101
FXUS65 KGJT 272246
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
446 PM MDT Thu Jun 27 2024

.KEY MESSAGES...

- Deep subtropical moisture will remain in place this evening
  with widespread showers and thunderstorms expected to continue.

- Stronger storms have produced heavy rain, gusty winds, and
  hail to 1 inch in diameter. Localized severe storms and flash
  flooding are expected to continue this evening.

- A Flood Watch remains in effect for much of eastern Utah and
  all of western Colorado from noon to midnight.

- A Severe Thunderstorm Watch is also in effect until 9 PM MDT
  for all of eastern Utah and most of western Colorado.

- Look for a slight downturn in thunderstorms on Friday before
  mostly dry weather returns for Saturday. The next push of
  moisture is expected to arrive Sunday.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 446 PM MDT Thu Jun 27 2024

Deep subtropical moisture remained in place today and a pre-
storm sounding launched at 20Z showed a precipitable water value
of 1.09" with MUCAPE of over 1500 J/kg which is highly unusual
for this area. DCAPE was 1100 and wind gusts to 52 mph have been
observed as of 21Z. The Flash Flood Watch remains in place,
and in addition, in coordination with SPC, a Severe Thunderstorm
Watch has been issued with large hail and wind the primary
threats. Storms which developed earlier this afternoon have
generated flash flooding west of the Green River while a strong
cell which moved through Moab prompted issuance of a Flash Flood
Warning which resulted in a report of a debris flow over Hwy
191 at the entrance to Arches NP which closed the road. In
addition to rain, this storm generated hail to 1" in diameter. A
number of storms across the forecast area have prompted
additional Flash Flood Warnings and Severe Thunderstorm
Warnings in addition to several Special Weather Statements for
near severe storm cells.

Numerous showers/thunderstorms were working across central Utah
and models suggested this activity is likely to hold together
into the evening as modest divergence associated with the jet
moving through the base of the trough over the northern Rockies
provides some lift. Otherwise, instability and outflows from
existing storms will continue to generate additional storms and
based on moisture and CAPE values, some of these storms are
likely to generate excessive rainfall and/or large hail and
winds to 60 mph. Therefore, the Severe Thunderstorm Watch will
continue to 9 PM MDT and the Flood Watch until midnight.
During the latter part of the night, showers and thunderstorms
will decrease and become more stratified in nature and therefore
it`s unlikely that the Watch will need to be extended. High
levels of moisture and lingering clouds will moderate lows which
should settle out about 5 degrees above seasonal norms.

Zonal flow sets in late tonight and will continue on Friday
causing the subtropical moisture plume to shift southeast of the
forecast area while drier air begins to infiltrate from the
west. However, residual moisture will be sufficient to generate
scattered to numerous showers and thunderstorms with the north
favored. Enough shear remains across the north for a marginal
threat of severe thunderstorms with wind and hail remaining the
primary threats. As moisture levels fall, excessive rainfall
becomes more localized, so a Flood Watch is unlikely, but will
not be surprised if a few cells produce heavy rain. Temperatures
are expected to rebound with highs for most locations climbing
back above normal, but not by much.

Drying continues into Friday evening, so expect storms to
diminish at a faster clip than tonight. Given zonal flow aloft,
overnight lows are expected to show little change as compared to
Friday morning`s lows.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Issued at 446 PM MDT Thu Jun 27 2024

High pressure remains centered over Texas while a trough will be
just off the West Coast at the start of the long term period.
Moisture will continue to stream up from the south but the
trough to our west does look like it`ll shunt the deeper
moisture further to our east. This follows as models indicate
some showers and storms but mainly along the spine of the
Continental Divide.

On Sunday, the trough will continue to slowly approach from the
west and allow another push of monsoonal moisture to reach our
CWA. By Sunday afternoon, PWATs will vary from 0.8 to 1.2 inches
for just about the entire CWA...more down south, less up north.
Weak impulses moving through the mean flow will be more than
enough to allow scattered showers and thunderstorms to form.
Favored areas include the southern valleys, and the San Juan and
central mountains (60 to 80% chance). Elsewhere, chances range
from 20 to 50%. The trough will have started filling as it
begins to move across the CWA on Monday allowing another round
of convection, in almost the exact same areas as those on
Sunday. As of now, models are depicting another day of scattered
convection but with the added support aloft, would think the
convection would be a bit more intense. This may be a bit
downplayed in current guidance so will need to keep an eye on
the forecast as we get closer to Monday.

Once the trough passes through, a wind shift will cause
northerly winds to setup across the area allowing drier air to
move in. Of note, models tend to dry out our CWA much too fast
when this is forecast to occur. Once the hi-res models get their
hands on this timeframe, there`s a good chance the PoPs will
increase for both Tuesday and Wednesday.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1151 AM MDT Thu Jun 27 2024

Widespread showers and thunderstorms will continue through much
of the TAF period. Storms will be capable of small hail and
gusty outflow winds, as well as heavy rainshowers that will
create TEMPO IFR conditions at terminals. Storms will taper off
near sunset and VCSH/VCTS conditions are expected until around
midnight MDT. VFR conditions are expected by daybreak at most
terminals, with a few mid cloud ceilings hanging around on the
terrain from leftover shower activity.

&&

.HYDROLOGY...
Issued at 446 PM MDT Thu Jun 27 2024

Deep subtropical moisture remained in place today as
precipitable water values have verified in the 200 to 250
percent of normal range. Additionally, surface dewpoints were in
the upper 40s and 50s with 60 degrees registered at KCEZ this
afternoon. Given the abnormally moist atmosphere, several
thunderstorms have produced excessive rainfall and Flash Flood
Warnings have already been issued for the Moab area, Piceance
Creek on the north end of the Roan Plateau and southwestern
Paradox Valley. Consequently, the Flood Watches were warranted
and will remain in effect for much of eastern Utah and all of
western Colorado until midnight. 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 until midnight MDT tonight for COZ001>014-017>023.
UT...Flood Watch until midnight MDT tonight for UTZ022-025-027>029.

&&

$$

SHORT TERM...NL
LONG TERM...TGR
AVIATION...TGJT
HYDROLOGY...TGJT