279
FXUS65 KGJT 292140
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
340 PM MDT Sat Jun 29 2024

.KEY MESSAGES...

- A Flood Watch for southeast Utah and southwest Colorado is in
  place through Monday afternoon. Abundant monsoonal moisture is
  expected to produce heavy rainfall rates and thunderstorms on
  Sunday and Monday.

- Monsoonal moisture gets shunted east on Tuesday, and
  afternoon showers will be far more isolated in nature.

- A drying trend develops later in the week in time for Fourth
  of July activities.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 324 PM MDT Sat Jun 29 2024

Clear skies this morning quickly eroded the capping inversion on the
Grand Junction morning sounding. Pockets of monsoonal moisture
continue to arrive, despite a `downturn` in moisture today. This has
culminated in widespread convection on our terrain with southward
exposure, including the Tavaputs, San Juans and Uncompahgre Plateau.
Storms have, thus far, been garden variety monsoon storms. Some
small hail, gusty outflows, and lightning will still pose weather
risks to those outside this afternoon and evening. Precipitation
rates remain low too, so flash flooding is off the table for now.
Unfortunately, that changes Sunday with the arrival of another rich
plume of moisture. Have gone ahead and issued a Flood Watch for our
southern counties including southeast Utah and southwest Colorado,
where the deepest moisture will create favorable conditions for
flash flooding Sunday and Monday. Flooding concerns will remain
heavily localized under the deepest convection. Outdoor enthusiasts
and travelers are expected to keep up with local forecasts and real-
time conditions in flood prone areas Sunday afternoon through
Monday. While PWAT values have remained above normal today, by this
evening, a plume of 200 percent of normal PWAT begins to invade the
Four Corners and spreads across the southern half of the CWA and
remains there through Monday. An open wave coming ashore tonight is
expected to tighten the pressure gradient and should keep breezy
conditions in place overnight. This should serve to keep us rather
mixed and likely keep inversions on the minimal side. With minimal
capping in place Sunday, I expect convective initiation to engage
early. Showers should be well fueled by then and have no problem
dropping some generous rainfall across the region. Favored areas for
heavy rainfall remain mostly south of I-70 and along the Divide to
the north. Flash flood concerns include flood prone areas of
southeast Utah and southwest Colorado. Low temperatures tonight will
remain on the high side in the well mixed regime, while Sunday
afternoon highs will get knocked down with cloud cover and
rainshowers in most locales. A few desert valleys will likely surge
up, if they avoid the cloud cover of surrounding terrain-based
thunderstorms.

&&

.LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/...
Issued at 324 PM MDT Sat Jun 29 2024

Convection will be ongoing Sunday evening as plentiful moisture will
continue to be worked on by an approaching upper level trough. The
high pressure over eastern Texas will continue to pump in monsoonal
moisture as guidance suggests a wave rotating around the high. This
wave, as of now, is progged to move through the area from southwest
to northeast from 6PM through just after midnight. A secondary wave
will be moving across Arizona and will get pulled into our CWA as
the approaching trough moves through. This will get another round of
precip going and possibly continuing through the day Monday. The
NAMNEST does show a line of stronger showers and storms moving
across the region from roughly 3AM through 10AM Monday morning with
a second line of weaker convection moving shortly after the first
push. Then another line associated with the trough moves in from the
northeast Monday afternoon. Looks busy. Important to note that there
should be plenty of cloud cover from the overnight rain and it may
temper the strength of convection on Monday though we`ll still have
to deal with heavy rain. PWATs are forecast to be around 1 inch
across most of the CWA so flash flooding will certainly be a
possibility under and near the stronger convection or if any
training occurs.

PWATs are expected to start dropping on Tuesday but models always
bring in this drier air too fast so look for convection, firing off
the higher terrain with the usual hail, gusty winds, and heavy rain
being possible. The NBM is downplaying the extent of convection but
do expect PoPs to increase as short range models get their hands on
this period.

Midweek onwards, an area of high pressure builds in from the West
Coast and shunts the plume of moisture to the east. As the high
continues to build in, anticyclonic flow will then bring northerly
winds to the CWA and keep moisture over Arizona and New Mexico. A
passing trough may clip our CWA and bring some isolated convection
to the Divide on Saturday but coverage is minimal at best.

Clouds and precip will determine temperatures through the long term
period. As of now, MOS guidance is showing most areas hovering near
normal or reaching 5 degrees above more seasonal early July values.
Again, more cloud cover/precip will limit those daytime highs.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1121 AM MDT Sat Jun 29 2024

Showers and thunderstorms will increase in coverage this
afternoon. Any showers and storms that form will be capable of
producing gusty and erratic winds, frequent lightning, and brief
heavy rain. Following HRRR guidance, included VCTS or -TSRA on
station for many TAF sites. Will amend as necessary as the
afternoon continues. Brief MVFR is possible under and near any
of the stronger showers or cells with widespread VFR expected.
ILS breakpoints may also be met near the stronger convection.
Activity will diminish, though probably not completely end,
after sunset.

&&

.HYDROLOGY...
Issued at 324 PM MDT Sat Jun 29 2024

Abundant monsoonal moisture returns Sunday through Monday and
is expected to produce heavy rainfall rates. Flood Watches are
in place for southeast Utah and southwest Colorado Sunday and
Monday. Flooding is expected to be heavily localized under
stronger thunderstorms. Slick rock areas, slot canyons, and burn
scars are at greatest risk for flooding, but urban flooding is
also a concern. Be sure to monitor the latest forecasts and be
prepared to take action should Flash Flood Warnings be issued
for your area!

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...Flood Watch from Sunday afternoon through Monday afternoon for
     COZ009-011-012-014-017>023.
UT...Flood Watch from Sunday afternoon through Monday afternoon for
     UTZ022-028-029.

&&

$$

SHORT TERM...LTB
LONG TERM...TGR
AVIATION...TGJT
Hydrology...TGJT