Heat in Oregon, August 16 to 17, 2020

Area Forecast Discussion
National Weather Service Portland OR
255 AM PDT Sun Aug 16 2020

SYNOPSIS...Strong high pressure over the western US will maintain a
hot air mass over the Pacific Northwest today. A southerly flow
aloft will bring a chance of thunderstorms today and tonight, mainly
to the Cascades during the afternoon and evening. Temperatures will
slowly cool down closer to normal this week as the high pressure
weakens, and there is a chance for showers to return to the
northwest part of the area late in the week as a trough of low
pressure aloft over the east Pacific nudges closer to the coast.
&&

.SHORT TERM...Today through Tuesday...A very warm air mass remains
over most of the forecast area early this morning as a ridge of high
pressure aloft extends north from the Great Basin through the
Columbia Basin. With the ridge axis off to the east today, southerly
flow aloft will bring mid and upper level instability north over the
region. Models suggest the potential for shortwaves to lift north
over the region today and tonight as well, but are not consistently
handled by models. The most notable one seen in satellite imagery is
off the central California coast early this morning, which
potentially could act as a trigger over the forecast area late today
and tonight. Convective allowing models and the National Blend
tending to favor the Cascade crest and areas east in depicting
thunderstorms today and tonight, but model soundings show mid level
instability extending further west, so prefer to include a slight
chance of thunderstorms west into the Willamette Valley tonight.
Model 850 mb temperatures expected to be little changed today
compared to Saturday, suggesting many low level areas inland still
potentially making a run for 100F today. Southerly flow spreading up
the coast gets a boost late today and tonight as a surface ridge of
high pressure presses north. This also strengthens onshore gradients
through the Coast Range late today, but it may be too late in the
day to have an appreciable effect on high temperatures today in the
Willamette Valley. Southerly flow aloft today also brings the
potential for smoke and haze to spread up from fires over southwest
Oregon as seen in HRRR smoke modeling, so will keep a mention of
haze today for much of the area.

While there has been a dearth of marine clouds along the Oregon
coast overnight, meso models indicate a moist marine layer spreading
clouds up the coast tonight, and potentially spreading marine clouds
through some of the Coast Range gaps by Monday morning. The marine
air will bring some relief from the heat to inland areas, but it
will still be a slow cool down for Monday and into Tuesday as upper
heights are slow to fall in the southwest flow aloft. Model 850 mb
temperatures are still expected to be around 20C over the Willamette
Valley Monday afternoon which should be good for lower 90s, and then
down another degree or two Tuesday. Will keep a slight chance for
thunderstorms over the southern part of the Cascades late Monday as
the flow aloft veers around to the south-southwest, and the unstable
air mass extending from southwest to eastern Oregon clips the Lane
County Cascades.

.LONG TERM...Tuesday night through Saturday...High pressure aloft
is generally forecast by models to remain entrenched over the Great
Basin through the week as a trough of low pressure over the east
Pacific nudges closer to the coast. A modest onshore low level flow
coupled with a slow lowering of heights aloft will generally allow
temperatures to drop back closer to normals through the course of
the week. By Thursday to Friday models including the National Blend
suggest increasing chances for showers, particularly with a weak
front expected Friday. As the operational models continue to show a
general reluctance to drop 500 mb heights much under 580 dm, will
include low pops for showers mainly over the northwest part of the
forecast area, but will scale back pops some from the National Blend
values.

&&

.FIRE WEATHER...A thernally induced torugh of low pressure early
Sunday morning extended from near the mouth of the Columbia river
south through the Central Coast Range of Oregon. This trough will
shift further inland today, and east of the Cascades tonight. A hot
dry and unstable air mass will remain in place for much of today for
inland areas, but will see slightly cooler and more stable marine
air gradually spread inland from west to east late today and
tonight. Mid-level haines 6 conditions will persist into the evening
today for the Willamette Valley and the Cascades along with minimum
relative humidity values bottoming out from 15 to 25 percent again
in many areas, warranting a continuation of the Red Flag Warning for
these areas. The main fire concern with these conditions is for
rapid growth of any existing or new fires. The chance of
thunderstorms over the Cascades, and to a lesser extent west into
the Willamette Valley late today and tonight adds a threat of new
fire starts as most storms would potentially produce less than a
tenth of an inch of rain, although it is not out of the question
that isolated storms may produce more, from a tenth to a quarter of
an inch.
&&

