633
FXUS65 KGJT 291122
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
522 AM MDT Sat Jun 29 2024

.KEY MESSAGES...

- Decreased shower and storm coverage and warmer temperatures
  are expected today as drier air moves in. The higher terrain,
  especially along the Divide, will be most likely to see
  showers or storms.

- A return of deep subtropical moisture tomorrow into early next
  week will bring a return to widespread showers and storms and
  cooler temperatures.

- The main threats with storms will begin as gusty winds and frequent
  lightning, transitioning to include heavy rain and flash
  flooding as moisture returns starting tomorrow.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 236 AM MDT Sat Jun 29 2024

Showers and thunderstorms have continued well into the overnight
hours, focused mainly across central eastern Utah and central
western Colorado. A lingering pocket of 500-1000 J/kg of CAPE and
rich surface moisture have created a favorable environment in this
area despite the time of day, and outflows from previous convection
earlier in the evening has acted as a lifting mechanism to keep
things rolling along. These storms have been capable of producing
frequent lightning and heavy rains. As this convection continues to
use up the lingering instability, look for a gradual downturn in
activity over the next several hours, with skies clearing out
somewhere between 6AM and 8AM.

The center of the subtropical high is forecast to drift further east
today, with the tap of deep subtropical moisture also shifting east
to follow the circulation. In addition, a ridge of high pressure
will build in overhead today, pulling in drier air aloft and really
limiting the amount of moisture available for afternoon convection.
This is already ongoing, as indicated by the 0.84 inch PWAT on last
nights 00z GJT sounding...down 0.20 inches from that morning alone
and the lowest value we`ve seen since Tuesday evening. Enough
surface based moisture remains to fuel convection today, with
forecast dew points still in the 40s to low 50s, but with the lack
of deep moisture, the threat of heavy rains will be at its minimum
today. Instability is also looking less than thrilling, with most
model guidance indicating around 500 J/kg of CAPE, with a few
embedded pockets of near 1000 J/kg. Additionally, forcing looks to
be relying solely on differential heating of the terrain this
afternoon. All that is to say, look for a significant downturn in
storm coverage this afternoon, with the higher terrain most favored
to see showers or storms, and for the main threats to be gusty winds
and lightning. With drier air in place and lower coverage of clouds
and showers, look for afternoon highs to run a few degrees above
normal and around 5 degrees warmer than we`ve seen the last few
days.

As with the last several days, look for afternoon and evening
showers and thunderstorms to linger into the overnight hours, though
hopefully we`ll see things taper off by midnight. Tonight will be
relatively quiet and mild as high pressure remains overhead. This
high slides east of the Divide tomorrow as a strong Pacific trough
moves onto the West Coast, bringing our little reprieve from
widespread convection to an end. The center of the subtropical high
begins to slide westward once again on Sunday, nudging that
subtropical moisture plume back toward the Four Corners. Moisture
will begin rapidly returning to eastern Utah and western Colorado,
as PWATs steadily climb back up to an inch or better across much of
the region. Better instability makes its return, with forecast CAPE
around 1000 J/kg on Sunday afternoon, a weak little wave rounding
the high combined with a jet draped across the north will provide
some enhanced lift. This jet will also bring some gustier winds to
northeast Utah and northwest Colorado on Sunday afternoon. The
higher terrain will be most favored to see showers and
thunderstorms, in particular the central and southern Divide
mountains. The return of deep subtropical moisture will also favor
an increased threat of heavy rain and even flash flooding, in
addition to the usual threats of gusty winds and hail. Temperatures
will once more drop to near normal values thanks to increased
moisture and more widespread afternoon clouds and showers.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Issued at 236 AM MDT Sat Jun 29 2024

Anomalous moisture will continue to be fed around the periphery of
the Southern Plains SubTrop High into the 4 Corners and Central
Rockies into early next week. This pattern will not break until a
stronger upstream trough works out of the Northern Rockies by
midweek to force the SubTrop High eastward and usher in drier air
from the West. This upstream trough will be moving dropping into the
Intermountain West by Sunday Night which will provide the upper
support necessary for nocturnal thunderstorms to become more
widespread on Monday. We have recently seen what more focused
dynamics and PWAT of 2 to 3 Std Dev can do...bring a threat of
strong to severe thunderstorms along with flash flooding threats due
to excessive rainfall rates. This certainly appears to have a least
a low probability (~30%) of occurrence over the southeastern 2/3 of
the CWA going into Monday which will be the peak of this next
rainfall event. The northwest 1/3 of the CWA looks to be on the
periphery of the main moisture plume. This doesn`t mean totally dry
conditions but as we move farther into northeast Utah the
probability of impacts will be less than 15 percent. The trough axis
looks to move through Monday night and likely keep nocturnal
convection active with a noticeable downturn Tuesday and much drier
conditions mid to late week as PWAT anomalies move in the other
direction with much drier air moving in behind the trough. A lot of
finer details still need to be worked out and depending on the
evolution of the convection...some of these details won`t be found
in model solutions but rather satellite and radar trends. Blended
model temperature forecasts keep things near to slightly below
normal through late week in a northwest flow pattern. Relatively
moist soil conditions from the recent moisture push may also help
contribute to moderate temperatures but looking farther out heat
dome over the West looks to be migrating into the Great Basin by the
weekend to bring on the July heat.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 519 AM MDT Sat Jun 29 2024

A lingering area of shower activity will continue to affect KMTJ
and KGUC over the next hour or two. Once this area dissipates,
skies will be generally clear and winds will follow typical
terrain driven patterns. Showers and thunderstorms are expected
to form on the terrain this afternoon and evening, with impacts
confined to higher elevation terminals mainly south of I-70.
Showers and storms will be capable of producing gusty and
erratic winds, frequent lightning, and brief heavy rain.
Activity is expected to decrease after sunset. VFR conditions
will prevail, but drops to MVFR and/or below ILS breakpoints
will be possible with showers passing directly over a terminal.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...None.
UT...None.

&&

$$

SHORT TERM...BGB
LONG TERM...15
AVIATION...TGJT