704
FXUS66 KPQR 161038
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
338 AM PDT Tue Jul 16 2024

.SYNOPSIS...Rather hot today with highs approaching triple digits
for inland portions of SW Washington and NW Oregon. Slight chance of
thunderstorms for the central Oregon Cascades this afternoon. Then,
upper level disturbance brings increasing chances for elevated
convection later this evening and tonight from the Cascades to the
coastal waters. Above average temperatures expected to continue
through the weekend while onshore flow persists at the coast.

&&

.SHORT TERM...Today through Thursday...Hot and dry today, followed
by a chance for elevated convection tonight. An upper level cutoff
low has developed off the northern California coast this morning and
is expected to lift northeastward today toward the Oregon coast.
More southerly flow aloft ahead of the incoming shortwave is still
expected to draw in very warm 850 mb temperatures peaking around 23-
25C late this afternoon into NW Oregon and SW Washington. Expect
this to translate to highs around 95-100 degrees across the interior
valleys, including the Columbia River Gorge and Hood River Valley.
As the upper level disturbance approaches the region this evening,
high pressure building along the southern Oregon coast will drive a
strong onshore push of marine air at the surface. Expect a southerly
wind reversal to begin to cool temperatures in the southern
Willamette Valley by late afternoon/early evening, gradually
expanding northward overnight. With the warmer air lingering across
the Portland/Vancouver Metro to Hood River, will maintain the Heat
Advisory.

Along with the heat, the approaching shortwave will bring potential
for thunderstorms, initially surface based in the central Oregon
Cascades, eventually evolving into elevated nocturnal convection
later this evening and overnight. The upper level shortwave will
take on a negative tilt which is a favorable pattern for enhancing
lift, while mid level moisture and instability are advected
northward. Any thunderstorms that do develop will have the potential
to produce prolific lightning, presenting a threat for heightened
fire weather concerns given the recent stretch of hot and dry
weather. Currently have capped thunderstorm chances at around 20%,
though trends are certainly indicating the actual probability may be
higher than that, as models sometimes struggle with elevated
convection.

Temperatures are once again expected to warm to around 85-90 degrees
for Wednesday and Thursday across inland valleys as the ridge builds
back in. Onshore flow will maintain mild temperatures for the coast.
Upper level troughing will likely deepen the marine layer late
Thursday. /DH

.LONG TERM...Friday through Monday...Cluster analysis indicates the
ensemble guidance showing good agreement of the upper level ridge
amplifying and shifting back west for Friday and Saturday. This will
likely cause temperatures to bump back up a few degrees with highs
in the 90-95 degree range. NBM guidance suggests a slight cooling
trend into early next week as upper level troughing begins to nudge
into the Pacific NW. Something to watch will be the potential for
another cut-off low and shortwave trough to slide across the area
again this weekend, which could produce another round of
thunderstorms, but confidence remains low at this time. /DH

&&

.AVIATION...IFR/LIFR marine stratus continues to cling to the
coast this morning, affecting mostly just the coastal terminals.
The marine layer has become shallower over the past 24 hours,
returning to a depth of only 1000-1500 ft. This means it is
unlikely to reach any inland terminals this morning, and hi-res
guidance looks a little too high with the chances in showing a 20%
chance of IFR reaching KPDX and a 30% chance for KTTD. So, most
likely scenario is that inland terminals remain VFR through this
afternoon. Coastal terminals should clear out shortly after
sunrise as weak low pressure off the northern CA coast begins to
pull easterly offshore flow as it moves northward along the coast.

The forecast becomes quite a bit more complicated after about 23z
as instability and moisture aloft become involved with the low
pressure moving up the coast. There is a 20-30% chance of
SHRA/TSRA developing over SW Oregon and moving north into our
forecast area beginning around 17/00z. Any SHRA/TSRA will move
into Lane County around 00z then progress northward across KSLE
and KPDX metro area overnight. This convection will be high-based
so conds should remain VFR for the most part, though there will be
gusty and erratic outflow winds with any TSRA. If a line of
showers forms, it will likely come with a sharp wind shift from NE
winds ahead of it to S-SW winds behind it, which will persist but
ease for the remainder of the night.

To further complicate things, the disturbance triggering the TS
threat will also deepen the marine layer and initiate a robust SW
surge of marine air into inland areas. Mixed IFR/MVFR conds will
and areas of -DZ will spread northward along the coast behind the
disturbance overnight. MVFR stratus appears likely to reach KEUG
sometime overnight, but high-res guidance is more stingy with the
stratus farther to the north - perhaps too stingy.

PDX AND APPROACHES...VFR through this evening, with only a 10%
chance of low MVFR stratus affecting KPDX around sunrise.
Increasing chance of high-based TSRA after 17/03z, with best
chance focusing around 17/09z. High-based nature should keep
cigs/vsbys VFR in showers, but winds near any showers will be
gusty and erratic.  Weagle

&&

.MARINE...North to northeast winds are slowly easing over the
coastal waters, enough to allow the SCA for winds to expire for
the inner waters. Upper level disturbance over the N CA waters
will lift northward today and tonight, increasing the chance of
thunderstorms over the waters overnight. This is the type of
system that has the ability to produce prolific lightning and
gusty/erratic outflow winds, so these thunderstorms could be more
hazardous than the weaker springtime storms that are more typical
for the coastal waters. Mariners planning on being out on the
waters tonight should pay close attention to the skies to the
south, and listen to NOAA weather radio for any Special Marine
Warnings. These showers/thunderstorms appear most likely to move
into our southern waters PZZ253/273 around 5-6 PM then gradually
progress northward through the coastal waters (and NW Oregon)
overnight.

N-NE winds today will shift suddenly to a gusty S-SW wind near the
line of showers/thunderstorms. This will cause confused seas for a
bit this evening, but fortunately combined seas should only be 5-6
ft at the time. S-SW winds may briefly gust 20-25 kt, but then
slowly ease overnight as the disturbance passes. South winds will
persist into Wed, but should only be 10-15 kt, easing from there
as high pressure returns Wed night and Thu. Thermal low pressure
re-strengthens over SW Oregon Friday, likely bringing a return of
gusty N winds to the Oregon and southern WA coastal waters.

One last thing - with the marine layer only about 1000 ft deep,
marine stratus/fog may be locally dense today. Webcams along the
coast suggest the dense fog isn`t widespread enough to warrant a
Dense Fog Advisory, but patches of dense fog reducing visibility
to just a few hundred yards are certainly possible.  Weagle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Heat Advisory from 11 AM this morning to 8 AM PDT Wednesday for
     ORZ111-112-120-122.

WA...Heat Advisory from 11 AM this morning to 8 AM PDT Wednesday for
     WAZ206-207-209-210.

PZ...Small Craft Advisory until 9 AM PDT this morning for PZZ272-273.

&&

$$

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