FXUS66 KMFR 302346

Area Forecast Discussion
National Weather Service Medford OR
346 PM PST Mon Nov 30 2020

.UPDATE...Updated Aviation Section.


SHORT TERM...Tonight through Wednesday night..A cold front has
moved through the forecast area this morning. Winds aloft
associated with the front are of moderate strength as indicated by
mountain wave clouds in visible satellite imagery this afternoon.
Precipitation with the front has diminished with only a few
hundredths being reported over the past 3 hours, focused from the
Cascades westward in lingering showers. Showers will continue to
diminish this afternoon, and then upper ridging will build through
the week. We expect a stagnant air mass to be in place from late
Tuesday through Saturday morning at least, with morning valley
fog/freezing fog and a minimal amount of mixing in the atmosphere.
Conditions will be favorable for pollutants to accumulate in
valleys. We're keeping the coast out of the Air Stagnation
Advisory for now because we think some north winds will keep that
area somewhat mixed, and some isolated areas west of the Cascades
(Ashland) and east of the Cascades could see some breezes that
keep things mixed better than other valley areas. That said, most
inland valleys west of the Cascades and east of the Cascades will
be included in the Air Stagnation Advisory, which will be issued
today and go into effect Tuesday evening.

Please take extra time for your travels during these periods of
fog/freezing fog. Additionally, watch out for slippery walking
surfaces where thin layers of ice can accumulate over time when
daytime high temperatures in valleys struggle to get out of the

LONG TERM...Thursday morning through Monday night...Not much change
in the extended period. Strong upper level ridging remains in the
forecast and another period of stagnant air with dry conditions and
overnight/morning fog and freezing fog for west side valleys remains
the story. Models show a trough approaching the area on Thursday,
however, it weakens considerably as it moves into the strong upper
ridge. The energy from this trough gets shunted to the south and any
moisture gets absorbed by the dry air from the ridge. It does,
however, help to shift the pattern some by knocking the ridge down a
bit. Another, stronger trough follows late Saturday into Sunday, but
it also weakens as it moves into the area. Deterministic models as
well as the vast majority of the ensembles maintain dry conditions
with this frontal passage. However, it may be enough to mix out some
of the stagnant air that will have settled and persisted in the
valleys during the previous week.

Looking past next weekend, the National Blend of Models, as well as
the vast majority of model ensembles for both the GFS and EC,
support mostly dry conditions continuing into the second week of
December. Model ensembles indicate a more appreciable change around
December 9th-11th. /BR-y

AVIATION...For the 01/00Z TAFs...Conditions have improved this
afternoon for most areas. However, MVFR/IFR conditions are occurring
in the Rogue and Klamath River Valley, with some higher terrain
likely obscured, especially along the Cascades. Expect stratus to
redevelop over west side valleys overnight, lowering to fog by
morning, resulting in LIFR/IFR conditions. With the recent rain,
conditions are favorable for fog at KLMT, but confidence of the
extent and duration is not low-to-moderate.

Breezy and gusty winds will begin to ease across the east side and
higher terrain overnight. Moderate northerly winds are anticipated
to along the coast tomorrow afternoon. -Miles

MARINE...Updated 200 PM PST Monday 30 Nov 2020...Gusty northwest
winds will continue into this evening as a long period northwest
swell moves into the waters. Seas of 14 to 17 feet at 15 to 17
seconds will peak into early this evening. Seas diminish Tuesday
into early Wednesday morning, but a thermal trough will develop at
the coast. Therefore high and steep seas remain with increasing
north winds over the waters. These winds will be strongest south of
Gold Beach. Seas then remain steep over the outer waters as they
briefly diminish Tuesday night into Wednesday morning. It's possible
the steep wind driven seas in the southern outer waters could reach
hazardous seas conditions for a 3-6 hour period Tuesday evening, but
confidence was not high enough to upgrade at this time. We'll let
the next shift take another look at this should future guidance

Models remain consistent in indicating a building long period west
to northwest swell Wednesday into Thursday with seas rising rapidly.
This is likely to combine with incoming tide and a shorter period
west-northwest swell at 13 seconds to bring an increased chance of
sneaker waves...with highest risk Wednesday but a continued risk as
seas build into Thursday. -BR-y/Petrucelli


OR...Air Stagnation Advisory from 10 PM Tuesday to 10 AM PST Saturday
for ORZ029>031.
Air Stagnation Advisory from 10 PM Tuesday to 10 AM PST Saturday
for ORZ023>026.
Beach Hazards Statement from late Tuesday night through late
Wednesday night for ORZ021-022.


Pacific Coastal Waters...Small Craft Advisory until 4 AM PST Wednesday for PZZ370-376.
Small Craft Advisory until 4 PM PST Tuesday for PZZ350-356.

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