FNUS21 KWNS 281540

Day 1 Fire Weather Outlook  
NWS Storm Prediction Center Norman OK
1039 AM CDT Tue Sep 28 2021

Valid 281700Z - 291200Z


...Northern High Plains...
The forecast generally remains on track with only minor adjustments
needed based on recent observations and morning guidance. Surface
observations from eastern MT show lee troughing underway with MSLP
values already near or below the 10th percentile for this time of
year. This supports recent forecasts, which maintain reasonable
probabilities for critical conditions across eastern MT/WY and the
western Dakotas as the low deepens further this afternoon. The
eastern extent of critical and elevated conditions remains somewhat
uncertain due to the overlap of 20-25 mph boundary-layer winds with
weak moisture return across the central Dakotas. Brief elevated
conditions are likely behind the approaching cold front across
central MT, but will likely be limited in duration. 

...Northern California...
An Elevated risk area is introduced for the Sacramento Valley where
winds have already increased to near 15 mph in some locations.
Recent hi-res guidance has come into better agreement regarding the
potential for RH reductions into the 15-20% range, which has
increased confidence in the areal coverage of elevated conditions.

..Moore.. 09/28/2021

.PREV DISCUSSION... /ISSUED 0149 AM CDT Tue Sep 28 2021/

Troughing over the Intermountain West will becoming increasingly
broad through the day as a secondary upper low over the southern
Plains moves northward and merges with the main trough across the
northern Rockies. As the primary mid-level jet associated with the
trough moves north into southern Alberta, a lee low/trough should
develop in response, across portions of eastern Montana/Wyoming. A
cold front trailing from the surface low is forecast to rapidly move
eastward across the Great Basin and Northern Rockies bringing cooler
temperatures and an increase in humidity. Gusty south/southwest
winds, and warm/dry conditions ahead of the front are expected to
support elevated to critical fire weather conditions through the
afternoon and evening.

...Northern Rockies and High Plains...
As synoptic lift from the trough and the rear quadrant of the
departing mid-level jet move across the northern Rockies, lee
cyclogenesis is expected early in the day across eastern Montana and
far western North Dakota. The increasing westerly flow aloft should
also aid in the development of a lee trough in eastern Wyoming and
the western Dakotas. The rapidly deepening low, below 1000 mb by
midday, will support strong mass response in the low levels, driving
20-25 mph southerly surface winds across much of eastern Wyoming,
and western South Dakota. Here, the warm temperatures and dry
downslope flow near the lee trough will support widespread diurnal
RH minimums of 10-20%. A lingering low-level jet may also bolster
surface gusts to 30+ mph through the afternoon given deep mixing
heights on regional model soundings. With very dry fuels in place
and ongoing drought conditions, fuels are highly receptive to starts
and fire spread. Widespread elevated to critical fire weather
conditions are expected through the afternoon and into the evening
hours. The approaching cold front will end the fire weather threat
after sunset as winds shift to the west northwest and RH begins to

...Central and southern California...
Gusty winds of 15-20 mph are expected in the post-frontal airmass
across much of the Central Valley and portions of southern
California this afternoon and evening. Low-level trajectories from
the west northwest will be less favorable for lower humidity given
the presence of a stronger marine layer farther inland. However, the
gusty winds and dry fuels may still support a few hours of locally
elevated fire weather conditions. The fire weather threat should
rapidly diminish overnight as RH recoveries near 50-60% are expected
to develop along with weaker winds.

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