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Area forecast discussion...updated
National Weather Service Portland or
1000 am PDT Sat Sep 21 2019

Updated aviation discussion

Synopsis...brief high pressure will bring clearing skies this
afternoon. However, clouds will be on the increase again tonight
ahead of a cold front that will spread more rain across the forecast
area Sunday. Sunday night may be the coolest night so far this
season followed by dry and mild weather Monday. Additional frontal
systems may bring more wet weather Tuesday and again later in the
week with temperatures remaining a little cooler than typical for
late September.

&&

Short term...today through Monday...made a quick update this
morning to adjust cloud cover to better reflect reality. Also added
drizzle to the north coastal areas where Langley Hill radar is
getting support from kast and camera obs. Rest of the previous
discussion follows. /Jbonk

Otherwise, areas of morning low clouds and fog will give way to
increasing sunshine throughout the day, allowing temperatures to
climb close to late-September normals with lower to mid 70s for the
inland valleys and mid 60s to lower 70s along the coast.

Model time-heights and 00z href suggest high clouds will increase and
thicken this evening as a frontal system now crossing 135w-140w
approaches the region. Rain will likely move onshore by dawn Sunday,
then spread inland and south across the forecast area during the day
Sunday. Jet forcing continues to look weak ahead of this system, with
winds generally 50 kt or less above at 500 mb and above. This will
keep winds modest ahead of the front, and should limit the potential
for orographic enhancement rainfall as the front moves through.
Therefore we are expecting quantitative precipitation forecast to be more uniform across the area
than the past couple fronts, at least based on terrain. Most
locations in our forecast area should receive around 0.25" from this
system, plus or minus 0.10" or so. Model soundings suggest some weak
instability/thin cape along and just behind the front, which may
introduce convective elements which could locally push rainfall into
the 0.50 to 1.00" range. There will be a brief window for isolated
thunder Sunday afternoon and evening as the associated cold pool
aloft swings through. The 06z NAM continues to suggest a capping
inversion will quickly develop around 700 mb Sunday evening, which
should cause showers to taper off quickly overnight.

High pressure builds into the region Sunday night and early Monday,
which may lead to rapid clearing for the valleys. Given the cool air
mass left behind (with 850 mb temps around +5 deg c) and improving
conditions for radiational cooling, the stage may be set for the
coolest night so far this season for many locations within the
forecast area. Most locations will likely dip well into the 40s
Sunday night, with some higher elevation valleys possibly getting
into the upper 30s. Is this enough for records monday? Not even
close. The record low for Monday is a frosty 31 degrees at Hillsboro
and Eugene, while the record is 40 for pdx. Our forecast lows for
Monday are a good 10 or more degrees warmer than these temperatures.
Still, it will be a noticeable touch of autumn after a Summer where
mild to warm nights have been the norm.

After a cool start with areas of valley fog Monday morning, high
pressure should result in increasing sunshine Monday afternoon with
temperatures rebounding to the mid to upper 60s for The Lowlands. One
tricky aspect about Monday may be how slow morning fog/low clouds are
to clear; with little low-level flow to speak of west of the
Cascades, mixing may be slower than usual to occur. With Monday being
the first day of astronomical autumn, the sun isn't as strong as it
was a few weeks ago but should still be enough to poke some decent
sunbreaks into the stratus deck Monday afternoon. Weagle

Long term...Monday night through Friday...strong but flat upper
level ridging develops across the NE Pacific Monday and Tuesday,
resulting in fairly strong zonal flow into the British Columbia/Washington through midweek.
This may clip our primarily our northern zones with some light rain
at times, mainly late Monday night/early Tue and again Wed night.
Areas Portland-Tillamook northward will likely remain cloudier
through midweek than areas to the south, due to their proximity to
the above-mentioned zonal jet. Therefore areas Salem southward have a
better chance of reaching the 70s Tuesday and potentially Wednesday,
though our forecast for Wed is a little cooler due to the potential
for increasing high clouds. Zonal flow begins to buckle Wednesday
night as a deep upper trough begins to develop over western Canada,
pushing into the pac northwest Thursday and Friday. The 00z European model (ecmwf) remains
more amplified/less progressive with the late-week upper trough,
keeping the trough axis offshore through Saturday while the 00z GFS
pushes the trough axis onshore by Friday. Either solution would be
chilly for our forecast area, but the placement is important in
determining whether the bulk of associated shower activity remains
over our forecast area or east of the Cascades, and whether or not
snow levels will dip low enough to bring the first snow of the season
to many of our ski resorts and Cascade passes.Weagle

&&

Aviation...expect a mix of conditions to continue through midday
with widespread MVFR cigs and local IFR stratus in the interior
lowlands. Conditions then become VFR between 18-21z, with some
sunbreaks expected this afternoon and evening due to variable mid
and high clouds moving across the region ahead of an approaching
front. This front will bring the return of light rain and IFR
conditions to the coast tonight, with rain and reduced flight
conditions spreading inland during the overnight hours.

Kpdx and approaches...MVFR cigs will continue through midday. VFR
conditions is then expected through tonight with variable mid and
high clouds. MVFR cigs under light rain likely after 10z tonight.
/64

&&

Marine...a transient surface ridge over the waters will shift
inland this evening. A front will move across the waters tonight
and ashore Sunday morning with high pressure building over the
waters Sunday afternoon. Expect Small Craft Advisory conditions
over the northern marine zones ahead of the front. The northern
outer zones should have northwest wind gusting 25 kt into Sunday
evening as high pressure build behind the front. Right now, not
expecting Small Craft Advisory conditions for the central Oregon
marine zones but future model runs may increase winds into the
advisory category. The next front for later Monday looks to be a
little weaker than the Sunday front.

Seas will remain in the range of 4 to 6 ft through Saturday evening.
The cold front Sunday will boost seas into the 8 to 10 ft range
through Sunday night. A west to northwest fetch developing in the
northeast Pacific will bring building seas to above 10 ft around mid
week.

&&

Pqr watches/warnings/advisories...
or...none.
Washington...none.
Pz...Small Craft Advisory from 3 am to 11 PM PDT Sunday for waters
from Cape Shoalwater Washington to Cascade Head or from 10 to 60
nm.

Small Craft Advisory from 3 am to 1 PM PDT Sunday for coastal
waters from Cape Shoalwater Washington to Cascade Head or out 10
nm.

&&

$$

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