Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 212214

Area Forecast Discussion
National Weather Service Medford OR
314 PM PDT Mon Aug 21 2017

.DISCUSSION...Flow has shifted from northeasterly to southwesterly aloft,
and smoke from the Chetco Bar fire is streaming across the
forecast area west of of the Cascades. Smoke will continue to
impact much of the forecast area, but the smoke sources will be
different. Smoke in Medford will likely come from the Siskiyou
County fires, fires in Western Jackson and Josephine Counties, and
the Chetco Bar fire in Curry County. This smoke will be aloft at
times, but it will also mix down to the surface at times during
the day and cause low visibility and poor air quality. The latest
high-resolution model`s (HRRR) smoke output shows some of the
worst near-surface smoke concentrations of the past several
days...for this afternoon, overnight, and into Tuesday...for areas
west of the Cascades to include Siskiyou County. Not knowing how
much smoke will be produced from fires in the next several days,
we have maintained smoke in the forecast for many of the areas
that are seeing smoke today. We are maintaining an Air Quality
Alert for most areas along and west of the Cascades...and the
Klamath Falls area. As winds shift northwest this afternoon, the
decent air quality in Klamath Falls now should degrade some. We
will evaluate the coverage of this alert tomorrow to see how this
switch in the wind flow will pan out in terms of smoke
concentrations, but expect plenty of degraded air quality through
tomorrow morning at least.

We`ll turn our attention to thunderstorm chances Tuesday and
Wednesday. An upper low off the coast of Southern California will
send moisture and instability...and provide the trigger for storms
over portions of the forecast area. The forcing/trigger for
thunderstorms Tuesday will be focused over the southern portion of
the CWA, so mostly over Northern California, but will gradually move
north during the day. Smoke will certainly be a factor that could
inhibit thunderstorm formation in some areas, primarily extreme
Western Siskiyou County, but potentially into the Cascades too. That
said, the area where smoke meets clear air could be a focal point
for thunderstorms, and we`ll evaluate that in the morning. We have a
Red Flag Warning out for tomorrow, and a Fire Weather Watch out for
Wednesday afternoon to highlight the thunderstorms and chance for
new fires. Tuesday night the NAM in particular maintains enough
shortwave activity to keep nighttime thunderstorms in the forecast.
On Wednesday, broad diffluence ahead of an approaching front, in
combination with midlevel moisture and instability, will support
thunderstorm formation. Thunderstorm activity will be focused
further eastward than Tuesday. Drier southwesterly flow will bring
an end to thunderstorm chances from west to east late Wednesday.
Storms on Tuesday and Wednesday will feature some rainfall with
them, most likely heavier on Tuesday due to weaker steering flow.

A cooling trend is expected through Thursday, with the coolest day
on Thursday when the cold front pushes through. Winds will be gusty
Tues-Thu afternoon, especially east of the Cascades. Models suggest
a significant warmup and a return to a thermal trough pattern Friday
and Saturday...with warming continuing into early next week.


.AVIATION...21/18Z TAF CYCLE...High pressure prevails with primarily
VFR conditions over the region.  The biggest concern remains the
smoke and fires burning in Curry and Siskiyou county.  Low level
winds are shifting to the south west bringing the smoke from those
fires into the west side valleys.  Look for low visibility for the
remainder of today with the elevated smoke plumes.  Thinking OTH and
most of the coastal areas will stay VFR today with the stratus deck
staying slightly off shore. -Smith


.MARINE...Updated 230 PM PDT Sunday 20 August 2017...With low
pressure moving inland today north winds will continue to decrease
into the evening. Low pressure will deepen again on the southern
coast Wednesday with north winds increasing again towards to the
end of the week. Sven


OR...Red Flag Warning from 2 PM to 11 PM PDT Tuesday for
     Fire Weather Watch from Wednesday afternoon through Wednesday
     evening for ORZ623>625.

CA...Red Flag Warning from 2 PM to 11 PM PDT Tuesday for
     Fire Weather Watch from Wednesday afternoon through Wednesday
     evening for CAZ280-281-284-285.

Pacific Coastal Waters...Small Craft Advisory until 5 PM PDT this afternoon for
     Small Craft Advisory for hazardous seas until 5 AM PDT Tuesday
     for PZZ350-356-370-376.
     Hazardous Seas Warning until 5 PM PDT this afternoon for PZZ376.


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