288
FXUS66 KPQR 151050
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
350 AM PDT Mon Jul 15 2024

.SYNOPSIS...Above normal temperatures will persist for inland
portions of SW Washington and NW Oregon through the 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, best chances east of I-5.

&&

.SHORT TERM...Today through Wednesday...Marine stratus is
entrenched along the coast early Monday morning and satellite
imagery shows the stratus creeping up the lower Columbia River
valley. This may produce some lower cloud cover this morning across
the Portland metro area, but sunshine will return with temperatures
warming again into the upper 80s to lower 90s. With yesterday`s high
temperature of 96, the current streak of 90+ temps at Salem (SLE)
has grown to 11 days and has taken over sole place for the 2nd
longest streak of such. There continues to be around a 75% chance
that Salem continues that streak of reaching 90 degrees today.
Satellite imagery also shows some high clouds over southern Oregon
which may drift northward across the area today, possibly keeping
temps a degree or two cooler.

Temperatures are expected to heat up again on Tuesday as the flow
aloft becomes more southerly and 850 mb temps jump to around 23-25C,
with the potential of a few locations within the interior valleys
reaching triple digits. Latest NBM guidance suggests there is only a
20-30% chance for 100 degree temps for most interior areas, except a
40-50% chance from Salem to Canby, as well as the Hood River area.
The Portland metro area, as well as the Columbia River Gorge and
Hood River Valley, remains in the major heatrisk category for heat
related illness. 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 forecast challenge will concern a 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 and 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.

In the wake of the upper shortwave trough, there is good agreement
of a stout southerly marine push at the surface late Tuesday into
Wednesday. The upper disturbance will likely deepen the marine layer
with substantial onshore flow bringing the potential for increased
marine stratus and somewhat cooler temperatures for Wednesday,
albeit mid-to upper 80s for highs, with at least a 50% chance of
temps exceeding 90 degrees for Salem to Portland. DH/CB

.LONG TERM...Thursday through Sunday...Warm and dry conditions
expected through the remainder of the week and weekend. Ensembles
continue to show good agreement of upper level ridging shifting back
to the west while 500 mb heights climb back toward 590 dam Friday
and Saturday. An upper level trough off the coast on Thursday would
maintain a deeper marine layer, but the strengthening ridge over the
region would likely compress the marine layer to the point where it
can only effectively cool the coast and coast range. Expect inland
valley highs remaining around 85-95 through the weekend.

&&

.AVIATION...Marine stratus remains solid across the coastal waters
and along the coast early this morning, extending into coastal
valleys and up the Columbia River to about Ridgefield as of 09z.
Astoria PSL profiler shows a deeper marine layer this morning than
we have seen for a while, closer to its more typical depth of
2000-3000 ft. Onshore MSLP gradients persist, so spotty stratus
is developing along the Clark County Cascade foothills, a good
sign that stratus will make it into at least the eastern half of
the Portland metro area. 00z HREF probabilities only show about a
30-40% chance of MVFR cigs making it into KPDX/KTTD/KUAO, but I
would put the chances at about double that at this point.
Therefore, we will be including MVFR cigs for these terminals
12z-17z, but keeping the deck FEW- SCT for KHIO, maintaining clear
skies inland KSLE southward.

IFR/low MVFR cigs will likely persist a bit longer along the coast
than their inland counterparts, but strengthening thermal low
pressure along the coast near the OR/CA border will pull an
increasingly N-NE component to the flow, eventually pushing the
stratus off the coast sometime around 20z according to most
guidance. Offshore flow will probably keep the coast from being
affected by wholesale stratus tonight into Tue morning, though
there could be some patches of fog after 16/06z.

North winds will increase this afternoon, with a good chance
(better than 70%) of gusts 20-25 kt along the coast KTMK
southward. Northerly gusts will approach 20 kt for the southern
half of the Willamette Valley this afternoon as well.

While SW Washington and NW Oregon have largely avoided significant
fire activity thus far, low-level smoke from the Donkey Fire in
the Willapa Hills is possible, along with higher-level smoke from
more distant fires to our south.

PDX AND APPROACHES...Deeper marine layer and continued onshore
flow have a 70-80% chance of bringing MVFR stratus into KPDX/KTTD
12z-17z, after which skies will clear and VFR will return. Expect
light to moderate N-NW sfc winds, perhaps turning light easterly
after 16/06z.  Weagle

&&

.MARINE...High pressure persists over the NE Pacific, with
strengthening thermal low pressure near the OR/CA border. This
will support increasing N winds across the waters this afternoon,
turning more NE overnight tonight. As winds turn more offshore,
they will slacken along the immediate coast. Therefore we have a
SCA for various times for the coastal waters Cape Falcon southward
to Florence. Gusts up to 25 kt will spread northward, affecting
PZZ253/273 beginning around 11 AM, and PZZ252/272 around 2 PM.
These winds will slacken for the inner waters by around midnight
tonight, but continue for the outer waters before decreasing
Tue morning. Aside from the steep wind waves generated by the
gusty N winds, there is very little background swell to speak of;
mostly just long-period Southern Ocean swell.

There is one tricky part to the forecast; lightning could become
an issue Tuesday night as a weak disturbance lifts northward along
the coast, bringing instability and moisture. If there is any
significant organization to these thunderstorms, it could produce
gusty and erratic outflow winds up to low-end Gale Force and
confused seas.  Weagle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Small Craft Advisory from 2 PM this afternoon to 2 AM PDT
     Tuesday for PZZ252.

     Small Craft Advisory from 10 AM this morning to 2 AM PDT Tuesday
     for PZZ253.

     Small Craft Advisory from 2 PM this afternoon to 9 AM PDT
     Tuesday for PZZ272.

     Small Craft Advisory from 10 AM this morning to 9 AM PDT Tuesday
     for PZZ273.

&&

$$

www.weather.gov/portland

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