Fire Weather Outlook Discussion
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FNUS21 KWNS 271645

Day 1 Fire Weather Outlook
NWS Storm Prediction Center Norman OK
1145 AM CDT Tue Jun 27 2017

Valid 271700Z - 281200Z


Primary changes to the Day-1 Fire Weather Outlook are as follows:

1. A Critical area has been added for portions of the southern CA
mountains and adjacent foothills. Across these areas -- from
portions of San Bernardino County southward to the international
border -- the latest observational data and model guidance indicate
that strong low-level flow will continue supporting westerly surface
winds of 20-25 mph and higher gusts through the period. With PW
around 0.5 inch highlighting the dry air mass in place, hot surface
temperatures associated with deep vertical mixing will allow RH to
fall to 3-8 percent amid dry fuels in support of the Critical
designation. Critical conditions will continue into tonight, as poor
RH recovery and a continuation of strong winds are expected.
Surrounding the added Critical area, minor expansions to the
Elevated area have been made based on the latest model guidance.
North of the Critical area, Elevated fire-weather conditions are
expected, though relatively less coverage of critically strong winds
is anticipated.

2. The Critical area, previously encompassing portions of northern
AZ and northwest NM, has now been expanded to include a broader area
across the southwest states and Great Basin -- now including
portions of UT, AZ, western NM, and western CO. The latest VAD wind
profiles across the region are sampling 20 to locally 30 mph winds
in the lowest 3.5 kft across the updated Critical area. This
reflects a low-level response to a subtle midlevel wind-speed
maximum crossing the region. As a result, through vertical mixing,
west-southwesterly to westerly winds (becoming northwesterly in
northeast UT and northwest CO) of generally 20 to locally 30 mph are
expected amid RH falling to 3 to 10 percent across the updated
Critical area. It is possible that, in some parts of northwest AZ
into central and northeast UT, wind speeds may only approach
Critical thresholds. However, given the very dry fuels in place in
many locations, given recent days of hot and breezy conditions
further contributing to drying, and given continued hot conditions
with very low RH today, Critical designation is warranted.

No other changes have been made to the ongoing forecast.

..Cohen.. 06/27/2017

.PREV DISCUSSION... /ISSUED 0212 AM CDT Tue Jun 27 2017/

Shortwave trough currently moving through southern OR/northern CA
will continue northeastward through the northern Rockies and into
the northern Plains. Some shortwave ridging will move eastward just
ahead of this shortwave trough but the overall progression of the
system will complete the breakdown of the upper ridging which
persisted across much of the western CONUS over the past week.
Surface low related to the shortwave trough will move across the
Dakotas while an attendant cold front moves across the
northern/central High Plains.

...AZ...Great Basin...Northern Rockies...
Modestly enhanced mid-level flow coupled with a strong surface
pressure gradient will support westerly surface winds from 15 to 30
mph. The strongest winds are expected over southern WY, where the
strongest mid-level flow exists, and farther south across northern
AZ and adjacent far northwest NM. In these areas, sustained westerly
winds from 20 to 30 mph are anticipated. Across northern AZ and
adjacent far northwest NM, these gusty winds in tandem with very
warm and dry conditions (i.e. minimum RH values below 10%) and dry
fuels will support a critical fire weather threat. Similar
meteorological conditions are anticipated across southern WY, but
fuels do not support a critical fire weather threat. Afternoon RH
values less than 15% and winds around 15-20 mph are expected
elsewhere across the region, supporting elevated to locally critical
fire weather conditions.

Additionally, mid-level moisture will advect into the region ahead
of the approaching shortwave trough. Across southern WY and the far
western NE Panhandle, resulting moist midlevels will exist atop a
very dry and deeply mixed boundary layer supporting inverted-v
profiles and modest instability. Isolated high-based thunderstorms
are expected to develop within this environment with high cloud
bases and sub-cloud evaporation coupled with fast storm motions
keeping precipitation totals low. Consequently, increased fire
danger due to dry thunderstorms exists across this areas. Higher
probability of wetting rains and less-receptive fuels will preclude
a dry thunderstorm threat farther north.

...Mountains and western foothills of southern CA...
Another day of breezy onshore winds is anticipated across the higher
terrain and western foothills of southern CA. Poor overnight
recovery will result in a long duration of critical RH values (i.e.
less than 15%). Resulting combination of dry and windy conditions
will support an elevated to locally critical fire weather threat.

...Please see for graphic product...

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