Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 222226 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Medford OR
326 PM PDT Tue Aug 22 2017

.Edited portion of AFD.

.DISCUSSION...Over the next 7 days the weather conditions will evolve
from warm and unstable to cool and stable, to hot and dry...with
continual periods of smoke affecting the forecast area.

The next two days will feature thunderstorm chances, focused today
over northern California, extending into Southern Oregon later this
afternoon and evening. There is potential for nocturnal storms as
disturbances aloft make their way south to north into Oregon.
Tomorrow a cold front will approach the forecast area and
gradually introduce a westerly component to the upper level winds,
drying out the midlevels and stabilizing the atmosphere. This
will push the threat from thunderstorms eastward tomorrow,
primarily along and east of the Cascades. We have Red Flag
Warnings out at RFWMFR for the expected abundant lightning on dry
fuels today and tomorrow. Some of the storms will produce heavy
rain, and we have a Flash Flood Watch at FFAMFR to highlight the
threat for flash flooding on some of the recent wildfire burn
scars in Siskiyou and Modoc Counties. Note that we do expect
plenty of lightning outside of storm cores, so new fire starts are
still expected despite some wetting rainfall.

Smoke is another big concern, impacting a majority of the forecast
area. In general, air quality has trended similar or worsened
compared to yesterday. The HRRR smoke models suggest some
improvement tomorrow but with similar wind flow, we have maintained
an Air Quality Alert out through tomorrow afternoon for much of the
West Side, Cascades, and portions of the East Side to around Highway
97. Brookings and the Curry County coast should continue to see
periods of poor air quality through tomorrow at least, based on weak
winds, overnight drainage northeast winds down the Chetco drainage,
and the abundant smoke from the nearby large fire.

A cooler air mass, not surprisingly, will accompany the cold front
on Thursday, but this trend towards lower temperatures will be very
brief. 850mb temperatures, a good indicator for trends of surface
temperatures, rise starting Friday and peak Monday or Tuesday. 850mb
temperatures reach the top 5 percent for this time of year on
Monday per GEFS output, and temperatures could reach 100 degrees
in the Rogue Valley. The surface temperatures will partially
depend on the amount of smoke in the area, so that will have to be
continually evaluated. Needless to say, the fire environment will
become hotter and drier late this week into the weekend. We have
increased northeasterly overnight winds near the coast as the
thermal trough strengthens into the weekend. Looking at cross-
sections of the atmosphere on Monday, relative humidity values
throughout the column are all 10 to 25 percent...indicating an
extremely dry air mass.


.AVIATION...22/18Z TAF CYCLE...Onshore flow is keeping LIFR/IFR
cigs/vis at the coast and just offshore.  It is possible that the
OTH TAF site sees some clearing this late morning and early
afternoon with some breaks in the cloud decks.

Elsewhere smoke from wildfires blanket southern Oregon and this is
affecting visibility at most terminals.  Look for visibility to
drop down to to IFR at some sites and MVFR for inland areas because
of the smoke.  Isolated to scattered thunderstorms are possible
later this afternoon and evening with Klamath Falls at greater risk
for thunderstorms near or over the terminal. -Petrucelli/Smith


.MARINE...Updated 130 PM PDT Tuesday 22 August 2017...A weak
thermal trough near the coast will strengthen Thursday...bringing
increasing north winds and steep seas to the area into the


OR...Red Flag Warning until 11 PM PDT this evening for
     Red Flag Warning from 1 PM to 8 PM PDT Wednesday for ORZ624-625.
     Flash Flood Watch until 11 PM PDT this evening for ORZ028.

CA...Red Flag Warning until 11 PM PDT this evening for
     Red Flag Warning from 1 PM to 8 PM PDT Wednesday for CAZ285.
     Flash Flood Watch until 11 PM PDT this evening for

Pacific Coastal Waters...Small Craft Advisory from 11 PM this evening to 5 AM PDT
     Thursday for PZZ356-376.


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