Frost Advisory 27, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
1046 AM PDT Sat Oct 24 2020

Updated aviation discussion.

.AVIATION...For the 24/18Z TAFS...A cold front dropping through the
west side continues to bank clouds up against the Cascades and
produce a few light showers that are flying mostly under the radar.
As drier air works in, these clouds will dissipate over the
terminals, but occasional mountain obscurations will be possible
through the day. North-northwest winds will pick up in the wake of
the front today, then turn northeast overnight and become gusty over
the mountains and in exposed areas. This will lead to some mountain
wave turbulence tonight and Sunday morning. -Wright

&&

.PREV DISCUSSION... /Issued 901 AM PDT Sat Oct 24 2020/

DISCUSSION...The warm front that pushed into the area has stalled
out over the California border. As it moved through it did cause
rainfall in the area. The coast range received between 0.1" and
0.2", while lower elevations, Like Coos Bay, recieved between
0.05" and 0.1". Rain was more plentiful in the hills of Douglas
County, despite the Roseburg airport sensor reporting no rain,
with measurements from 0.05" to 0.4" in the surrounding hills.
Areas south of the Umpqua Divide were much more rain starved with
the only recorded rainfall just north of Grants Pass of 0.03".
Amounts in the Cascades are also quite light, but so is our obs
network, and with a few more hours of rainfall still possible
areas north of the Umpqua Divide from Roseburg to the Cascade
crest could still receive more light accumulations.

Clouds will be slow to recede today, with Siskiyou and Modoc
Counties being the only places likely to not have areas of clouds
throughout the day.

Have added areas of blowing dust to the forecast in Lake County,
mainly in the areas southwest of Summer Lake, Paisley, and Valley
Falls in anticipation of gusty NE winds ahead of the cold front.
The other area being around Christmas Valley.

For other impacts the approaching cold front will bring and
weather beyond this evening, please read the previous discussion
below.  -Miles

MARINE...Updated 200 AM PDT Saturday 24 Oct 2020...A thermal
trough will strengthen along the coast Saturday, then weaken
Monday. This will bring strong north winds and steep to very steep
seas to the area through Sunday night. The weakened trough will
remain in place through Wednesday, and gusty north winds and steep
seas will prevail into Wednesday, with very steep seas possible
over the southern outer waters.

PREV DISCUSSION... /Issued 345 AM PDT Sat Oct 24 2020/

DISCUSSION...Eventually, a cold front will push into the region
later today. This front should continue to bring light spotty
precipitation to locations along the Cascades. Cooler and drier
air will move in behind this front. The probability for
temperatures under 32 degrees Saturday night is about 70 percent
for the Rogue, Illinois ,Applegate, Shasta and Scott valleys. As
for the Umpqua Valley, the probability for temperatures under 32
degrees is around 20 percent. Models seem to be picking up on
warmer temperatures in that location. With the recent rainfall,
some patchy fog is also within the realm of possibility. Models
are not predicting fog at this time near Roseburg. Because of the
high potential for subfreezing temperatures, we'll keep the freeze
watch up.

As we move through Saturday night into Sunday, north east to east
winds will pick up across Oregon. This will lead to another round
of fire weather concerns. These winds will be pretty intense,
especially over the higher terrain and mid slopes. This is
discussed in more detail in the fire weather discussion below.

The cold dry air will remain settled in over southern Oregon and
northern California through Sunday night. Low temperatures will
trend lower by a few degrees. The probability of temperatures
less than 32 degrees is near 100 percent for most valleys west of
the Cascades and the probability of temperatures less then 28 is
about 50 percent. Sunday night will probably lead to a hard freeze
and the end of growing season for the west side valleys.

High pressure then begins to build off the coast into Monday. The
area of high pressure will start to nudge closer to the coast
throughout most of next week. Temperatures will trend warmer next
week with highs 5 to 10 degrees warmer than normal. We'll likely
remain dry through the end of October as high pressure remains in
control. There is a slight chance for some rain around Halloween
along the coast and into the Umpqua valley.

-Smith

FIRE WEATHER...Updated 200 PM PDT Friday Oct 21 2020...

The main concern ahead will be moderate to strong east to northeast
winds starting Saturday night and lasting into at least the mid
morning hours Monday.

A front will move into the northwest part of our area this evening,
then exiting Saturday morning. Strong high pressure building east of
the Cascades in combination with what we call an inside slider will
result in increasing east to northeast winds along and west of the
Cascades, especially over the mid slopes and ridges. At the same
time a strong thermal trough will develop along the coast and a
tight east to west oriented gradient will develop in the vicinity of
the Cascades, likely resulting in moderate to strong east winds,
especially for the mid slopes and ridges in Fire weather zone 623.

Breezy to gusty east winds, dry conditions, and widespread moderate
to poor recoveries are expected Sunday into Monday morning. Note:
ensembles are showing this to be possibly one of our more extreme
events for this time of year, with some model guidance suggesting
gusts up to 50 mph over the ridges of Curry, eastern Josephine, and
western Siskiyou Counties Sunday evening. Meanwhile, the Cascades
could see wind gusts to 45 mph and some of our valleys (southern
Rogue and most of of the Shasta Valley) could see wind gusts to 30
mph. Given confidence is high if this event, the Fire Weather Watch
has been upgraded to a Red Flag Warning. Please see RFWMFR for more
details. Areas not receptive to fire spread (the coast) will
continue to see a headline in the FWF.

Unlike the early September event, nights will be longer and
temperatures colder, especially at night which could result in a
stronger inversion in the valleys. If the inversion lingers into the
afternoon, then winds in the Rogue Valley could remain fairly light,
but we think the inversion will have a chance to break, especially
in the south end of the Rogue Valley near Ashland resulting in
gusty breezes there and possible up towards Talent and Phoenix.

The only caveat is precipitation. The front is expected to bring
some precipitation and the concerns could be mitigated somewhat
depending on how much rainfall there is, especially along and north
of the Umpqua Divide and Cascades from about highway 140 north.

There's good agreement the winds will diminish Monday morning as
the thermal trough weakens and moves inland. Therefore after Monday
in terms of winds, were not expecting any concerns. It will remain
dry for most, if not all of next week, although the ECMWF does show
a weak front approaching the area towards the end of the week.
-Petrucelli

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 8 AM Sunday to 11 AM PDT Monday for
     ORZ619-620-622-623.
     Freeze Watch from late tonight through Sunday morning for
     ORZ023.
     Freeze Watch from late tonight through Sunday morning for
     ORZ024-026.
     Red Flag Warning from 8 AM Sunday to 11 AM PDT Monday for
     ORZ621.

CA...Red Flag Warning from 8 AM Sunday to 11 AM PDT Monday for
     CAZ280.
     Freeze Watch from late tonight through Sunday morning for
     CAZ080>082.
     Red Flag Warning from 8 AM Sunday to 11 AM PDT Monday for
     CAZ281-282.

Pacific Coastal Waters...Small Craft Advisory until 5 AM PDT Monday for
     PZZ350-356-370-376.
     Hazardous Seas Warning from 5 PM this afternoon to 5 AM PDT
     Monday for PZZ370-376.
     Hazardous Seas Watch from late Sunday night through late Tuesday
     night for PZZ376.