Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 211744
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1043 AM PDT Sat Jul 21 2018

.Updated Aviation Discussion.

.DISCUSSION...Morning smoke covers areas south of the Umpqua Divide,
including southern Curry, Josephine, Jackson, Western Siskiyou,
and portions of Klamath County. Given another day of northerly
flow, we don`t expect this smoke coverage to change much.
Unhealthy air quality continues per observations for Jackson
County and Siskiyou County air quality sites. An Air Quality Alert
continues through Monday and highlights the expected continued
smoke impacts to air quality.

Today will feature gusty north winds again west of the Cascades
(similar to about 10 percent weaker than yesterday afternoon), and
there is a Red Flag Warning for the Illinois Valley and portions
of Eastern Curry County. These winds and dry conditions will
affect the Klondike wildfire.

These gusty northerly winds will shift on Sunday as we lose the
influence of the thermal trough. This represents a significant
change, and winds tomorrow for most areas will be lighter and more
from the west overall. We are introducing thunderstorm chances
tomorrow, and please see the previous discussion below for more
details on thunderstorm chances.

Temperatures today will be similar to yesterday, and in areas with
heavy smoke the high temperatures will be significantly lower than
guidance suggests. One area that won`t be impacted by smoke and
should reach temperatures that guidance suggests is the Umpqua Basin
to include Roseburg. Temperatures will be trending higher as the
week progresses, but again, smoke will likely keep most areas from
reaching the full potential that guidance suggests.

&&

.AVIATION...For the 21/18Z TAFs...Wildfire smoke continues over many
of the inland valleys west of the Cascades. Lower visibilities along
with MVFR conditions will exist near these fires will continue into
tonight. There`s a chance visibilities could be reduced further
tonight as smoke settles into the valleys.

Over the last three days, we have seen an initial burst smoke
reducing visibilities to IFR and LIFR around 0z when the fires north
of Medford get going. So look for very poor visibilities around 0z
at Medford Airport.

At the coast, the main concern will be gusty north winds, north of
Cape Blanco through this evening with gusts to 30 kts and VFR
conditions. Onshore flow is expected to increase enough for marine
stratus to form along the coast, north of Cape Blanco, including
North Bend tonight. The TAF shows lower conditions developing around
8z, but confidence on the timing is low to medium.

East of the Cascades and north of the Umpqua divide, VFR will
prevail for the TAF period. -Petrucelli

&&

.MARINE...Updated 230 AM PDT Saturday 21 July 2018...High
pressure offshore and a thermal trough at the coast will continue
gusty northerly winds with steep to very steep wind driven seas
through Sunday afternoon. Seas will be highest through this evening
with gales and very steep seas south of Cape Blanco beyond 5 nm from
shore, and advisory conditions elsewhere. Winds and seas will then
see a weakening trend into Wednesday, though steep seas may linger
in the outer waters south of Cape Blanco Sunday night through Monday.

The weakening will be due to the thermal trough moving inland with
high pressure building into the waters. Northerly winds may increase
on Thursday into Friday. -DW

&&

.FIRE WEATHER...Updated 430 AM PDT Saturday 21 July 2018...

North winds will trend lower on the west side of the Cascades but
west winds on the east Side of the Cascades will increase some.
Moreover, while moisture will increase some into the south, the
latest guidance keeps most of the 50percent 700-500mb layer RH in
Lassen County. We have removed the thunderstorm threat for this
afternoon.

Thanks to a disturbance to the north and some southerly flow to the
ridge levels, the thermal trough and gusty north wind pattern
weakens on Sunday. Models continue to show the thermal ridge moving
inland into the Cascades Sunday. This would create a very dry and
steep lapse rate near the ground. This will combine with cooler
temperatures moving in aloft and an unstable air mass developing
over much of the region. This will lead to a solid high level
Haines Index of 5 over SW Oregon and into the Cascades. This
covers just about all of our fire area.

Another concern for Sunday is increase in the H7-H5 layer into
Siskiyou and Modoc County. We`ve introduced slight chance of
thunderstorms from Mt Shasta northeast into the Medicine Lake area
for Sunday morning but confidence is not very high on this evening
until Sunday afternoon. Thunderstorm chances increase Sunday
afternoon but covers northern California zones and southern Jackson
County then east into southern half of Klamath and Lake County.
Currently we`ve kept thunderstorm threat out of Josephine and most
of Douglas County (except the Cascade crest of eastern Umpqua NF).
Most of Klamath and Lake Counties will be under thunderstorm
threat Sunday evening except the far northern end of those
counties.

Instability weakens but remains over the region Monday, but with
weak steering winds anything that develops should latch to terrain. -
FB

&&

.PREV DISCUSSION... /Issued 428 AM PDT Sat Jul 21 2018/

DISCUSSION...Widespread smoke from the Umpqua Divide south will
wreak havoc with high temperature forecast today. Had there been
no smoke, southwest Oregon would likely climb to around 100 again.
Outside the smoke area high temperatures are likely to be up to 5
degrees higher today than yesterday. Thunderstorm potential for
this afternoon and evening is even smaller with this newest model
run with relative humidity of 50% or more in the 700-500mb layer
confined south of Modoc County. We have removed the threat of
thunderstorm for today.

However Sunday is another matter. Southerly flow appears stronger
as the trough over the Pacific NW starts to lift. We are
concerned with Sunday morning as models bring the 700-500 layer of
60% into Siskiyou and Modoc County in an arc from southern Scott
Valley through Tulelake then back into SE Modoc County. The areas
of greatest lift and instability appears to be near the Medicine
Lake area and we have introduced slight chance of thunderstorms
there during the morning half of Sunday. The remainder of our
forecast area should see increasing mid and high clouds. By Sunday
afternoon the moisture spreads into southern half of Jackson
County and southern Oregon Cascades, then east into the Klamath
Basin and eastern part of the Fremont-Winema NF and southern half
of Lake County. This is in line with previous forecast. We`ve kept
thunderstorm threat out of Crater Lake or the Umpqua Basin during
the day on Sunday.

By Sunday evening more moisture spreads north and east. It appears
thunderstorms will miss Josephine County but southeastern and
eastern part of Jackson County could be under threat, as well as
Crater Lake and north. We`ve kept thunderstorm threat along the
Cascades north of Crater Lake, mainly in the far eastern part of
Douglas County, and kept the northern extent of thunderstorm
threat to around Chemult in northern Klamath County.

On Monday there will be less moisture to work with but there are a
few short wave disturbance that could cause a few thunderstorm
buildups over the Cascades and Siskiyous. One thing that could
mitigate this threat is the ridge axis is sharpening and moving up
to our area which could help to suppress upward movement a bit.

The first half of next week will be quite warm with 100-105 highs
possible, depending how much smoke shading the area will be
under. -FB

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Fire Weather Watch from Sunday afternoon through Sunday evening
     for ORZ621>625.
     Red Flag Warning from 1 PM this afternoon to 9 PM PDT this
     evening for ORZ619-620.

CA...Fire Weather Watch from Sunday afternoon through Sunday evening
     for CAZ280>282-284-285.

Pacific Coastal Waters...Small Craft Advisory until 5 PM PDT Sunday for PZZ356-370-376.
     Small Craft Advisory for hazardous seas from 2 AM to 5 PM PDT
     Sunday for PZZ350-356-370-376.
     Hazardous Seas Warning until 2 AM PDT Sunday for PZZ356-370-376.
     Small Craft Advisory until 2 AM PDT Sunday for PZZ350.
     Gale Warning until 2 AM PDT Sunday for PZZ376.

$$


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