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883
FXUS65 KGJT 212114
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
314 PM MDT Sat Sep 21 2024

.KEY MESSAGES...

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- An early fall storm will bring showers and storms through
  tonight with mountain snow, mainly above pass level, though
  Vail, Monarch and Red Mountain passes are likely to experience
  wet, slushy snow tonight.

- Unsettled conditions remain in place through Monday, with
  afternoon thunderstorms possible, mainly over higher terrain,
  before drier conditions move in Tuesday through the end of the
  week.

- Temperatures rebound to near normal Monday and Tuesday
  followed by a warming trend during the latter part of the week
  as high pressure builds over the region.
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&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
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Issued at 314 PM MDT Sat Sep 21 2024

Showers and thunderstorms came this morning across southeast
Utah and spread into southwest Colorado as advertised with these
storms starting off fairly robust on the leading edge of the
wraparound precipitation on the eastern and northern sides of
the low. Also hinted by the models, a dry slot has wrapped up in
the center of the closed low over the Four Corners this
afternoon, shutting down convection in that region. CAPE values
were fairly decent in the 500 J/kg range this morning across
southeast Utah and southwest Colorado, with that region in the
left exit region of the upper level jet, so forcing was
sufficient for thunderstorms. However, the latest HRRR indicates
CAPE reducing to less than 100 J/kg later this afternoon and we
are seeing a reduction in the strength of convection this
afternoon as showers are becoming more stratiformed with
embedded thunderstorms here and there. Gusty outflow winds of 30
to 40 mph have been observed and expect this to remain the case
through this evening as the wraparound moisture continues around
the closed low. Small hail was detected earlier in the day but
thinking this concern is waning as top down saturation begins to
occur with the advection of cooler air associated with the low. The
closed low will track through southwest to central Colorado
this evening, staying around central Colorado overnight before
exiting to the east Sunday morning.

Speaking of cooler air, snow levels look to lower to around
10kft this evening, which is the period that some snow will
accumulate at elevations above 10kft, focused on the central
Colorado mountains where the closed low appears to stall
overnight before moving out. Models were quicker with the
progression of this closed low today and this is seen on
satellite and radar trends. Snow accumulations look to be
around 2 to 4 inches with the best chances overnight but pass
level should remain wet to slushy in spots with limited impacts
if any. Some light snow will linger over the high terrain
through Sunday morning and models are indicating some isolated
to scattered shower and storm development over the western
Colorado divide mountains Sunday afternoon, but overall drier
northwest flow should be pushing in by Sunday afternoon with
little additional accumulation. Sunday night is expected to be
quiet with breezy northwest flow ahead of a quick moving
disturbance set to move through Monday. Temperatures on Sunday
will be markedly cooler with highs 5 to 10 degrees below normal
in the wake of this fall storm, right on cue with the official
first day of Autumn!
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&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
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Issued at 314 PM MDT Sat Sep 21 2024

Monday afternoon, a quick moving shortwave trough will pass
through the area from the northwest, leading to some isolated
showers and storms over the western Colorado mountains. Other
than that, should be a relatively quiet day as lower valleys
look to remain dry with the mountains picking up some convection
but probability and PoP chances seem to remain on the lower end.
Blended guidance is finally starting to fall in line with the
overall drying trend that was seen in the deterministic models
since Friday`s morning run with a strong ridge of high pressure
building across the western half of the CONUS Monday, shifting
overhead by mid week. Despite a weak shortwave trying to disrupt
the ridge next Thursday across the Northern Rockies, this really
has a minimal impact as the ridge rebounds and remains in place
through next weekend. So, confidence is higher that Tuesday
through the end of the coming week will remain dry with
temperatures warming back to near to slightly above seasonable
levels by Wednesday and perhaps as much as 10 degrees above
normal by the latter half of the coming week.
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&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1235 PM MDT Sat Sep 21 2024

A low pressure system will track northeast across the Four Corners
this afternoon, moving over the central Colorado mountains by
00Z/Sun. Expect showers and thunderstorms over southwest
Colorado to spread north and west around the low into the
central mountains and valleys after 21Z and into the northern
mountains by 00Z/Sun. The main threat from these storms will be
gusty winds 30 to 40 kts, small hail and potentially brief
moderate to heavy rain in the stronger cells. Snow levels start
around 12K feet lowering to 11K feet by the end of the TAF
period resulting in snow showers obscuring the higher mountain
peaks across the region. Expect brief periods of MVFR conditions
at KDRO and KTEX between now and 00Z, and at KGUC, KASE, KEGE,
KGJT and KRIL after 21Z through the evening hours. Ceilings are
expected to fall below ILS breakpoints at KRIL, KEGE and KASE
after 22Z through about 12Z before improvements may be seen from
west to east through Sunday morning.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

SHORT TERM...MDA
LONG TERM...MDA
AVIATION...TGJT