Fire Weather Outlook Discussion
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FNUS28 KWNS 172141

Day 3-8 Fire Weather Outlook
NWS Storm Prediction Center Norman OK
0439 PM CDT Thu Jun 17 2021

Valid 191200Z - 251200Z

An upper-level area of high pressure positioned over portions of the
Southwest will begin to break down as a mid-level trough enters the
Northern Rockies on D3/Sat and moves into the northern Great Plains
by D4/Sun. This will result in an increase in mid-level heights and
amplification of a ridge along the West Coast. By D5/Mon, an
upper-level cutoff area of low pressure will approach the Pacific
Northwest and is forecast to merge with the broader jet stream
thereafter. At the surface, a cold front will sweep through the
Northern Rockies and northern Great Plains on D3/Sat through D4/Sun,
with high pressure settling in thereafter. Farther west, subtropical
moisture will begin moving north into parts of Oregon by D5/Mon with
the approach of the upper-level low.

...D3/Sat - Dry Thunderstorms in portions of the Great Basin,
Central Rockies, and Southwest...
The latest high-resolution guidance (e.g., HRW FV3, NAM Nest)
continues to indicate the potential for dry thunderstorms over a
broad region of the West, including the potential for scattered dry
thunderstorms, within the residual subtropical moisture/upper-level
high pressure regime. Thunderstorms are expected to be mostly dry
given PWAT values at or below 0.5-0.75 inches and large dew point
depressions amid very dry to near record dry fuels across the
region. For now, 40% probabilities have been maintained for the
region. CAMs suggest scattered dry thunderstorm potential may extend
into portions of western Colorado and north-central New Mexico,
however, uncertainty in the mesoscale placement of such features
remains too high at this time to warrant any changes.

...D3/Sat - Snake River Plain and portions of eastern
Montana/central Wyoming...
Afternoon winds of 15-25 mph are expected as a result of the
mid-level trough overspreading the region, resulting in enhanced
mid-level flow mixing toward the surface, along with a cold frontal
passage across portions of Montana and northern/central Wyoming.
This will result in a well-mixed boundary layer over the region,
with RH values falling into the teens amid receptive fuels. The 40%
probabilities delineating critical fire-weather potential have been
maintained for this region.

...D4/Sun - D7/Wed - Portions of the Great Basin and Southwest...
On D4/Sun, an extension of enhanced northwesterly mid-level flow
will overspread portions of the region, as the mid-level trough
continues to move southeast within the broader upper-level trough
and lee cyclogenesis occurs along the southern High Plains. This
should result in a region of 15-25 mph surface winds amid a residual
dry airmass and RH values into the teens.

By D6/Tues, southwesterly mid-level flow will be present over
western portions of the Great Basin region and is expected to
continue into D7/Wed, as the cutoff low approaches the West Coast
and the ridge builds into the northern Rockies. This general pattern
should support dry/breezy conditions across the region, and thus,
potential for critical fire-weather conditions each day.

...D5/Mon - D7/Wed - Portions of the Pacific Northwest...
With the approach of the cutoff low, subtropical moisture beneath
the upper-level ridge should flow north into the region. This
pattern may support dry thunderstorm potential. However, details
regarding the track of the low and corresponding PWAT values remain
highly uncertain at this time, thus precluding any dry thunder
probabilities at this time.

..Karstens.. 06/17/2021

...Please see for graphic product...

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