Fire Weather Outlook Discussion
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FNUS28 KWNS 182146

Day 3-8 Fire Weather Outlook
NWS Storm Prediction Center Norman OK
0446 PM CDT Wed Oct 18 2017

Valid 201200Z - 261200Z

A large-scale upper trough will move across the western CONUS and
into the High Plains from Day 3/Friday to Day 4/Saturday. This upper
trough should continue to advance eastward across much of the
central/eastern CONUS through the upcoming weekend, and an upper
ridge will become prominent across the western CONUS by Day
5/Sunday. Surface high pressure is expected to develop across the
Great Basin this weekend and support a prolonged offshore flow event
across portions of southern CA beginning around Saturday night and
continuing through at least Day 7/Tuesday.

...Day 3/Friday: Portions of Central/Eastern WY...
A 40% marginal area has been included for portions of
central/eastern WY for Day 3/Friday. Enhanced mid-level
southwesterly winds associated with the approaching upper-level
trough described above will overspread this region by Friday
afternoon. Strong/gusty downslope winds appear likely, but the
degree of RH reduction is still somewhat uncertain based on latest
guidance. Regardless, elevated to locally critical fire weather
conditions should occur, and the inclusion of a critical delineation
may be needed in a future update if RH values are forecast to become
critically lowered.

...Day 4/Saturday - Day 7/Tuesday: Portions of Southern CA and the
Lower CO River Valley...
No changes have been made to ongoing 40%/70% probabilities for
critical fire weather conditions across portions of southern CA and
the lower CO River Valley in the Day 4/Saturday to Day 7/Tuesday
time frame. A prolonged period of moderate offshore winds is still
apparent, with the strongest surface pressure gradient likely to
occur from late Sunday to early Tuesday morning based on consensus
of medium-range deterministic and ensemble guidance. RH values will
become critically lowered by Saturday afternoon across much of
southern CA as the upper ridge builds across the western CONUS, and
poor overnight RH recovery will also likely occur. Very dry fuels
will remain receptive to large fire starts and spread. Some
consideration was given to upgrading probabilities to 70%/critical
for Day 7/Tuesday, but the surface pressure gradient is forecast to
begin weakening by Tuesday afternoon/evening, and could continue
weakening into Day 8/Wednesday as the upper ridge begins to break

..Gleason.. 10/18/2017

...Please see for graphic product...

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