Wildfire in Oregon, October 1 to 4, 2020

Area Forecast Discussion
National Weather Service Medford OR
910 PM PDT Wed Sep 30 2020

.DISCUSSION...The forecast has been updated to extend the coverage
of smoke and haze across the area through the short term portion,
through Friday night. As new model data comes in, the smoke
forecast will be adjusted and extended out in time. An Air
Quality Advisory /PDXAQAMFR/ remains in effect through Saturday
afternoon. There will be relatively little day-to-day change in
conditions through Saturday. The smoke coverage is a complicating
factor in temperature forecast. It generally limited heating by 5
to 10 degrees this afternoon.

A very weak shortwave on Saturday does not even look to be
sufficiently strong to generate any high level clouds. But, it
will put a dent in the persistent ridge and bring a few degrees of
cooling. A stronger (though still very dry) cold front to follow
on Sunday will bring additional cooling and a stronger westerly
flow, which is likely to help improve air quality...though more
in Oregon than in northern California. A new ridge looks to build
in early next week. It will not be as strong or long-lasting as
this week's ridge. A change to a seasonable pattern is likely to
begin beyond day 7...during the second half of next week.

&&

.AVIATION...For the 01/00Z TAFs...Over the coastal waters and along
the coast...widespread LIFR/IFR cigs and MVFR/IFR vsbys due to low
clouds and fog is expected through Thursday morning, then conditions
will improve to MVFR cigs by early afternoon with partial clearing
to VFR in the afternoon.

Inland...VFR conditions will prevail through the TAF period. Smoke
aloft may result in some low end VFR cigs, and possible MVFR vsbys,
but confidence was not high enough to include in the forecast. -CC

&&

.MARINE...Updated 800 PM PDT Wednesday 30 September 2020...Seas at
or below 6 feet will persist through tonight with light north winds.
Coastal stratus and fog will produce periods of reduced visibilities
through the morning hours. Although sub-advisory conditions are
expected to continue into the weekend, seas will build Thursday as a
long period northwest swell arrives early Thursday morning.
Northwest swell of 8 to 9 ft at 14 to 15 seconds peaks Thursday
afternoon and evening, then seas gradually subside early Friday.
Seas will transition to wind wave dominated late this weekend as a
thermal trough develops late Saturday night and strengthens into
Sunday. This will mark a return of gusty north winds and steep wind
driven seas, especially south of Cape Blanco, Sunday into Monday.

&&

.PREV DISCUSSION... /Issued 246 PM PDT Wed Sep 30 2020/

DISCUSSION...Dry weather will continue for the next 7-10 days.

The latest satellite image shows mainly elevated smoke moving from
the south into the area. However there has been signs of the smoke
thinning out. The exception is on the Klamath River, Scott and the
Shasta Valley and in and around Mount Shasta City where smoke is
more concentrated and has settled in. Northern Klamath and most of
Lake County is clear, but some elevated smoke will work into these
areas late this afternoon.

The elevated smoke was more extensive in southwest Oregon for most
of the morning before thinning out and this has resulted in a much
slower warm up compared to yesterday. For example, Medford is 15
degrees, Montague 10 degrees and Roseburg 7 degrees lower compared
to this time yesterday (noon pdt).

With that said, it appears we could be dealing with the smoke for
the next couple of days and possibly into next weekend. The high
resolution smoke models show the smoke through this evening will be
elevated which could end up being the case tomorrow. However the
details beyond tonight is not as clear, so stay tuned for the latest
updates.

Meanwhile marine stratus cover most of the marine waters, the coast
including the coastal valleys and Coquille Basin. However the marine
stratus is showing signs of retreating towards the coast, so we
expect the coast to clear out for a brief time (an hour or two) late
this afternoon and evening. Marine stratus will return to the same
areas tonight and linger through the morning hours Thursday.

Overall, little variation is expected in the weather for the next
couple of days. Friday night, the upper ridge will flatten out some
as weak shortwave rides over the ridge. The main affect from this
will be an increased onshore flow which could push marine stratus
into the Umpqua Basin and possibly get banked up along the Umpqua
Divide. Afternoon temperatures Saturday will be cooler, especially
north of the Umpqua Divide due to the marine stratus expected to
linger for most of the morning.

Another weak disturbance will move north of the area Sunday with
basically the same net result as Saturday. A thermal trough will set
up along the south coast Sunday night into Monday with breezy
northeast winds near and at the ridges.

There's fairly good agreement it will remain dry for most of next
week with above normal temperatures. The operational models do show
cooling and non zero chance of pops by next Thursday. Beyond next
Thursday, several of the individual ECMWF ensemble members are
showing increasing chance of precipitation by the following weekend.
This is also supported by the ECMWF and GEFS ensemble mean which
shows a possible shift in the pattern with an upper trough setting
up over the Pac Northwest. It's still a ways out there so this could
change, but the past couple of runs have been showing this. Thus,
it's possible we could be heading into a cooler and wetter pattern
by the following weekend. -Petrucelli

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...None.

CA...None.

Pacific Coastal Waters...None.

DW/CC