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FNUS28 KWNS 202043
FWDD38

Day 3-8 Fire Weather Outlook
NWS Storm Prediction Center Norman OK
0343 PM CDT Sun Aug 20 2017

Valid 221200Z - 281200Z

At the start of the extended period, a mid/upper ridge is expected
to temporarily build into portions of the interior Northwest and
northern Rockies, ahead of a strong upper trough approaching the
British Columbia coast. Further south, a weak and slow-moving upper
trough is expected remain centered off of the California coast
through Wednesday, then weaken with time and perhaps meander back
over the eastern Pacific later in the period. With time next week,
the strong upper trough is expected to move into southwest Canada
and adjacent portions of the Northwest and northern Rockies/High
Plains, sweeping a cold front through the region by the end of the
week. Next weekend, a mid/upper-level ridge is expected to rebuild
over much of the West, resulting in a return to primarily hot and
dry conditions.

...D3/Tuesday - D5/Thursday: Dry Thunderstorm Threat from the
interior Northwest into the Northern Rockies/High Plains...
On D3/Tuesday, moisture returning northward in advance of the weak
system off of the CA coast will result in an increased chance of
thunderstorms over portions of northern CA and southern OR. While
coverage may be relatively limited and tied to terrain-based
circulations, antecedent warm/dry conditions and deep, well-mixed
afternoon boundary layers will result in a threat for isolated dry
thunderstorms.

On D4/Wednesday, a larger area of the Northwest and northern Rockies
is expected to be impacted by thunderstorms, as moisture spreads
northeastward and a weak midlevel trough is drawn northward to the
east of the weak eastern Pacific trough and in advance of the
stronger system approaching British Columbia. Moisture may be
sufficient to support locally wetting rainfall, but given dry fuels
over the region, some fire weather threat from dry thunderstorms is
expected to develop from OR/northern CA northeastward into western
MT.

On D5/Thursday, the thunderstorm threat will continue but begin to
end from west to east, as a cold front sweeps through the region.
The greatest coverage will likely shift into portions of ID/MT, but
with uncertainty regarding rainfall potential and timing of the cold
front, only 10% dry-thunderstorm probabilities will be maintained at
this time.

...D4/Wednesday - D5/Thursday: Critical Wind/RH threat over the
interior Northwest...
On D4/Wednesday, a modest increase in low-level flow is possible as
the strong upper trough approaches, resulting in the potential for
elevated conditions given very warm and dry conditions. At this
time, the threat for critically strong wind speeds appears too low
to include any critical probabilities.

On D5/Thursday, the dry and windy post-frontal regime will result in
a threat of elevated/potentially critical wind/RH conditions across
portions of central/eastern OR/WA, in the Columbia Gorge and the lee
of the Cascades. Some uncertainty remains regarding the potential
for critically low RH in the cooler post-frontal airmass, but it is
quite possible that a critical area will be needed for some portion
of the area in subsequent outlooks.

...D6/Friday - D7/Saturday: Northern High Plains
Elevated to potentially critical conditions will be possible in the
post-frontal regime across portions of MT and perhaps the western
Dakotas on D6/Friday, and a 40% area has been added to cover this
threat. Some extension of this regime may extend into D7/Saturday,
but low predictability precludes any additional probabilities at
this time.

..Dean.. 08/20/2017

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

$$


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