209
FXUS65 KGJT 210913
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
313 AM MDT Sat Sep 21 2024

.KEY MESSAGES...

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

- Showers and thunderstorms become likely for much of the region
  as a Pacific storm passes over southeast Utah and southwest
  Colorado.

- Unsettled conditions remain in place through much of next week
  with daily thunderstorms possible, mainly over higher terrain,
  each afternoon/evening.

- 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.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 313 AM MDT Sat Sep 21 2024

As models had predicted, a deep closed mid-level low moved to
western Arizona early this morning. As of 0215Z, there were few
clouds over the region as water vapor imagery showed a dry slot
ahead of the low was directly overhead. However, that is soon to
change.

Models were in very good agreement in shifting the low center
northeastward to almost directly over the Four Corners by
00Z/Sun. As the system moves to central Arizona this morning
clouds will increase and lower across the south. Showers begin
to develop this morning in response to good mid-level forcing
indicated in model Q fields combined with jet divergence.
Showers spread into the north during the afternoon and become
widespread for much of the region as upward forcing becomes
centered over central Colorado. Northeast Utah and far northwest
Colorado are likely to miss out on the action as the track of
the low keeps these areas on the fringes of its reach. The San
Juan Mountains may see a couple of organized thunderstorms due
to more favorable shear, but most storms will produce gusty
winds to 40 mph along with small hail and lightning. In the
higher elevations, expect some snow. This will be primarily
above 10,000 feet, but convective cells will be capable of
driving levels down to mountain passes, but wouldn`t expect more
than wet roads before sundown. Unsurprisingly, temperatures
will be markedly cooler across the region in response to cloud
cover and shower activity.

Shower and thunderstorms continue tonight as the low tracks
slowly east-northeastward to the Colorado Plains by 12Z/Sunday.
Widespread precipitation will gradually diminish from west to
east in the wake of the low leaving scattered showers over the
northern and central Colorado mountains by sunrise Sunday. Snow
becomes more likely to accumulate at pass level overnight and
light accumulations are possible, but expect more of a heavy wet
slushy composition on road surfaces. Lows overnight will be
cooler than in recent days, but not significantly so.

Precipitation continues to wind down Sunday morning as the low
tracks farther east over the central Plains during the day.
However, a strongly positive tilt trough is left draped over the
forecast area in the storm`s wake, which when combined with
daytime warming and lingering low level moisture should yield
some isolated to low end scattered thunderstorms during the
afternoon. In the wake of the storm temperatures are expected to
cool further, dropping to around 5 degrees below normal Sunday
afternoon.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Issued at 313 AM MDT Sat Sep 21 2024

Northwest flow sets up Sunday evening ahead of another
disturbance projected to move through the area as an open wave
positively tilted trough on Monday afternoon. Unlike previous
model solutions 24 hours ago, the latest guidance shows a quick
moving weaker wave passing from northwest to southeast as
opposed to a more robust system dropping down the western slope
from north to south. So, expect some scattered showers and
unsettled conditions, but mostly favoring the mountains with
little in the way of accumulation.

My oh my how the models change even after 24 hours. This is why
we cannot hang our hat on one particular solution beyond 3 days
as models have a tendency to change. The latest GFS solution now
indicates drier northwest flow following the quick moving
shortwave by Tuesday with high pressure ridge building out
west and moving overhead by mid to late week. A weak wave
brushes the north on Thursday but even this looks dry. Lo and
behold, the ECMWF is fairly similar in this drier trend. So,
temperatures will start off the week much cooler than we`ve
seen with highs 5 to 10 degrees below normal on Sunday following
Saturday`s closed low system. Highs stay relatively 5 degrees
below normal for the early part of next week with temperatures
moderating towards near to slightly above seasonable by mid to
late next week. This could be a tad milder if the drier solution
holds true. Blended model guidance keeps isolated to scattered
PoPs in the forecast through mid week, probably a carryover from
previous solutions but if this drier pattern holds in future
model runs, anticipate these PoP chances will dwindle. Low
confidence exists in PoPs beyond Monday.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1113 PM MDT Fri Sep 20 2024

An approaching low pressure system will track northeast across the
Four Corners after 18z reaching the San Juan Mountains by 00Z
Sunday. Expect isolated showers to start after 10Z across
southeastern Utah and spread east into the San Juan Mountains by
about 15Z. Showers and thunderstorms become scattered to widespread
across the southern areas after 18Z spreading north to the I-70
corridor by 21Z and into the northern mountains by 00Z Sunday. These
storms may produce periods of moderate to heavy rain and wind gusts
of 30-40 kts. 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 below ILS breakpoints at KDRO and KTEX between 15Z
and 00Z, and at KGUC, KASE, KEGE and KRIL after 20Z through the TAF
period.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

SHORT TERM...NL
LONG TERM...MDA
AVIATION...DB