Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 142200

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
400 PM MDT Mon Aug 14 2017

.SHORT TERM...(Tonight through Tuesday night)
Issued at 107 PM MDT Mon Aug 14 2017

Afternoon GOES satellite imagery shows a potent shortwave trough
traversing the Great Basin in the western CONUS. Broad ascent
ahead of this trough in the mid and upper levels of the atmosphere
has resulted in a rather classic baroclinic leaf structure,
evident in infrared and water vapor channels today. Ahead of this
system, height falls over eastern Utah and western Colorado have
lead to an uptick in convective development this afternoon.
Mesoanalysis indicates around 500 to 750 J/Kg of MLCAPE in place
along and north of the I-70 corridor this afternoon. 25-30 knots
of 0-6km speed shear has aided convective organization today,
resulting in a few briefly strong cells capable of some small
hail. Gusty winds will also remain a threat for the remainder of
the evening, considering DCAPE values around 1000 J/kg sampled by
the morning KGJT sounding. The more intense convection should die
down after sunset this evening as heating is lost...however with
dynamic support approaching from the west and height falls
continuing overnight, pockets of lighter precipitation may linger
through the morning hours especially in the north.

On Tuesday afternoon, the aforementioned shortwave trough pivots
through the western slope. Guidance is in excellent agreement
indicating the passage of this trough and associated vort max
early in the afternoon - slightly too early to maximize the
thunderstorm threat over the region. Dry air in the lower levels
advecting in from the south will also temper the thunderstorm
threat. That being said, with plenty of forcing in place,
thunderstorms are still expected through the afternoon across
eastern Utah and western Colorado. The most favorable location for
stronger thunderstorms will once again be north of I-70, where
PWAT values are modeled to be a bit higher than in the south. By
midnight Wednesday, nearly all CAM guidance shows the trough and
any lingering moisture well to the east, with a dry overnight
forecast area-wide. Have adjusted PoP values downward late Tuesday
evening and early Wednesday morning to account for the slightly
faster storm progression in today`s afternoon guidance cycle.

.LONG TERM...(Wednesday through Monday)
Issued at 107 PM MDT Mon Aug 14 2017

A drier airmass moves in from the west for the middle and end of
the week as heights briefly rise behind Tuesday`s trough. The
flow can not be quite classified as a broad, weak
trough will remain over the southwestern CONUS. However that
being said, a more `zonal flow` pattern will set up over the Four
Corners characterized by light westerly flow aloft and only
isolated diurnal convection in the higher terrain. High
temperatures will run near seasonal averages by the end of the
week. The next notable change in the weather pattern appears to
arrive by late this weekend. All three major global models
indicate a trough enhancing over the central Pacific coast. Flow
over the Four Corners turns a bit more southwesterly and high
pressure in the southern plains shifts eastward. While exact
timing is still uncertain, the net result of these factors will be
a return of a more classic monsoonal pattern over the west. PWAT
values and afternoon convection rise by Sunday, with Monday and
next Tuesday appearing possibly more unsettled. Subtle differences
in global models casts a shadow on the rather important Monday


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1111 AM MDT Mon Aug 14 2017

Showers and thunderstorms will develop again today and be
focused mainly north of Interstate 70. The southern half of the
area will receive only isolated storms. Any cells that move over
TAF sites will bring some gusty winds and heavier rain. A quick
drop to MVFR conditions is possible if/when storms move directly over
TAF sites. ILS breakpoints may also be met from time to time but
should not stay down for too long. Storm should dissipate after




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