-- Discussion containing changed information from previous version are highlighted. --
999
FXUS65 KGJT 160450
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1050 PM MDT Sun Sep 15 2024

.KEY MESSAGES...

- Showers and thunderstorms decrease this evening, then
  redevelop in the south late tonight.

- 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.

- Temperatures dip to near normal Monday in response to clouds
  and increased shower/thunderstorm coverage. A cold front
  moving through the region on Tuesday is expected to bring
  cooler than normal readings for the latter part of the week.

- Expect a break in the action Wednesday through early Thursday
  afternoon, then another storm system impacts the region
  Thursday night into Saturday.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 313 PM MDT Sun Sep 15 2024

As models had depicted, remnant moisture from Ileana, which has
been downgraded to post-tropical cyclone remnants, reached the
forecast area during the early morning hours. Looks like the GFS
solution from 12Z/Sat, which featured better chances for showers
and thunderstorms over the Four Corners region was on the right
track with the other models a bit dry when compared to the
activity apparent on radar around 13Z. Storms now favored the
central and northern part of the forecast area as lift provided
by a low-amplitude short wave trough was moving over that area.
Meanwhile, clearing behind the wave has allowed good surface
heating and, as a result, new cells have formed over the
Uncompahgre and the western San Juan Mountains.

As the short wave trough mentioned previously shifts to the
north of the forecast modest subsidence and diurnal cooling will
result in a downturn in moist convection during the evening.
However, later tonight another slug of moisture arrives over the
Four Corners region. Jet divergence associated with the 90+ Kt
just moving through the base of a deepening closed low pressure
system over the San Francisco Bay area coincides with the deeper
moisture which will cause showers and thunderstorms to redevelop
in the early morning hours. This activity spreads northward as
the closed low moves east over northwest Nevada which increases
the spread of moisture northward. This coincides with a more
favorable area of difluent flow aloft, which when combined with
and diurnal warming is expected to result in numerous showers
and thunderstorms as far north as the Flat Tops with more
scattered activity to the north and west. Additionally, decent
shear and MUCAPE values near 1000 J/Kg over southeast Utah and
southwest Colorado bring the potential for organized storms
cells with one or two capable of producing severe weather. The
main threats will be strong outflow winds to 60 mph and hail
approaching an inch in diameter. Meanwhile, with PWATs climbing
toward an inch the potential for localized excessive rainfall
exists which could lead to debris flows or mud slides in areas
of unstable terrain.

Showers and embedded thunderstorms will continue through Monday
night for much of the area. Forcing not all that impressive and
mainly driven by difluent flow aloft, but it won`t take much
lift with the amount of moisture available. Lower levels of
CAPE point to reduced storm intensity.

Expect milder overnight lows tonight and Monday night as clouds
and higher moisture levels buoy readings to above normal levels.
Highs on Monday, on the other hand, will be cooler than over the
past several days due to fairly widespread shower and
thunderstorm activity, though readings only dip to near normal
levels.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Issued at 313 PM MDT Sun Sep 15 2024

Models were in good agreement Tuesday with a strong 100kt south
to southwesterly jet streak sliding east across eastern Utah
and western Colorado through the day supporting a frontal
boundary moving west to east under it. Expect southerly winds
gusting 30 mph ahead of the front turning more westerly behind
the front that will sweep the moisture out of the region
bringing clearing skies and a quick end to convection. Ahead of
the front, moisture remains ample for showers and thunderstorms
with PWATs running above normal at 0.6 to 0.8 inches. Though
CAPE will be on the low end, <500 J/kg, there will be plenty of
shear to support some of the storms becoming well organized.
The main threats from these storms will be frequent lightning,
strong winds, large hail, and localized flooding in areas of
training storms. The southern faces of the southern and central
Colorado mountains may pick up on average another half inch
precip through the day mostly in the form of rain as the snow
levels remain above 12 Kft ahead of the front. Behind the
front, snow levels drop below 9 Kft with the higher peaks
possibly picking up a quick dusting to maybe an inch of snow
before the dry air moves in.

As the low continues to track northeast across Wyoming and
eastern Montana Wednesday, another deep low descends through
the Pacific Northwest into the Great Basin. The deterministic
models diverge on timing of this low ejecting east across the
region with the GFS bringing it through Friday into Friday night
with the European about 18 hours behind and the Canadian
splitting the difference. The European and GEFS ensembles narrow
the difference to about 12 hours leaning towards the GFS, but
both indicate wide dispersion among the members, so either could
be right. There is a little moisture associated with this
system with PWATs slightly above average at 0.5 to 0.7 inches,
so expect widely scattered showers and thunderstorms over the
higher terrain between Thursday night and Saturday. Stay tuned
over the next couple day to get the timing nailed down.

Daytime temperatures through the extended period will generally
run five to ten degrees below normal behind the front as the
longwave trough remains in place over the western States.
Morning lows will be near normal to slightly below normal for
this same period.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
-- Changed Discussion --
Issued at 1050 PM MDT Sun Sep 15 2024

Clouds will increase overnight with a line of showers and
embedded thunderstorms arriving near the Four Corners by 12Z and
tracking northeastward across the area through the morning
hours. Added VCSH for many TAF sites affected by this line but
cannot rule out some morning thunder along the leading edge.
Some clearing will work in behind this line allowing time to
widespread to numerous storms to fire and become more organized
this afternoon. Expect VCTS at most TAF sites Monday afternoon
into early evening with convection winding down to scattered
Monday evening. Periods of light to moderate rain is possible
with the initial line with brief heavy rain, hail and strong
gusty winds possible with afternoon convection. VFR to MVFR
expected with brief periods of IFR/ILS conditions possible in
stronger storms.
-- End Changed Discussion --

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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