washington and NW Oregon through next weekend,
though onshore flow should help to guard against more extreme
temperatures. Flow aloft will turn more southerly Tuesday,
potentially carrying moisture and instability northward from
California. This will lead to about a 10-20% chance of
thunderstorms Tuesday afternoon into Tuesday night, with the
best chance being east of Interstate 5.

&&

.SHORT TERM...Now through Tuesday Night...The upper level pattern
has changed little over the past 24 hours as strong high
pressure remains anchored over the Four Corners region while a
cutoff low continues to sit off the California coast. Warm and
stable west-southwest flow remains in place over the Pacific
Northwest to the north of these features, yielding another hot
and sunny day with marine stratus limited to the coastal waters
and the immediate coastline. Temperatures well into the 80s as
of mid afternoon remain on track to peak in the low 90s across
the Portland metro area and closer to the mid 90s from Salem to
Eugene later this afternoon.

Models continue to depict a slight deepening of the marine
layer tonight, resulting in probabilistic guidance showing a
50-60 percent chance for stratus to make it up the Lower
Columbia River Valley and into Portland and the northern
Willamette Valley Monday morning. This will have implications
on the high temperature for Monday and represents the best
chance for highs to top out below 90 within the coming week.
That said, latest guidance still gives around a 75 percent
chance to hit 90 at Salem and over 90 percent at Eugene where
marine relief is less likely to occur.

Largest forecast concerns arise on Tuesday as the upper level
pattern amplifies and results in a pivot to more southerly
flow over our area. First, the stout ridge building over the
western CONUS will send 500 mb heights upward and 850 mb temps
into the 23-25C range, coinciding with highs potentially in the
mid to upper 90s in the interior valleys on Tuesday. The
probability to reach triple digits has crept upward to around 30
percent in the Portland metro and 35-45 percent through the
rest of the Willamette Valley. HeatRisk continues to show
Portland , the Gorge, and the Hood River Valley back in the
Major category as a result, which may eventually warrant Heat
Advisories for the areas on Tuesday. The complicating factor
with southerly flow remains the potential for additional high
level cloud cover, but even if temperatures are a few degrees
cooler than currently forecast the relief would likely be
minimal as the influx in moisture would result in slightly more
humid conditions than we are accustomed to in the Pacific
Northwest.

The other primary forecast challenge will concern the
aforementioned cutoff low near the California coast as models
continue to depict it becoming dislodged by the amplification of
the pattern and moving northward as a negatively tilted
shortwave trough Tuesday into Tuesday night. This will bring
concern for lightning strikes as mid level moisture and
instability are advected northward into our area, yielding
surface based convection Tuesday afternoon transitioning to more
of an elevated nocturnal threat overnight Tuesday into Wednesday
morning. Ensemble based probabilistic guidance is still
somewhat underplaying the thunder threat as it tends to struggle
with scenarios, but still has shown signs of catching on to
what the deterministic runs have been signaling, with the
chance for thunderstorms increasing to 10-20 percent across the
Oregon ans south Washington Casacdes Tuesday evening through
Tuesday night. Will continue to monitor how this threat evolves
over the coming forecast cycles, but any lightning will
obviously present heightened fire weather concerns given the
recent prolonged stretch of hot and dry weather. /CB


.LONG TERM...Wednesday through Sunday...Guidance has backed off
of previously advertised mid to upper 90s for Wednesday as areas
south of Salem, as well as the coastal valleys, have a better
chance of seeing substantial onshore flow Tuesday night into
Wednesday as the upper disturbance will likely deepen the marine
layer, potentially even pulling stratus inland behind it. If
the upper disturbance indeed lifts through western Oregon
Tuesday night, Wednesday, could be cooler than the lower 90s we
are generally showing for the interior lowlands Wednesday
afternoon.

For those hoping for an end to the unusually warm weather this July,
looking at the long term forecast may be a bit of a
disappointment as GFS/EC ensembles show good agreement on 500
mb heights climbing back toward 590 dam Friday and Saturday. If
this occurs, it would again compress the marine layer to the
point where it can only effectively cool the coast and coast
range, and low clouds would likely be limited to the coast. So,
expect inland valleys to see highs remaining around 90 degrees
on average through next weekend, likely with mild to warm nights
in between. Weagle/CB

&&

.AVIATION...Marine stratus has been more persistent than the last
few days, with continued MVFR at the northern coast, and IFR at
southern terminals (KONP). Expect this to continue into Monday
with improvement in the afternoon as the surface ridge pokes
inland a bit more offshore flow component. Deeper marine layer
should move up the lower Columbia River a further than recent days
with a 40-60% chance of for MVFR cigs reaching KPDX area, and
20-40% chance as far as KUAO area.

Winds at the coast can expect gusts up to 20 kt, while inland
terminals will see sustained winds up to 10 kt. All inland
terminals remain VFR with clear skies throughout the period.

PDX AND APPROACHES...Slightly deeper marine layer may be enough
to bring some stratus to the PDX area as onshore flow remains
steady. HREF guidance has about a 40-60% chance for MVFR cigs in
the 1000-2000 ft range and a 20-40% chance for IFR cigs from
13-17Z Mon. Diurnally driven winds, northwest to 10 kt subsiding
overnight then picking up again Mon afternoon./mh


&&

.MARINE...High pressure will maintain northerly winds across the
waters for much of the next week. Winds will generally be
strongest off the central coast of Oregon and lightest off the
mouth of the Columbia River each day. Winds will typically peak
each day in the late afternoon and evening hours, with gusts at
times just barely meeting Small Craft Advisory criteria at 20-25
kt.

Probabilities of 20 kt winds or greater do decrease substantially
(to less than 20%) across the waters off even the central coast
of Oregon towards Tuesday and Wednesday. In the meantime, seas
will primarily be a hybrid of fresh swell/wind-driven waves
through the weekend.
-JH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory from 3 PM to 10 PM PDT Monday for PZZ252-
     253.

     Small Craft Advisory from 3 PM Monday to 9 AM PDT Tuesday for
     PZZ272-273.
&&

$$

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