-- Discussion containing changed information from previous version are highlighted. --
806
FXUS65 KGJT 162332
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
532 PM MDT Mon Sep 16 2024

.KEY MESSAGES...

- Widespread showers and storms are expected today and tomorrow
  as a system moves through the region.

- Some storms may be capable of producing large hail, damaging
  winds and heavy rainfall. Therefore there is a chance for
  localized flooding.

- Temperatures dip to near normal today in response to clouds
  and precipitation. A cold front moving through the region
  tomorrow is expected to bring cooler than normal readings for
  the latter part of the week.

- Expect a break in the action Wednesday and Thursday before
  another system impacts the region Friday into the weekend.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 307 PM MDT Mon Sep 16 2024

Satellite imagery shows the low already shifting east into the Great
Basin with the models running about six hours behind. THe high res
models are doing a little better, but are also lagging a bit. This
could have impacts on the timing of the front tomorrow, so we`ll
want to see how this tracks on the 00Z model runs. Currently
scattered showers and thunderstorms are moving up from Arizona into
southeastern Utah and southwestern Colorado with a deep band of
moisture and pwats ranging 0.7 to about an inch. Models put the nose
of the jet still to the west, but it will move in over eastern Utah
and Western Colorado through the afternoon and evening to support
continued overnight convection. Imagery also shows the moist band
extending to the west to support convection through eastern Utah.
Included some HRRR and NAM in the PoPs to increase probs in these
areas. A mid-level shortwave ascends out of the trough across the
eastern half of the region through the late evening and overnight to
keep nocturnal showers and isolated to widely scattered
thunderstorms favoring the higher terrain going. These could
stratify out across as any available energy gets expended, but with
the dynamics of the shortwave will likely keep some convection going
especially over the western areas of the CWA.

Models start lifting the low out of the Great Basin overnight with
it moving into Western Wyoming by Tuesday afternoon. The cold front
is progged to transit the region between about 15Z and 21Z spawning
a wave of strong, possibly severe thunderstorms ahead of it. The
question is the timing of the front as an earlier passage as
discussed above would mean storms wouldn`t have a chance to draw on
the diurnal heating and would likely tame the resulting storms; a
later passage would align the frontal dynamics with max diurnal
heating leading to more widespread severe thunderstorms sweeping
west to east across the region through the afternoon.

The cold front moves through quickly with gusty westerly winds
behind it bringing a dry airmass into the region to shut down
convection and clear out the skies by Tuesday evening. Look for
about a five degree drop in temperatures behind the cold front.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Issued at 307 PM MDT Mon Sep 16 2024

Models appeared to be in excellent agreement Thursday and Thursday
night but begin to diverge on Friday and by Sunday consensus was on
the low end of fair. The pattern on Wednesday features a closed mid-
level low over eastern Montana with another closed low spinning just
off the coast of San Francisco. Between these systems relatively dry
southwest flow moves over the forecast area with PWAT values near
0.3" for all but a small portion of the eastern Uinta Mountains
where values were near 0.5" according ensemble means. As a result,
skies will be partly cloudy/mostly cloudy over the Uintas Wednesday
with a slight chance for afternoon showers/thunderstorms. Warm air
advection and sunshine will help temperatures rise toward normal
late Wednesday afternoon. On Thursday models move the Pacific low
southeastward, though the ECMWF keeps it centered closer to the
coast just west of Bakersfield. The Canadian and GFS shift it
farther east along the border of Nevada northwest of Las Vegas.
Despite these differences, the forecast looks similar to Wednesday
with slight chances redeveloping for the eastern Uintas Thursday
afternoon while dry weather continues elsewhere. Temperatures climb
to near normal Wednesday and may rise a degree or two above the norm
on Thursday.

As noted previously, models fall further out of sync Friday, but
regardless, the region will see a modest increase in moisture with
PWATs climbing to near 0.5" for much of the area by midday. This
should be sufficient to generate low end scattered afternoon/evening
storms over higher terrain. The main threats will be gusty outflow
winds and lightning as the subcloud was indicated to be dry.
Nocturnal storms are likely Friday night as the low moves across
northern Arizona providing dynamic lift, though timing does vary
between models, so don`t want to get too hung up on positioning of
best chances. Looking at NBM compared to operational runs, it does
appear the blends are chasing the latest guidance a bit and won`t be
surprised to see the PoP field shift toward the southern zones in
later runs. By Saturday, positioning of the low is clearly up for
debate, though Canadian and GFS solutions were more similar. The
Canadian moves the closed low to the northeast Colorado Plains while
the GFS fills the low, though trough positioning was similar. In
contrast, the ECMWF carries the closed low over southwest Colorado
Monday afternoon. So again, the forecast is fairly generalized with
showers favoring the north in the NBM and while the ECMWF favored
the south and the Canadian and GFS were fairly equally distributed
along the Continental Divide. With the EC solution, showers may
linger overnight and persist into Sunday across the north as the low
center hurdles the Divide and moves to northeast Colorado then to
the northern High Plains on Sunday. The other models push the low
farther east over the central Plains and suggest precipitation
chances end ahead of midnight Saturday with potentially enough
lingering moisture for a few storms along the Continental Divide
Sunday afternoon.

Temperatures dip with increased clouds and showers and the trough
passage this weekend, running 3 to 5 degrees under seasonal norms
with the coolest temperatures arriving Saturday. Meanwhile,
overnight lows are expected to run within a few degrees of normal
throughout the long term period.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
-- Changed Discussion --
Issued at 529 PM MDT Mon Sep 16 2024

Widespread showers and thunderstorms will continue through the
evening and into the overnight. Main impacts from storms will be
gusty outflow winds, brief heavy rain reducing visibility,
frequent lightning, and maybe some small hail. As the night
continues, the threat will transition more toward heavier rain
without the convective component. Additionally, outside of
storms, strong southerly to southwesterly winds will continue
through the TAF period, with gusts in excess of 35 knots
possible. VFR conditions will prevail, but occasional drops to
MVFR/IFR will be possible through the period thanks to lowering
ceilings and reduced visibility.
-- End Changed Discussion --

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

SHORT TERM...DB
LONG TERM...NL
AVIATION...TGJT