Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 152321

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
521 PM MDT Tue Aug 15 2017

.SHORT TERM...(Tonight through Wednesday night)
Issued at 118 PM MDT Tue Aug 15 2017

As of 19z this afternoon, the axis of a 500 mb trough is
analyzed just west of the Green River valley in central Utah. This
trough axis will pivot eastward through the remainder of the day,
crossing the CO/UT border by 0z and exiting the forecast area east
of the Continental Divide after 06z tonight. Low level dry air
with notably lower PWAT values continue to round the base of this
trough and advect into the southern half of Utah and Colorado.
This has essentially eliminated any chance of meaningful
convective development south of Interstate 70 for the remainder of
the day. To the north of I-70 toward the Wyoming border, enough
moisture remains and numerous showers and thunderstorms have
developed...partially aided by positive vorticity advection on
the leading side of the trough axis. This vort max will head
northeast and exit the area by nightfall with convection rapidly
coming to an end area-wide after sunset. Mesoscale analysis indicates
around 1000 J/kg of MLCAPE and 35 knots of shear along and just
south of the CO/WY border this afternoon and this corridor will
need to be monitored for a stronger thunderstorm or two over the
next few hours.

A noticeable change overspreads the Four Corners region on
Wednesday as light westerly flow returns. PWAT values will fall
area-wide to the 0.3-0.4" range, which is around 1 to 2 sigma
below climatological normals for mid-August: a notably dry
airmass for sure. Given the dry air and light flow aloft,
thunderstorm chances in the higher terrain will be quite low, and
valley locations will see a dry, mostly sunny afternoon with
temperatures running right around seasonal averages.

.LONG TERM...(Thursday through Tuesday)
Issued at 118 PM MDT Tue Aug 15 2017

Light westerly flow continues on Thursday with another mostly dry
day expected. A very weak shortwave trough will pass through
overnight Thursday and into Friday, perhaps touching off a shower
or two along the Continental Divide Thursday night. Weak ridging
to the north of the central Rockies moves in for Friday and
Saturday with some hints of monsoonal flow creeping into the
southern portions of Arizona and New Mexico by the weekend.
Eastern Utah and western Colorado will remain between these two
main influences, with mostly sunny skies and very isolated
afternoon showers and thunderstorms in the highest terrain each

Details regarding the end of the weekend and early next week
continue to evolve in todays 12z guidance cycle. The GFS develops
a strong closed low along the California coast on Sunday
afternoon, which remains nearly stationary through Tuesday. The
ECMWF also develops a trough over the west coast, however the low
pressure is further south and not as deep, resulting in weaker
monsoonal flow on Monday and Tuesday. The Canadian indicates a
split-flow/Rex block situation, developing a ridge over the PACNW
and a trough over southern California on Monday and Tuesday. With
such a wide range in deterministic model solutions, it is
important not to let specific details eclipse the overall
message: monsoonal moisture will return on Sunday through Tuesday
with an increase in cloud cover and thunderstorm chances each


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 500 PM MDT Tue Aug 15 2017

The focus for showers and thunderstorms will remain north of
Interstate 70, over northeast UT and northwest CO through this
evening as the airmass is much drier farther south and east.
However, isolated storms are still possible over and near the
Divide. Ceilings could briefly drop below 070 across the north
and winds could be erratic and gusty.




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