Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 290335 AAA

Area Forecast Discussion...Update
National Weather Service Portland OR
834 PM PDT Wed Oct 28 2020

Updated aviation discussion

.SYNOPSIS...High pressure aloft will remain over the region through
Thursday. Low-level inversions tonight and Thursday morning will
result in greater fog and stratus coverage. A weak frontal system
slides over the area Friday. High pressure and dry weather returns
this weekend leading to renewed valley inversions resulting in night
and morning fog. A change to a more progressive pattern looks to
begin around the middle of next week.


.SHORT TERM...Tonight through Saturday...Goes-17 RGB product valid
20Z showed generally clear skies. There was a small patch of low
cloud at the mouth of the Columbia River. Water vapor imagery
indicated strong 500 mb high pressure over the eastern Pacific, with
the main jet stream well to the north.

The upper level ridge remains in place tonight and Thursday morning
and then starts to weaken late Thursday as a frontal system
approaches Vancouver Island. Low-level inversions tonight will be a
little stronger compared to last night. Thus, anticipate greater fog
and stratus coverage late tonight and Thursday morning. Would not be
surprised if dense fog coverage is enough for a Dense Fog Advisory.
Min temps have generally been lower than guidance and see no reason
this will be any different for Thursday morning. Have gone a little
below guidance, especially for the interior valleys. Model forecast
850 mb temps valid 00Z Friday show little change from today. However,
interior valley max temps will be tricky, dependent on how
persistent the fog/stratus ends up. The 12Z HREF guidance suggests
stratus in portions of the Willamette Valley may persist into early
afternoon. Went slightly below most guidance for the interior

The aforementioned weak cold front is expected to reach the south
Washington and far north Oregon coast around 12Z Friday. Precip
chances gradually increase through the day Friday, with the highest
QPF, on the order of a quarter inch, focused on the south Washington
coast, Willapa Hills and far north Oregon Coast Range. The southeast
quarter of the forecast area, generally Albany to Willamette Pass,
may not get anything from this system. A few lingering showers will
be possible over the south Washington and north Oregon Cascades and
foothills Friday evening. The Friday frontal system will be a one-hit
wonder as high pressure quickly begins to rebound Friday night.
Models show the surface high pressure axis generally near a Newport
to Sweet Home line, with light northeast flow to the south of the
line and minimal surface flow to the north.

High pressure, surface and aloft, strengthen Saturday. The NAM, GFS
and ECMWF indicate thermally-induced surface low pressure over the
far south part of the forecast area expanding north during the day.
The air mass Saturday does not appear to be as warm as today or
tomorrow, based on model forecast 850 mb temps. However, the
low-level northeast flow should provide a little extra boost.

.LONG TERM...Saturday night through Wednesday...High pressure holds
firm Saturday night through Monday. Low-level valley inversions
become more established Saturday night. Sunday and Monday remain dry,
with daytime temperatures generally a little above average. However,
some areas in the interior valleys could end up a few degrees below
normal due to persistent inversions. A change in the large scale
pattern looks to begin Monday night or Tuesday. The operational GFS
and a majority of the ensemble members hold on to the ridge Monday
and then show more zonal flow beginning 12Z Tue.

The operational GFS and ECMWF drag another front across the area late
Monday night and Tuesday. The total mean 500 mb cluster   valid 00Z
Wed maintains slightly higher-than-normal heights. The ECMWF ensemble
mean shows around one-quarter inch QPF for KPDX and KSLE Tuesday. A
high majority of the individual ensemble members indicate some
measure of QPF. The zonal pattern continues through Wednesday, per
the deterministic and associated ensemble runs. However, based on the
pattern over the past several weeks, would not be surprised if future
model runs trended weaker with these fronts and maintained some
degree of ridging. With this in mind, opted to go with Superblend
POPS Tuesday night and Wednesday, which were lower than the NBM
guidance. Weishaar


.AVIATION...Skies are clear across the majority of the forecast
area early this evening, though there are some areas of marine
stratus near the coast KTMK northward. Given light winds and
already fairly tight surface dewpoint depressions, valley fog
should begin to develop by 06z, becoming fairly widespread by
12z. Fog may be locally dense, reducing visibility to less than
1/4 mile in some areas. VFR conditions should return most areas
by midday, though light northerly flow may keep low cigs locked
in a bit longer near KEUG.

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

KPDX AND APPROACHES...VFR conditions likely to persist through
around 08z, then expect deterioration as fog develops in the
area. IFR/LIFR is likely between 12z-16z, then a gradual return
to VFR is likely by midday Thursday as fog clears.  Weagle


.MARINE...High pressure offshore will maintain northerly winds
across the waters through Thursday, with the strongest winds
expected over the coastal waters south of Cascade Head. Seas will
stay in the 4 to 6 ft range through Thursday. Minimal change in
these conditions is expected through Thursday evening.

A front approaches the region late Thursday night and into
Friday morning. Expect light to moderate southerly winds through
Friday afternoon, with winds around 10 to 20 kt with potential
gusts up to 25 kt. Model agreement is improving but, each
successive model run is weakening the winds that will impact the
waters. Therefore, will not issue a small craft advisory at this
time as confidence is low. Will pass these observations to the
oncoming shifts. Seas will build on Friday but, will likely peak
between 7 and 9 ft Friday for most waters. The exception to this
is zone PZZ270, which could have seas close to 10 ft during this

Seas will subside and winds will return to a more northerly
direction through the weekend as high pressure rebuild across the
area. /42





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