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FNUS21 KWNS 281643

Day 1 Fire Weather Outlook
NWS Storm Prediction Center Norman OK
1143 AM CDT Wed Jun 28 2017

Valid 281700Z - 291200Z


No changes have been made to the ongoing forecast, with elevated and
critical wind-driven fire-weather risk and isolated dry-thunderstorm
potential affecting portions of a broad region from the southwest
states to the central Great Basin and central Rockies. While
critical fire-weather conditions will be likely across the entire
Critical area, there is particular concern for potentially locally
extreme fire behavior, enhanced fire spread, and explosive fire
growth across parts of southern UT and northern AZ this afternoon
into the evening.

The 12Z Las Vegas NV and Flagstaff AZ soundings this morning sampled
90th percentile to near-record (warm) 500-mb temperatures combined
with 75th percentile to 90th percentile (enhanced) 500-mb wind
speeds -- based upon the SPC sounding climatology. These factors
reflect the anomalous combination of substantial mid-tropospheric
warmth and enhanced winds. Water vapor imagery depicts subtle
disturbances grazing the southern periphery of a broader belt of
enhanced midlevel zonal flow across the western states -- locally
enhancing the height gradient / winds aloft over some of the driest
surface conditions and driest fuels (i.e., across southern UT and
northern AZ). The aforementioned soundings indicate 30-40 mph of
flow in the midlevels, highlighting the enhanced momentum aloft that
will be manifest to some extent at the surface. This will be the
result of mixed layers deepening to over 10 kft above ground in some
areas -- aided by surface temperatures warming well into the 80s
across the higher elevations and into the 90s to near 100F at the
lower elevations. In addition to supporting an area of locally
enhanced winds -- i.e., 22-25 mph sustained west-southwest winds
with gusts of 30-35 mph across southern UT and northern AZ -- the
deep vertical mixing will also support minimum RH of 3-8 percent.
This anomalous combination of very warm to hot temperatures / very
low RH at the base of deep, well-mixed boundary layers / very dry
fuels -- and the enhanced boundary-layer and surface winds -- will
all exacerbate the critical fire-weather risk in southern UT and
northern AZ.

..Cohen.. 06/28/2017

.PREV DISCUSSION... /ISSUED 0257 AM CDT Wed Jun 28 2017/

An upper-level shortwave trough will move across the Intermountain
West today, enhancing westerly flow aloft over much of the central
Great Basin and Southwest.  At the surface, low pressure will
persist over much of the West with continued hot and dry conditions.

...Arizona...Utah...and adjacent areas...
A persistent hot, dry airmass over the area will allow for deep,
well-mixed boundary layers yielding afternoon RH values in the
single digits for most locations.  Deep vertical mixing in the
presence of enhanced flow aloft will result in sustained afternoon
wind speeds of 20-25 mph across much of northern Arizona and
southern Utah, resulting in critical fire weather conditions.  For
the surrounding elevated fire weather areas, sustained wind speeds
are expected to remain just below 20 mph.

Despite limited moisture across the region, sufficient instability
will develop during the afternoon across portions of northern Utah,
southwestern Idaho, southern Wyoming, and northern Colorado to
support isolated thunderstorm development.  Given the expected high
cloud bases and dry sub-cloud layers of these thunderstorms, little
precipitation is expected to reach the ground, supporting the
potential for isolated dry thunderstorms.

...Southern California mountains and adjacent foothills...
Continued low RH coupled with modestly enhanced winds will also
prolong elevated fire weather conditions through today across
portions of Southern California.

...Please see for graphic product...

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