.AVIATION...Expect VFR conditions through Sun. Early this morning
along the coast there is a very slight chance for some haze to
develop. However, with the atmosphere being quite dry with a
large dew point depression, there is low confidence of any haze
developing. Because winds have reversed to a southerly
direction, there is a chance that some smoke and haze could seep
into the Willamette Valley from nearby fires. Not expecting the
smoke to impact visibility and should remain elevated.

By Sun afternoon a more defined area of low pressure and associated
short wave will migrate north bringing in more moisture and slightly
stronger southerly winds. The intensified pressure gradient along
the Coast Range will also cause winds to be more southeasterly
pushing this moisture inland. Models are showing a chance for some
mid-level stratus and haze to move in along the coast towards the
end of the period. Likely will start first around KONP around 03Z
Mon then spread north along the coast. Could see IFR conditions
with vis and cigs, however, stuck with MVFR for now. Scattered
mid-level clouds are possible inland and along the Cascades. In
addition to the moisture, the atmosphere with this low pressure
will become more unstable thus providing a slight chance of
thunderstorms through the southern Willamette Valley, along the
Cascades and foothills.

METAR note: KTTD ASOS has several missing elements and will
likely not be repaired until Monday.

Refer to http://weather.gov/zse for detailed regional aviation
weather and hazard information.

KPDX and APPROACHES...VFR conditions through the next 24-hr.
There is a very slight chance that some nearby smoke may filter
in near the terminal but likely it will remain elevated with no
impacts. -Muessle

&&

.MARINE...Early this morning, weak low pressure will advance
northward causing a southerly wind shift. The southerly winds 10
to 15 kt with the highest gusts up to 20 kt will peak Sun night
and persist through Mon. As high pressure once again builds
across the Pacific NW by Tue, winds will become mostly northerly
with a slight onshore flow likely prevailing.

Northwesterly swell will be the dominate through the next
several days and will likely remain around 4 to 6 ft with a 10
second period. There is also a very slight southeasterly fresh
swell at around 2 ft with a 16 second period which will begin
Tue evening. Later in the week models suggest a stronger low in
the Gulf of Alaska may drive in increasing westerly swell with
seas increasing to 10 feet by late Fri but confidence remains low
with the timing and track of the system. -Muessle/DDH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Excessive Heat Warning until 9 PM PDT this evening for Cascade
     Foothills in Lane County-Central Columbia River Gorge-
     Central Willamette Valley-Greater Portland Metro Area-
     Northern Oregon Cascade Foothills-South Willamette Valley-
     Upper Hood River Valley-Western Columbia River Gorge.

     Red Flag Warning until 11 PM PDT this evening for Central
     Oregon Cascade Foothills-Mt. Hood National Forest West of
     Cascade Crest-North Oregon Cascade Foothills-Willamette
     National Forest-Willamette Valley.

     Heat Advisory until 9 PM PDT this evening for Cascades in Lane
     County-Central Coast Range of Western Oregon-Coast Range
     of Northwest Oregon-Lower Columbia-Northern Oregon
     Cascades.

WA...Excessive Heat Warning until 9 PM PDT this evening for Central
     Columbia River Gorge-Greater Vancouver Area-Western
     Columbia River Gorge.

     Red Flag Warning until 11 PM PDT this evening for Clark County
     Lowlands-Eastern Gifford Pinchot National Forest Mt Adams
     Ranger District-Extreme South Washington Cascades and
     Foothills.

     Heat Advisory until 9 PM PDT this evening for I-5 Corridor in
     Cowlitz County-South Washington Cascade Foothills-South
     Washington Cascades.

PZ...None.

&&

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