270
FXUS65 KGJT 182310
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
510 PM MDT Wed Sep 18 2024

.KEY MESSAGES...

- Conditions will remain dry and cooler than normal through late
  Friday.

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

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 302 PM MDT Wed Sep 18 2024

Satellite imagery shows the models initialized well this morning and
are tracking well with the weather with the last low over eastern
Montana moving to the northeast towards Saskatchewan and the next
low descending just off the California Coastline starting to turn
east to make landfall. Cool dry air in the lower levels and a weaker
gradient aloft is keeping winds light and mostly terrain driven in
the lowers elevations across the region under clear skies today. The
higher terrain will see a few gusts above 20 mph through this
afternoon and again tomorrow afternoon. Temperatures today are
generally running a few degrees below normal, but will warm about
five degrees tomorrow under continued mostly sunny skies. The low
off the coast will continue to dig to the south as it makes landfall
through the day Thursday and will be over Southern California by
Friday morning. This is a slower progression of the low than
the previous model runs, but otherwise little change as
discussed below.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Issued at 302 PM MDT Wed Sep 18 2024

Little changed in terms of both the weather pattern and model
consensus in the latest runs. As pointed out in the previous
discussion, models remained in good agreement through Saturday.
Variance between models becomes noticeable Sunday, though
differences don`t really have an impact on weather over the area,
but by early next week significant differences appear.

Models were in excellent agreement in moving the Pacific mid-level
closed low to southwest Arizona late Friday. Ahead of the low the
influx of dry air will continue, leaving little to no chance for
afternoon moist convection. However, as the low shifts toward the
Four Corners moisture wrapping around the system combined with
modest mid-level lift will begin to generate light showers over the
southern half of the forecast area. Expect much of this will be
virga with some light sprinkles reaching the ground here and there.
Chances ramp up quickly on Saturday as the low pushes across the
Continental Divide to the eastern Colorado Plains. As mentioned in
the previous discussion, PWAT values rise to around 0.5" to 0.6" for
much of the area which, when combined with daytime warming is
expected to result in scattered to numerous showers and
thunderstorms favoring the mountains in general and the Divide in
particular. Moisture levels not high enough for concerns of
excessive rainfall and dynamic and thermodynamic elements are
unlikely to yield severe weather either. Wrap around moisture is
expected to keep showers and embedded storms going over the northern
and central Colorado mountains through Sunday morning.

Models diverge Sunday, though all indicated a drier northwest flow
settling over the area reducing precipitation chances further over
Colorado`s northern Divide ranges. As mentioned previously, there
was a lot of variance in model solutions early next week. In
general, what they agreed to was a fairly vigorous shortwave moving
out of northern Rockies and sweeping across the area sometime
Monday. Not much moisture associated with this system so blended
solutions generated slight chance PoPs over much of the area Monday
and Monday night with 25 to 50% PoPs Tuesday, favoring the Divide.

Ahead of these systems temperatures will be warmer than normal
across the area Friday. However, cooler temperatures arrive Saturday
with the passage of the Pacific low. It appears from the latest
guidance that the cold air stays over the eastern side of the Divide
early next week, so highs will stay fairly persistent at just a
degree or two below seasonal norms. Morning lows show little
variance throughout the extended period.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 510 PM MDT Wed Sep 18 2024

Light terrain winds and VFR conditions can be expected over the
next 24 hours.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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