336 
FXUS66 KPQR 131655 AAA
AFDPQR

Area Forecast Discussion...UPDATE
National Weather Service Portland OR
340 AM PDT Mon Sep 13 2021

Updated Aviation Discussion

.SYNOPSIS...Seasonable mid-September weather is expected through
Thursday with varying degrees of cloudiness and onshore flow.
Significant changes will begin Friday as an autumn-like front brings
widespread rain and blustery winds to the Pac NW, with showers and
cool temperatures lingering through the weekend. Over an inch of rain
is possible by the end of the weekend, especially along the coast and
in orographically favored portions of the Coast Range and Cascades.

&&

.SHORT TERM...Today through Wednesday...The next three days will be
seasonable with a mix of sun and clouds and comfortable temperatures
as seasonable westerly flow persists aloft. A flat upper level ridge
will gradually increase 500 mb heights and 850 mb temperatures today
through Tuesday, yielding a slow warming trend. Temperatures were
generally stuck in the 60s and 70s for highs Sunday. Expect similar
to slightly warmer temperatures today, with more highs in the 80s
probable for Tuesday if the 00z HREF is correct in holding off clouds
for much of the day. Another decaying front from the Pacific will
fall apart while moving onshore Tuesday night. Latest model runs have
increased the low-level moisture associated with this front, while
some shortwave energy may wring out some drizzle through Wed morning.
Accumulations should generally be less than a tenth of an inch, and
primarily for our north coastal zones. Clouds will linger into
Wednesday, which combined with a cool air mass should hold
Wednesday's highs down in the mid 60s to mid 70s for much of our
forecast area, beginning a prolonged period of near to below normal
temperatures.  Weagle

&&

.LONG TERM...Thursday through Sunday...Strong and cool high pressure
will build into the Pac NW Wednesday, ushering in a maritime polar
air mass that keep temperatures a few degrees below normal Thursday
afternoon despite plenty of sunshine. In fact, excellent radiational
cooling is expected Wed night, which may result in the coolest
morning so far this season Thursday morning. It is not out of the
question that a few Sep 16 record lows would be challenged Thursday
morning if skies remain clear - especially some of the "weaker"
records such as Hillsboro's 38 degrees and Astoria's 41 degrees. 

The cool air mass will also serve a purpose as we head into Friday:
It will provide a cool overrunning surface for isentropic lift as an
incoming front spreads subtropical moisture into the area later
Friday into early Saturday. This will help maximize precipitation
potential as the first truly autumn-like front of the season barrels
onshore Friday/Friday night. Models have been very consistent the
last several cycles that a potent Pacific frontal system will spread
rain onshore sometime Friday, becoming locally heavy during the
afternoon and overnight as an atmospheric river becomes involved with
the front. Latest CW3E analyses based on 00z EC/GFS ensembles now
suggest a moderate to borderline strong atmospheric river with IVT on
the order of 750 kg/(ms). Fortunately, most models suggest a fast
progression of the front through the forecast area, so it does not
appear QPF will be excessive from this event. That said, rain rates
may briefly reach levels sufficient to cause debris flow and/or flash
flooding issues in recently burned areas. The front may also have
some decent wind associated with it as it moves onshore late Friday,
as several ensemble members hint at gusts in excess of 40 mph along
the coast and gusts in excess of 30 mph for Portland and Salem
sometime late Friday through early Saturday.

Post-frontal showers will be significant through Sunday in
orographically favored areas. With snow levels lowering to near 6000
feet, a brief dusting of snow is possible as low as Timberline Lodge.


Between Friday and Sunday, expected rainfall ranges from 0.50 to 1.50
inches for the inland valleys, 1-2 inches for the coast, and 1.50 to
3 inches for elevated terrain, with possible locally higher amounts
in places with best orographic enhancement. Given rivers are at
seasonal baseflow, no river flooding is expected from this event,
though fast rises may occur on some of the flashier rivers like the
Grays in Wahkiakum County.  Weagle

&&

.AVIATION...Dry northwesterly flow aloft will continue across the
region through early Tuesday morning. MVFR stratus across the
northern Willamette Valley and southwest Washington is expected
to dissipate by around 18Z-19Z today. Otherwise VFR conditions 
are expected to persist over the next 24 hours. The main 
exception will be along the northern coast later this evening and
overnight. Shallow marine stratus and IFR conditions will be 
possible near KAST after 08Z Tuesday. HREF guidance currently 
shows only a 20-30% chance that low clouds develop through the 
interior between 12Z to 16Z Tuesday morning. 

For detailed regional Pac NW aviation weather information, go 
online to: https://weather.gov/zse

KPDX AND APPROACHES...MVFR stratus with cigs around 025 this
morning will likely last to around 18Z-19Z. VFR and mostly clear
skies then expected through the remainder of the period.
Northwest wind should remain under 10 kt. /DDH

&&

.MARINE...Weak high pressure will build back in across the 
region today and will allow the thermal low to build to on the
south Oregon coast. Northerly winds increase again across the 
central waters in the afternoon for another round to Small Craft
winds gusting to 25 kt through early evening.  High pressure 
will weaken Tuesday into Wednesday allowing a series of weak 
fronts to move across the waters during the second half of the 
week. Seas will trend around 6 ft at 10 seconds through the week.
/mh

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory from 2 PM this afternoon to 8 PM PDT this 
     evening for coastal waters from Cascade Head OR to Florence OR out 60 nm.

&&

$$