355
FXUS65 KGJT 271138
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
Issued by National Weather Service Salt Lake City UT
538 AM MDT Thu Jun 27 2024

.KEY MESSAGES...

- Deep subtropical moisture will remain in place today with
 widespread showers and thunderstorms expected.

- Stronger storms will be capable of producing heavy rain, gusty
  winds, and hail. Localized severe storms and flash flooding
  are possible. A Flood Watch remains in effect for much of
  eastern Utah and all of western Colorado from noon to
  midnight.

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

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 345 AM MDT Thu Jun 27 2024

All signs point to an exciting and busy day across western
Colorado and eastern Utah. This will be in response to a strong
trough of low pressure digging into the Intermountain West this
afternoon, as well as the abundance of subtropical moisture in
place across the area. Precipitable water values are still on
track to exceed 200 to 250 percent of normal today with the 00Z
GJT sounding recording a PWAT of 1.08 inches. Additional, deep
moisture has translated to surface dewpoints in the 50s and 60s
overnight, and this trend will continue going through the day.
Early this morning we`ve seen scattered showers with embedded
thunderstorms focused north of I-70, thanks to the passage of a
shortwave in zonal flow aloft. CAm guidance projects this
shower activity will dissipate by 12Z, ushering in a period of
daytime heating which will aid in thunderstorm development later
this morning. As the upper level trough continues to progress
farther east into western Montana, modest ascent and steepening
lapse rates overhead will aid in afternoon destabilization. ML-
CAPE is forecast to reach 1000-1500 J/kg across much of the area
this afternoon which will lead to increase potential for severe
thunderstorms. In fact, with this morning`s package, the Storm
Prediction Center upgraded much of western Colorado and eastern
Utah to a Slight (level 2 of 5) risk for severe thunderstorms.
The primary concerns from storms will be gusty outflow winds in
excess of 60 mph and large hail. Outside of the more organized
convection, pea to half-inch hail will be common as well as wind
gusts of 45 to 55 mph.

But, that`s not the only hazard on the table today. As has been
advertised the last several days, the presence of anomalously
high moisture will result in moderate to heavy rain with showers
and thunderstorms. And, subsequently, the potential for flooding
and flash flooding will be there, particularly for burn scars,
slick rock, slot canyons, and steep terrain. Out forecast area
remains under a Slight (level 2 of 4) risk for excessive rain
today and, as a result, made no changes to the Flood Watches. Be
sure to monitor the latest forecasts and be prepared to take
action should Flash Flood Warnings be issued!

Even with the loss of daytime heating, showers and storms are
expected to continue into the overnight hours. There are some
notable differences regarding overall coverage in the CAm
guidance but, either way, unsettled weather will persist as an
embedded wave tracks to the north. The upper level trough will
become positively-tilted as it reaches the Dakotas on Friday.
This will drag some slightly drier air into the area as we
remain cut off from the main tap of subtropical moisture to the
south. All the same, PWATs will stay elevated across the
forecast area and, once daytime heating and orographics has
their way, at least scattered showers and storms will develop
Friday afternoon. Even though we won`t have dynamics conducive
to stronger thunderstorms, any storms that do develop will be
capable of producing gusty winds in excess of 50 mph, small
hail, and heavy rain. Localized flash flooding will be possible
given already saturated soils.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 345 AM MDT Thu Jun 27 2024

The wave tracking across the northern Rockies Friday afternoon and
evening will keep showers and thunderstorms going across the
northern and central mountains, before finally tapering off some
time after midnights. Drier air aloft and ridging looks to build in
for Saturday, and models are very keep to dry things out fast,
limiting chances of afternoon convection exclusively to the Colorado
Divide mountains. And yes, with forcing limited to differential
daytime heating, any afternoon convection that does fire will be
tied strongly to the terrain, leading to a comparative downturn in
coverage compared to days previous. However, models are notorious
for trying to scour out surface moisture far faster than reality,
and this has occurred multiple times in the last few weeks alone. So
it is likely that there will be more moisture available no Saturday
than is currently projected, and showers and thunderstorms will fire
off all terrain features rather than just along the Divide. The main
threats with these showers will continue to be heavy rains, gusty
winds, and frequent lightning.

By Sunday, the subtropical high will have made its way to the
southern Plains, centered somewhere over the Texas/Oklahoma border.
In the meantime, a trough is forecast to move onto the west
coast, leaving the Desert Southwest sandwiched between these
features. This is a prime setup for a tap of subtropical and
tropical moisture to be drawn northward into the Desert
Southwest and, while I can neither confirm nor deny that this is
the start of the Monsoon, it is certainly reminiscent of a
monsoonal surge. As this rich tap of tropical moisture makes its
way northward Sunday, look for an expansion in storm in storm
coverage during the afternoon hours. This pattern will rinse and
repeat Monday before the west coast trough sweeps through and
disrupts the tropical moisture tap for Tuesday onward. One
concern is how much of an impact overnight convection and
morning cloudiness will have on afternoon storms. Too many
clouds will limit instability and lead to a more stratiform
precipitation form, compared to the more typical afternoon
convection. This will also limit threats from gusty winds and
flash flood producing heavier rains. Something to be watched
closely in the coming days.

As the trough sweeps through late Monday into Tuesday and cuts
off, eastern Utah and western Colorado from the tropical
moisture tap, models are once again quick to dry things outs. At
this far of a remove, its difficult to say whether this is
unrealistically quick drying or not. Either way, a downward
trend in shower and storm coverage is likely, as high pressure
builds in from the west for the late week period.

With increased moisture, showers, and cloud cover, daytime highs are
forecast to run near to around 5 degrees above normal through the
period. Lows will remain mild, running 5-10 degrees above
normal.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
Issued at 442 AM MDT Thu Jun 27 2024

|Showers and a few embedded thunderstorms will continue to
impact areas north of I-70 through early morning. More
widespread shower and thunderstorm activity is expected this
afternoon and evening. All TAF sites will see at least VCSH /
VCTS during the period. Storms will be capable of producing
gusty winds in excess of 40 kts as well as heavy rain that could
cause brief MVFR / IFR conditions. Showers and some storms will
continue overnight and into Friday morning. Outside of showers,
VFR will prevail at TAF sites.

&&

.HYDROLOGY...
Issued at 345 AM MDT Thu Jun 27 2024

Deep subtropical moisture remains in place today as precipitable
water values run 200 to 250 percent of normal. Additionally,
surface dewpoints will remain in the 50s and 60s. Given this
juicy atmosphere, any thunderstorms that develop today will be
capable of producing heavy rain and, subsequently, flood or
flash flooding, especially as soils become more saturated and
steering flow remains moderate. The Flood Watches remain in
effect for much of eastern Utah and all of western Colorado
from noon to midnight MDT today. 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!

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...Flood Watch from noon MDT today through this evening for
     COZ001>014-017>023.
UT...Flood Watch from noon MDT today through this evening for
     UT002-025-027>029.

&&

$$

SHORT TERM...MMS
LONG TERM...BGB
AVIATION...TGJT
HYDROLOGY...TGJT