436
FXUS66 KPQR 151802 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1101 AM PDT Mon Jul 15 2024

Updated aviation discussion...

.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...Satellite imagery as of 1730z Mon depicts marine
stratus beginning to dissipate along the coast and Lower Columbia
River Valley. The north OR coast is still holding on to low stratus,
and guidance suggests conditions will improve to VFR at KAST around
21-23z Mon. Otherwise, high pressure will maintain VFR conditions
under clear skies today with northerly to northwesterly winds.
Expect wind gusts up to 25 kt along the coast, and gusts up to 20 kt
inland. Winds weaken below 10 kt this evening.

High confidence for marine stratus returning along the coast tonight
after 03-05z Tue, returning IFR/MVFR conditions or lower. Guidance
also suggests a 15-30% chance of MVFR stratus returning to the
Portland/Vancouver Metro Area again after 12z Tue. These
probabilities may be higher as MVFR stratus has been observed at
KPDX/KTTD even though previous chances have been low. Thus, MVFR
cigs were included in the recent TAF package for KPDX/KTTD for
tomorrow morning.

PDX AND APPROACHES...VFR conditions under clear skies through most
of the TAF period. Chance for low stratus returns for the Portland-
Vancouver Metro Area around 12-16z Tue. Northwesterly winds today
around 6-8 kt, weakening further tonight.   -Alviz

&&

.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 until 2 AM PDT Tuesday for PZZ253.

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

     Small Craft Advisory until 9 AM PDT Tuesday for PZZ273.
&&

$$

www.weather.gov/portland

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