880
FXUS65 KGJT 121153
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
553 AM MDT Mon Aug 12 2024

.KEY MESSAGES...

- Showers and thunderstorms continue each afternoon and evening
  until further notice with gusty outflow winds, small hail,
  and frequent lightning possible.

- Heavy rain rates with storms will bring an enhanced threat of
  flash flooding and debris flows through Tuesday. Saturated
  soils and steep terrain will be most susceptible to flooding.

- Temperatures will remain near to several degrees below normal
  thanks to the abundance of clouds and showers.

- Drier air will result in a downturn in storm coverage mid to
  late week with a potential return of moisture by this weekend.

&&

.SHORT TERM /THROUGH TUESDAY/...
Issued at 359 AM MDT Mon Aug 12 2024

Storms are still firing on the southern edge of the cloud shield
across southwest Colorado where brief moderate to heavy rain and
frequent lightning is still occurring. Most of the showers have
stratiformed out down south or come to an end for all areas
north of the San Juans and Abajo Mountains. These storms across
the southern foothills are moving towards the east so anticipate
the duration to be short-lived with MRMS currently indicating
0.5 to 1.0 inch an hour rates. Hi-res CAMs that are picking up
on this activity indicate a downward trend over the next couple
hours with most activity coming to an end towards sunrise. We
are still left with plenty of cloud cover across the area, much
like early yesterday, which could result in a bit of a delay in
convective initiation and later start.

For today, we will kind of be in a transition phase between the
westerly flow and switch to southerly flow for Tuesday. Hi-res
CAMs and even deterministic models indicate a bit of a downturn
in convection across western Colorado. Even though deep
monsoonal moisture remains in place, what we are lacking is
sufficient instability (probably due to more CIN and more cloud
cover this morning) where CAPE values are maybe up to 200 J/kg.
Also, we lose the jet support today as the jet lifts north of
the region. However, better jet support due to the incoming
PacNW trough is seen across Utah today ahead of a rather robust
shortwave trough that is projected to move through on Tuesday.
Higher CAPE values up to 1000 J/kg are also seen across
northwest Utah today. So, we will still see some scattered
storms today but mainly forming over the high terrain late this
afternoon with better chances across eastern Utah, and
especially northeast Utah due to better instability and forcing.
Any storms will still be capable of moderate to heavy rain as
well as gusty winds, small hail and frequent lightning...but the
flooding threat appears more localized and isolated today due to
less storm coverage, instability and forcing than seen in
previous days. Therefore, opted not to issue a Flood Watch at
this time.

Storms actually look to increase this evening across eastern
Utah and spread into western Colorado overnight due to the
approaching shortwave trough and increasing moisture in the
southerly flow. On Tuesday, a rather robust trough moves
through, so we gain back the jet support as well as the
instability as CAPE values climb higher up towards 500 to 1000
J/kg for Tuesday afternoon. Tuesday looks more widespread with
storms and a better potential for heavy rain and flash flooding
as training storms look possible, so stay tuned for updates on
the latest forecast as Tuesday appears more active. As far as
temperatures are concerned, we continue to remain a few degrees
below normal today with the coolest day occurring Tuesday due to
increased cloud cover and better chances and coverage of showers
and storms. Highs on Tuesday look to drop to 6 to 12 degrees
below normal as a result.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Issued at 359 AM MDT Mon Aug 12 2024

The trough passage on Tuesday will nudge high pressure into
eastern Texas/Louisiana by Wednesday, which, in turn, shifts
both sources of moisture east of the Divide. A new area of high
pressure builds back in over the Desert Southwest in the wake of
Tuesday`s wave. This hot and dry air mass will work its way
into the CWA from the southwest, reducing storm coverage across
the Four Corners and lower valleys on Wednesday. However,
lingering moisture will still be sufficient for afternoon
convection to flourish atop the high terrain. Ridge of high
pressure shifting overhead on Thursday and Friday will continue
to support dry air advection across the Western Slope, as such,
we`ll see a downturn in terms of storm coverage and likely
rainfall intensity for the end of the work week. Confidence in
forecasting the evolution of the high decreases leading into
the coming weekend. Half of the ensemble cluster analyses show
above average moisture across the Western Slope by Days 6 and 7,
which would allude to the monsoon redeveloping. However, subtle
shifts in the high`s center may keep the monsoon plume to our
west, and a dry air mass overhead.

Temperatures during the long term start off approximately 3 to
8 degrees below normal, thanks to cloud cover and the abundance
of moisture. We`ll see a rise mid week through Friday as the new
area of high pressure builds overhead, with highs hovering
right around normal as we end the work week.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 553 AM MDT Mon Aug 12 2024

Some showers will linger this morning across the southern
valleys but should dissipate shortly after sunrise. Plenty of
cloud cover this morning should result in a later start to
convective initiation this afternoon. Expect scattered storms
developing over the high terrain around 20Z with better
potential across eastern Utah. Storms increase this evening
after 00Z across lower elevations of eastern Utah and northwest
Colorado. Storms may continue to spread into western Colorado
after 06Z with nocturnal activity possible due to a disturbance.
Kept VCTS at higher elevation TAF sites of KASE, KTEX and KGUC
as well as eastern Utah sites of KVEL and KCNY where confidence
was higher. Confidence in lower elevations of western Colorado
was less certain so left VCTS out of the TAF for now. Expect
primarily VFR conditions with MVFR and ILS conditions where
storms occur due to moderate to heavy rain. Gusty winds are also
possible with storms but overall winds should remain light and
terrain driven outside of storm activity.

&&

.HYDROLOGY...
Issued at 359 AM MDT Mon Aug 12 2024

Monsoonal moisture will keep showers and thunderstorms over the
region through at least Tuesday with rainfall rates of 1 to 2
inches per hour possible with storms. The flooding threat
appears more localized today given less storm coverage expected
but storm coverage and flooding potential increase Tuesday as a
low pressure trough moves through the area. Given how saturated
soils have become over the recent days, there is increased
potential for flash flooding and debris flows across much of the
area. Stay weather aware over the coming days, especially if
you live in terrain vulnerable to flooding, or are planning to
recreate outdoors.

Stay tuned to the latest forecast updates for any additional
flood watches, warnings, and / or advisories over the coming
days.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

SHORT TERM...MDA
LONG TERM...MDA/ERW
AVIATION...MDA
HYDROLOGY...TGJT