Area Forecast Discussion
Issued by NWS Grand Junction, CO

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261
FXUS65 KGJT 150511
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1111 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 zonal...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
forecast.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1107 PM MDT Mon Aug 14 2017

Scattered showers and thunderstorms will continue to be focused
mainly north of I-70 through the early morning hours. Most
activity will diminish by 09Z but some nocturnal showers will
persist overnight. Another round of scattered showers and storms
are expected for Tuesday as a cold front works through the region.
As has been the trend the past several days, storms will be
capable of producing gusty outflow winds, heavy rain, and frequent
lightning. Any storms that move over TAF sites could bring brief
MVFR conditions. Mountain tops may be obscured at times in heavy
rain and low clouds.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...None.
UT...None.
&&

$$

SHORT TERM...MAC
LONG TERM...MAC
AVIATION...TGJT



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