Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 301024

Area Forecast Discussion
National Weather Service Portland OR
316 AM PDT Fri Jul 30 2021

.SYNOPSIS...Hot weather continues today with a warm night and
Saturday morning. Temperatures cool several degrees Saturday and a
few more Sunday, but highs are expected to remain slightly above
normal through mid week.  Thunderstorms in the Cascades should refire
this afternoon for the Cascades, and possibly to the Willamette
Valley into the late evening. Off and on chances for showers look to
continue for the Cascades through early next week.


.SHORT TERM...Today through Sunday...The PacNW remains on the
periphery of an upper high pressure centered over the Great Plains.
Meanwhile an upper low near the Gulf of Alaska will swing a trough
that will approach the PacNW.  This will turn the upper flow more
southerly over the area today. Currently showers are east of the
Cascade crest, but may see some elevated showers approaching Lane
County early this morning.  Thunderstorm chances increase for the
Cascades this afternoon as a batch of mid level moisture combines
with the developing unstable air mass. Can`t rule out elevated
thunderstorms over Willamette Valley & southwest WA lowlands this
afternoon into tonight. Most models of HREF simulated radar
reflectivity have showers extending over the valley late this
afternoon and tonight. Model soundings also support convection over
the valley.  Some models are suggesting there may be some convection
into the Coast Range, but confidence remains very low so have left
out of the forecast. QPF amounts will probably be low a few
hundredths to maybe an isolated tenth of an inch or so.
Thunderstorm threat diminishes Saturday and Sunday, and should be
limited to the Lane and Linn Co. Cascades.

The heat continues today with max temperatures a few degrees higher
than Thursday. So expect to see more locations reaching/exceeding 100
degrees and a few locations in the 100-103 degree range. NBM guidance
has been verifying close to the 80th percentile and the current
forecast reflects that.  A little cooling is expected Saturday with
850 mb temps down a notch. But with more cloud cover expected into
Saturday afternoon, expect max temps in the mid 90s despite warm
overnight lows in the upper 60s. This will be a tricker forecast
because of the potential variability of the cloud cover.

Increasing onshore flow Saturday into Sunday along with lowering
upper height will lead to noticeably cooler Sunday.  But temperatures
will still be above seasonal averages./mh

.LONG TERM...Sunday night through Wednesday...Long range models agree
that a northward-moving, negatively-tilted shortwave trough will pass
through the area during the Sunday night through Monday afternoon
timeframe. This feature could promote another round of showers
Sunday night.

Beyond Sunday night, the deterministic models diverge with the
Canadian and GFS suggesting a more progressive pattern to bring in
the quasi-stationary upper-level low currently in the Gulf of Alaska.
The ECMWF is holding that system in place through midweek. A more
progressive pattern would favor cooler temperatures, and perhaps a
low chance for widespread rain (without thunder). On the other hand,
a slower pattern - such as the one depicted by the ECMWF - would keep
us warmer, and would likely favor a continued chance for thundery
weather along the Cascades through the end of the period. With
confidence lacking, it seems prudent to stick with the NBM`s guidance
at this time, but chances are the forecast will swing in one of two
directions depending on which scenario plays out. -Bumgardner


.AVIATION...Satellite fog product shows a solid area of IFR marine
stratus over the coastal waters, affecting coastal areas and
some adjacent valleys. The marine layer remains shallow at just
over 1000 ft deep, so we are not expecting stratus to reach the
Willamette Valley this morning. The shallow marine layer should
allow clouds to clear along the coast for the midday and
afternoon hours, only to return by late evening.

Meanwhile, VFR conditions should prevail inland through this
evening. Scattered thunderstorms are expected to develop along
the Cascades this afternoon. Related high-based showers and
isolated thunderstorms may drift as far west as the Willamette
Valley this evening, but conditions should remain VFR.

For detailed regional Pac NW aviation weather information, go
online to:

KPDX AND APPROACHES...VFR through the forecast. Light N-NW sfc
winds in the morning will increase to around 10 kt later this
afternoon and evening. Ground crews can expect a hot day, with
afternoon high temperatures approaching 100 degrees. Isolated
high-based showers and thunderstorms are possible after 22z, but
confidence is low in the exact timing and location.


.MARINE...Typical summertime N-NW wind pattern continues across
the coastal waters, with high pressure offshore and thermally
driven low pressure inland. Hot temperatures across the
Willamette Valley will likely strengthen thermal low pressure,
potentially weakening northerly pressure gradients along the
central Oregon coast. As such, our southern inner waters have
been left out of the Small Craft Advisory this afternoon, but all
other zones should see gusts to 25 kt. High pressure looks to
strengthen well offshore this weekend, likely resulting in
stronger N-NW winds in general for the coastal waters.

Seas will be mainly comprised of wind waves, with little to no
background swell through early next week.  Weagle


OR...Red Flag Warning from 11 AM this morning to midnight PDT
     tonight for Central Oregon Cascade Foothills-Mt. Hood
     National Forest West of Cascade Crest-North Oregon Cascade
     Foothills-Willamette National Forest.

     Heat Advisory until 11 AM PDT Saturday for Cascade Foothills in
     Lane County-Central Columbia River Gorge-Central
     Willamette Valley-Greater Portland Metro Area-Lower
     Columbia-Northern Oregon Cascade Foothills-South
     Willamette Valley-Upper Hood River Valley-Western Columbia
     River Gorge.

WA...Heat Advisory until 11 AM PDT Saturday for Central Columbia
     River Gorge-Greater Vancouver Area-I-5 Corridor in Cowlitz
     County-South Washington Cascade Foothills-Western Columbia
     River Gorge.

PZ...Small Craft Advisory until 11 PM PDT this evening for Waters
     from Cape Shoalwater WA to Cascade Head OR from 10 to 60

     Small Craft Advisory from 11 AM this morning to 11 PM PDT this
     evening for Coastal waters from Cape Shoalwater WA to
     Cascade Head OR out 10 NM-Waters from Cascade Head to
     Florence OR from 10 to 60 NM.



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