-- Discussion containing changed information from previous version are highlighted. --
005
FXUS65 KGJT 222341
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
541 PM MDT Sun Sep 22 2024

.KEY MESSAGES...

- Showers linger over the western Colorado mountains this
  afternoon with embedded thunderstorms possible.

- 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 towards well above normal levels
  mid to late week as high pressure builds over the region.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 204 PM MDT Sun Sep 22 2024

The low pressure system that moved through the area last night
is currently cyclonically spinning over northeast Colorado, near
the Kansas-Nebraska-Colorado triple point. Plenty of clouds
still exists across the western Colorado mountains with
lingering light showers but this is limiting convective
instability and potential for storms this afternoon as the sun
is struggling to break through over the mountains. Any storms
that do form, which looks like a low probability at this point
unless these clouds could clear and allow for surface heating,
look to favor the San Juans according to the hi-res CAMs.
However, at this time, nothing much is developing on radar aside
from maybe some shallow light showers that are below the beam.
Precipitable water on this morning`s 12Z GJT sounding was 0.83
inches, which is largely an effect of lingering moisture from
last night`s low. Conditions are beginning to dry out from the
northwest and expect this drier air to move in tonight with
skies eventually clearing out, leading to a cooler night as the
low moves further east into the Plains due to efficient
radiational cooling. Along with clearing skies, we could see the
formation of some fog in higher mountain valleys and basins
along the Continental Divide.

Drier northwest flow ushers in behind the departing low on
Monday with a quick moving shortwave passing through. This
shortwave comes through with enough forcing to produce some
isolated storms over the San Juans but that appears to be about
the best it can muster as it will be much drier. Temperatures
on Monday will warm a bit towards near normal compared to
today`s below normal highs in the wake of last night`s storm
system that sure made it feel much more like autumn on this
official first day of Fall.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Issued at 204 PM MDT Sun Sep 22 2024

Tuesday morning starts off with potentially breezy conditions
thanks to a tighter pressure gradient aloft, as well as a modest
upper level jet. These conditions won`t last long, however, as
this jet sweeps out into the Plains, and a large dome of high
pressure builds into the Great Basin. This high will be in
charge of the weather across much of the western US, including
here in eastern Utah and western Colorado, through at least
Thursday morning. Much warmer air builds in with the ridge,
seeing temperatures jump from 3-5 degrees above normal on
Tuesday to 8-12 degrees above normal by Thursday. Lower desert
valleys will see mid-to-upper 80s, with even some pockets of low
90s, with the higher elevation valleys are looking at upper 70s
to low 80s. Mountain towns can expect temperatures in the 60s
through the period. Seems that summer needed to get one more
word in before fall could really take over. Although, longer
nights combined with calm winds and clear skies will allow for
some good radiational cooling that will keep lows a bit closer
to normal. Cool nights to temper the unusually warm days, all
together making for a fairly pleasant middle of the workweek.

Even as quiet weather dominates the western CONUS, the forecast
for the eastern half of the US is much more complicated.
Wednesday is where uncertainty begins to creep in, as an
elongated trough over the middle of the country cutoff a low
over the Missouri/Arkansas/Oklahoma region, while at the same
time a tropical entity enters the southern Gulf of Mexico. The
real source of uncertainty arises from how these features play
off each other and what that does to the overall continental
pattern. Current guidance uses the interplay of these two
features to block up the large scale pattern, squeezing the
ridge in the west between the cutoff eastern low and a large
Pacific trough moving into the Northwest. This won`t make a
great deal of difference to our sensible weather, at least at
this point, as we`ll remain under a col between all these
features...leaving us high and dry. There is some potential for
breezier conditions Thursday and Friday, depending on how far
south the Pacific trough`s jet is able to make it, meaning a
close eye will need to be kept on this period for fire weather
concerns. This blocked pattern is forecast to persist through
the weekend, with temperatures remaining well above normal and
generally quiet conditions prevailing. However, we are in a
transition season. Models are notorious for struggling during
these times of the year, and as we`ve seen with the recent storm
over the Western Slope, guidance can change drastically not
just from day to day but from model run to model run. So stay
tuned, while the forecast for late week looks warm and quiet
now, it may change quite a bit in coming days. Until then, enjoy
a late taste of summer.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
-- Changed Discussion --
Issued at 537 PM MDT Sun Sep 22 2024

Clouds will slowly scour out this evening and mostly clear, VFR
conditions with light winds will return to the terminals. Skies
will remain mostly clear into the afternoon Monday, with some
scattered to broken clouds possible on the mountains again. A
few isolated showers can`t be ruled out Monday afternoon, thanks
to better sunshine and instability on the terrain. KEGE, KASE,
KGUC, and KTEX will once again be good targets of opportunity
for watching shower development. Gusty winds will be the biggest
threat Monday in and around showers. Elsewhere, winds remain
light and terrain driven through the day, with sunny skies.
-- End Changed Discussion --

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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