Dense Smoke Advisory 2, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
932 AM PDT Fri Sep 11 2020

.DISCUSSION...The latest visible image shows smoke pretty much
over the entire forecast area and it's thickest west of the
Cascades. the concentration of smoke near the surface may end up
less in the afternoon as the mixing height increases, but that
does not mean improvement. No updates are needed for this morning.
Please see previous discussion for more details.

&&

.AVIATION...For the 10/12Z TAFs...Wildfire smoke will be the main
impact to aviation for the next 24 hours. Right now, IFR/LIFR smoke
is affecting most areas near and west of the Cascades, including
KOTH, KRBG and KMFR.  Marine stratus/fog also continues to limit
ceilings and visibility over the coastal waters and along the coast.
Expect conditions in these areas to generally waver from LIFR to IFR
(perhaps MVFR briefly) and back to LIFR through Friday.

Expect smoke to spread east of the Cascades overnight with
visibility reduced to IFR/LIFR, even at KLMT.  This spread of smoke
to the east will be more prolific Friday.

&&

.MARINE...Updated 900 AM PDT Friday 11 September 2020....Weak
southerly winds today will become a mix of light northerly and
variable winds on the weekend. Low seas will persist through
Sunday. Marine stratus or fog will affect the area today and into
this weekend. Visibility may continue to be reduced by wildfire
smoke at times. Early next week, south winds will increase as a
front approaches the area and steep, choppy seas may develop
across the waters. -CC/Petrucelli

&&

.PREV DISCUSSION... /Issued 345 AM PDT Fri Sep 11 2020/

DISCUSSION...Adjusted MaxTs to account for smoke in the valleys
using todays observed MaxT and reducing a few degrees further for
increasing smoke concentrations. Wind information can be found in
the fire weather section below.

Smoke impacts will continue to spread across the entire forecast
area through the weekend, from west to east as we get more
westerly flow on area fires. One area that may eventually see a
reprieve from significant smoke impacts in the coast, but smoke
that spans hundreds of miles offshore will still need to push
through the area before any clearing.

Attention turns to an upper trough that will be approaching early
next week. The impacts will begin Sunday, as winds begin to pick
up along and east of the cascades. This bring another round of
critical fire weather conditions. Rain chances enter the forecast
on Monday, and for now our messaging is this: There's a 60%
chance of rain generally from the Umpqua Valley south to the
Illinois Valley and westward. Not currently anticipating much if
any rain reaching eastern Lake County.

Thunderstorm potential...Tues., Wed., and Thurs...Have kept it
out on Tuesday because of smoke; confidence is moderate .
Wednesday, there could be less smoke after a few days of windier
conditions. There is also agreement in the GFS and EC for negative
LIs with an area of 500+ J/kg of CAPE. Even if smoke exists in
valleys, it seems reasonable that ridges will be more cleared by
increasing winds to support better thermal heating. With low-to-
moderate confidence (NBM Tstorm probs 15-20%), introduced a slight
chance for Tstorms from Roseburg to Chemult and south from
Chemult along the Cascade ridge to Howard Prairie. Climatology
suggests that areas south of here are not climatologically favored
for lightning, however the area of CAPE and LIs does south to the
Trinity horn. Will look for this signal to remain in the next few
runs before extending south. Thursday is a difficult forecast
situation, while low potential does exist cut off lows are
difficult for models to predict and the ensembles are struggling
to resolve how the low and ridge will respond to each other. While
the potential is not zero, confidence is far too low to introduce
Tstorms at this time.  -Miles

FIRE WEATHER...Updated 200 AM PDT Friday 11 September 2020...The
area will remain under the influence of an upper ridge with
westerly/onshore flow through Saturday. West to southwest afternoon
breezes will develop in the afternoons and evenings, especially for
areas west of the Cascades today then for most all inland valleys on
Saturday. For the most part, we're not expecting gusty winds, but
valleys west of the Cascades will see an uptick in breezes late in
the afternoon and evening. Winds in these areas will drop off pretty
quick around sunset with generally light winds expected for the
overnight hours.

Overall, humidity recoveries will gradually improve from west to
east and recoveries this morning will be significantly better than
recent nights. Daytime humidities, however, will remain low through
the weekend with teens expected across inland west side valleys and
single digits for valleys east of the Cascades. Warm weather will
persist into the weekend. No critical fire weather patterns are
currently expected through Saturday. This will change Sunday, as the
upper ridge shifts east due to an upper trough approaching the area.
Southwest winds ahead of this trough will increase, and gusts of 30
to 40 mph are possible Sunday afternoon for the areas east of the
Cascades. (portions of FWZ 624, 625, 284, and 285).

Monday night into Tuesday could end up being the first day in quite
some time where we could see some measurable rainfall at the coast
and inland westside locations. Operational models are starting to
come into better agreement with the timing of the initial front
(which could be at the coast late Monday afternoon, then inland
westside Monday night). Confidence is low in the details at this
time, however, since typically, the first front of the season
weakens considerably as it moves into the very dry air from the
ridge. However, numerous ensembles are in agreement with their
operational counterparts in showing precipitation arriving to the
area late Monday into Tuesday. Confidence in precipitation is higher
for coastal locations and north of the Umpqua Divide, then
confidence decreases for locations farther east and south. At the
least, we'll see a trend towards cooler temperatures and slightly
higher humidities, especially daytime humidities, but the one thing
we'll have to keep a watch on is continued gusty winds along and
east of the Cascades. Overall, expect a transition to a cooler
pattern with increased chances for precipitation.

Wednesday, the upper low may remain just off the coast or near
the northern California - Oregon border. This may result in
additional moisture and some instability moving into the area,
resulting in additional chances for rain along with a slight
chance of thunderstorms for central portions of the area.

-CC

AVIATION...For the 10/06Z TAFs...Wildfire smoke will be the main
impact to aviation for the next 24 hours. Right now, IFR/LIFR smoke
is affecting most areas near and west of the Cascades, including
KOTH, KRBG and KMFR.  Marine stratus/fog also continues to limit
ceilings and visibility over the coastal waters and along the coast.
Expect conditions in these areas to generally waver from LIFR to IFR
(perhaps MVFR briefly) and back to LIFR through Friday.

Expect smoke to spread east of the Cascades overnight with
visibility reduced to IFR/LIFR, even at KLMT.  This spread of smoke
to the east will be more prolific Friday.

-Spilde/Smith

MARINE...Updated 200 AM PDT Friday 11 September 2020....Weak
southerly winds today will become a mix of light northerly and
variable winds on the weekend. Low seas will persist through Sunday.
Marine stratus or fog will affect the area today and into this
weekend. Visibility may continue to be reduced by wildfire smoke at
times. Early next week, south winds will increase as a front
approaches the area and steep, choppy seas may develop across the
waters. -CC

&&

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

CA...None.

Pacific Coastal Waters...None.