Fire Weather Outlook Discussion
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FNUS28 KWNS 192057
FWDD38

Day 3-8 Fire Weather Outlook
NWS Storm Prediction Center Norman OK
0356 PM CDT Thu Oct 19 2017

Valid 211200Z - 271200Z

Large-scale critical fire weather concerns should remain centered on
southern CA through much of the extended forecast period. As a
large-scale upper trough moves eastward from the Rockies to the
Plains on Day 3/Saturday, surface high pressure will become
established across much of the northern Rockies and Great Basin. The
surface pressure gradient will strengthen between this surface high
and lower pressure off the coast of southern CA, leading to a
multi-day period of offshore flow across parts of southern CA. An
upper ridge will amplify across much of the western CONUS from Day
4/Sunday through at least Day 6/Tuesday, and the surface high should
remain prominent beneath the upper ridge in this time frame.

...Day 3/Saturday - Day 6/Tuesday: Portions of Southern CA and the
Lower CO River Valley...
A prolonged period of at least moderate offshore winds and critical
fire weather conditions will be likely across much of southern CA
from Day 3/Saturday through Day 6/Tuesday. Confidence has increased
in a sufficiently strong surface pressure gradient across portions
of Santa Barbara, Ventura, and Los Angeles counties to support
likely critical conditions Saturday afternoon/evening and continuing
into early Sunday morning. Have therefore added a 70%/critical area
for Day 3/Saturday where the best overlap of strong/gusty offshore
winds and critically lowered RH values are expected to occur. The
40%/marginal area across the lower CO River Valley has also been
extended southward to the international border based on latest
guidance.

No changes have been made to the 40%/70% areas for Day 4/Sunday or
Day 5/Monday, as the best potential for strong/gusty offshore winds
and RH values below 15% should persist each day across the
mountains/foothills of southern CA, as well as some adjacent
valleys. 70%/critical probabilities have been added to the same area
for Day 6/Tuesday, as latest deterministic and ensemble guidance
suggest the strongest surface pressure gradient and offshore winds
should occur both Sunday night into Monday morning, and again Monday
night into Tuesday morning. If current model trends continue, the
70%/critical areas for Day 5/Monday and Day 6/Tuesday may need to be
extended to the coast, and include major cities such as Los Angeles
and San Diego.

By Day 7/Wednesday, medium-range guidance suggests a shortwave
trough may act to break down the upper ridge across the western
CONUS, which should lessen the surface pressure gradient, resultant
offshore winds, and the critical fire weather threat across southern
CA. Have therefore not included probabilities for critical
conditions across southern CA on either Day 7/Wednesday or Day
8/Thursday.

..Gleason.. 10/19/2017

...Please see www.spc.noaa.gov/fire for graphic product...

$$



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