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524
FXUS65 KGJT 192023
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
223 PM MDT Thu Sep 19 2024

.KEY MESSAGES...

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- Conditions will remain dry with near to slightly above average
  temperatures through Friday.

- The next system arrives on Saturday bringing scattered showers
  and storms as well as a reinforcing shot of cooler air.

- The central mountains look to be favored for a majority of the
  precipitation with some in the form of snow.

- The weather pattern remains cooler and unsettled early to mid
  next week as additional disturbances move through, providing
  at least scattered shower activity.
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&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
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Issued at 223 PM MDT Thu Sep 19 2024

Satellite imagery shows the models struggled to initialize the
closed low just off the West Coast on the 12Z run this morning
with 100-200 mile variation between models. The GFS seems to
best fit with the satellite observations down through the mid
level, but none of the models picked up on the low-level
features. By 18Z, the models are already tracking ahead of
observations indicating the models are a little fast bringing
this low into the region. The deterministic models are running
ahead of the ensemble means pushing the low well into Arizona by
Saturday morning as is the NAM, but the HRRR looks much better
placing the low near the Nevada-California-Arizona triple point
more in line with the ensembles. The NBM guidance seems to also
be leaning towards the HRRR solution with wrap-around moisture
pushing up into southern Utah sparking isolated to scattered
nocturnal convection overnight Friday. Models are also hinting
the southerly flow will pick up some of the Gulf of Mexico
moisture over Chihuahua and push it north through New Mexico
into the San Juan Mountains along the Divide Friday evening and
overnight, presenting a slight chance for nocturnal showers and
thunderstorms in south central Colorado mountains though
confidence in these is lower. Otherwise, look for temperatures
running near normal to a little above normal this afternoon and
overnight with a band of high clouds moving up from the south
into the central mountains. Tomorrow will be a few degrees
warmer under mostly clear skies, but the sunshine will make it
feel much warmer. Clouds will start to move in from the south
and southwest Friday evening ahead of this next system.
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&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
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Issued at 223 PM MDT Thu Sep 19 2024

By Saturday morning, the low that appears cut off from the main
upper level trough will lift northeast through the Four Corners from
Arizona and move through much of southeast Utah into western
Colorado. This low packs a bit of a punch and is fairly compact,
closed and vertically stacked with a 100 kt jet wrapping around
the base providing needed orographic and dynamic lift Saturday
morning into the afternoon for showers and storms. Moisture
increases more on the right and northern sides of this low as it
pulls up some moisture from the Gulf of Mexico late Saturday
afternoon and evening through Sunday morning with wraparound
precipitation. In fact, PWAT anomalies increase to 160 to 200
percent of normal late Saturday afternoon and evening across the
eastern and northern areas of this low where the wraparound
precipitation occurs. The best precipitation favors the central
mountain areas with easterly upslope flow, which favors the
mountains along the central Continental Divide on the eastern
sides, but downslopes a bit into our western sides. That being
said, storms will still develop out ahead of this low, but
precipitation amounts may not reach the amounts that the eastern
slopes would receive. This time of year, we are considering
snow levels and impacts there. At this time, it looks like snow
levels Saturday start off above 12kft and lower to around 10.5
to 11kft Saturday night, so snow is possible at the higher peaks
with maybe around 1 to 3 inches with locally higher amounts
above timberline. Not expecting much impact though at pass
level as roads remain wet given milder afternoon temperatures.
Wraparound northwest flow will keep some light snow lingering
through Sunday morning over the peaks with light rain in the
lower elevations along the Divide, before clearing out by
Sunday afternoon, with the peaks revealing some white with
perhaps colorful foliage in the foreground as the cooler
temperatures are starting to allow the leaves to change in the
higher elevations.

Drier northerly flow takes hold Sunday afternoon with fast
moving northwest flow moving in Sunday night into Monday as
another shortwave trough drops in from the northern Rockies.
Conditions Sunday afternoon and evening could remain breezy in
this pattern between systems. This low again becomes cut off
from the main flow and drops north to south across western
Colorado Monday afternoon through Tuesday afternoon, keeping
conditions unsettled with more shower activity. High pressure
builds in overhead behind this wave, leaving somewhat of a Rex
Block with the low potentially lifting back over the Four
Corners region by mid next week. Models are in somewhat decent
agreement with the overall synoptic pattern but the jury is out
as run to run consistency has not quite been there. Will have to
wait and see how this evolves with future model runs, but
something to keep an eye on. The blended solution keeps
scattered PoPs and unsettled conditions, favoring the high
terrain for now which seems reasonable. Temperatures this
weekend through mid next week remains around 5 degrees below
normal in the lower elevations with around 10 degrees below
normal over the mountains with temps modifying to near or
slightly above seasonable mid to late next week, however this
all depends on the weather pattern and what happens with this
cut off low.
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&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1140 AM MDT Thu Sep 19 2024

VFR conditions will remain in place for the next 24 hours with
mostly clear skies and terrain, driven winds. Winds could be
gusty at times in the afternoon particularly at high terrain TAF
sites.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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