414
FXUS65 KGJT 151755
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1155 AM MDT Sun Sep 15 2024

.KEY MESSAGES...

- Showers and thunderstorms return to the area today as the
  leading elements of remnant moisture from TS Ileana moves into
  the region.

- Conditions may become favorable for strong, organized
  thunderstorms Monday afternoon/evening which could result in
  localized wind gusts in excess of 60 mph and hail approaching
  an inch in diameter.

- Above normal temperatures persist through Monday before a cold
  front moving through the region on Tuesday brings cooler than
  normal readings for the latter part of the week.

- Aside from Wednesday, off and on showers can be expected
  across eastern Utah and western Colorado next week.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 300 AM MDT Sun Sep 15 2024

A low amplitude trough approaching the region from the southwest is
drawing moisture from the remnants of tropical storm Ileana that is
currently over Arizona and New Mexico. The trough is expected to
track over the forecast area today along with the moisture. The lift
provided and moisture will support scattered showers through out the
day. Low levels are fairly dry so cloud bases will be relatively
high, which will limit rainfall amounts. The mountains have the best
chance of measurable rain while valleys may only see sprinkles.
Strong wind gusts could not be ruled out due to the high DCAPE.
Subsidence behind the short wave trough combined with diurnal
cooling will bring a downturn in shower/thunderstorm coverage this
evening. Tonight a stronger trough moves southward over California
and taps into even better remnant moisture currently in northern
Mexico. The associated jet begins to nose over the region as well as
diffluent flow aloft. Modest moisture advection (PWAT upwards of 200
percent of normal) together with the dynamics should support
widespread showers and storms especially in the morning hours. The
high-res models show a large swath of convection and stratiform that
favors the southwest half of the area. The models continue
convection into the afternoon as the low pressure begins to lift
over Nevada. Wherever the clouds can break up CAPE could reach 500-
1000 j/kg, which appears like it may be the western half of the
CWA. That in combination with shear supports the chance for
stronger and more organized storms. These storms will be capable
of gusty winds, large hail and heavy rainfall. Scattered
convection looks to linger into the overnight hours as moisture
advection does not let up. The highest rainfall amounts by late
tomorrow evening are expected to be in the San Juans where
upwards of an inch are possible (NBM probs of 50%). This poses a
slight risk for localized flooding. There may also be a dusting
of snow on the higher peaks across the forecast area.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Issued at 300 AM MDT Sun Sep 15 2024

Showers and thunderstorms look to continue through the overnight
hours Monday night as the deep low over the Great Basin lifts
northeastward into Idaho by Tuesday afternoon. The cold front will
be right on our doorstep moving across eastern Utah Tuesday morning
and across western Colorado Tuesday afternoon. Moisture still
remains to the east of the front with a much cooler and drier air
mass advecting in behind the cold front. We will still see potential
for some showers and storms, some of which could be strong, firing
along the cold front and moisture gradient. Areas along and to the
east of the cold front will be in the warm sector as well as the
left exit region of a 100 kt jet streak. Gusty winds in the 30s and
40s remain possible as this trough axis and cold front swing through
Tuesday afternoon. PWAT anomalies will drop from 120 to 150 percent
of normal ahead of the front to 50 to 80 percent of normal behind
the front. So, safe to say, this cold front will sweep out whatever
moisture is left. Temperatures drop to around 5 to 10 or so degrees
below normal for highs on Tuesday. Clearing skies Tuesday night
along with the drier air mass post-frontal will allow for efficient
radiational cooling and our lows quite cool and fall-like.

On Wednesday, we will be in a transition period with much drier
conditions as one low moves out and another dives into the Great
Basin. There is quite a bit of model variance beyond Wednesday with
how to handle this next system. The GFS brings this low quickly
across our CWA as an open wave trough on Thursday, while the ECMWF
keeps this trough over the Great Basin Thursday before bringing it
across our CWA on Friday. One thing the models seem to agree on is
that this system seems to be lacking in the moisture department
unlike the previous system. Despite timing differences, this system
also helps reinforce the cooler air as highs in the wake of this
system will drop from around 5 degrees below normal to between 5 and
10 degrees below normal. Snow is possible on the higher peaks in the
wake of this system due to the cooler air mass. The question really
is when this occurs. The blended guidance maintains chance PoPs with
unsettled conditions Thursday through Saturday with the best
potential Thursday night into Friday...kind of a consensus between
the models. Northwest flow takes hold behind this system as a ridge
builds just off the West Coast, keeping cooler below normal
temperatures in place.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1135 AM MDT Sun Sep 15 2024

Scattered showers and isolated thunderstorm currently east of
the line KVEL-KDRO is moving northeast across the region through
the afternoon will exit the region to the east by about 02Z.
Another round of stronger showers and thunderstorms will move
up into the Four Corners area from Arizona and New Mexico after
07Z, spreading northeast across the region and becoming more
widespread by about 15Z. Primary threats from this latter band
of storms are lightning, strong wet or dry downbursts with gusty
outflow winds and large hail.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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