445
FXUS65 KGJT 210542
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1142 PM MDT Thu Jun 20 2024

.KEY MESSAGES...

- Scattered to widespread showers and storms continue today
  through tomorrow night.

- Significant moisture supports the potential for heavy rain and
  flash flooding especially across far western Colorado and
  southeast Utah. Those chances exist today, but are higher
  tomorrow.

- Strong storms capable of producing wind gusts in excess of 60
  mph are possible each afternoon/evening. Hail is also
  possible with the stronger cells.

- Drier conditions with more general afternoon thunderstorms are
  possible daily Saturday onward.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 302 PM MDT Thu Jun 20 2024

Deep tropical moisture has moved up into the region with showers and
thunderstorms firing off on schedule about noon today and is
becoming widely scattered to widespread this afternoon. Several
cells have already produced large hail and/or near 50 kt winds over
the San Juans and along the Roan Cliffs with more severe storms
moving up from Arizona and New Mexico. CAPE and shear decrease this
afternoon which may let the storms settle down, but there is still a
lot of energy available for severe storms to continue. These storms
are also producing frequent lightning, so if you are out today, be
aware of your surroundings and take shelter when needed.

The activity looks to stratify out to mostly widespread light rain
with a few embedded showers and an occasional thunderstorm in the
late evening and overnight. A shortwave moves up from the southwest
Friday morning with increased shear and cool air aloft to kick up
the CAPE again. Moisture deepens in southeastern Utah with pwats
around 1.5 inches. Look for another round of organized convection
starting as early as 8AM with another chance for some severe storms.
Concerns for tomorrow is the increased moisture leading to heavy
rain and possible training storms across far Western Colorado and
southeastern Utah along and south of the I-70 corridor. Moved up the
start-time on the Flood Watch in place for these areas running to
8AM, running to midnight for possible flash flooding from heavy rain
and training of these storms. Considered a Severe Thunderstorm Watch
for mid morning through mid afternoon with the shortwave passage and
based on todays activity, but held off as confidence isn`t quite
there. Stay tuned on this one. Behind the shortwave, heavy rain is
still likely with the deep moisture with pwats staying up between an
inch and an inch and a half well into the evening before convection
diminishes northwest to southeast overnight.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...
Issued at 302 PM MDT Thu Jun 20 2024

The ridge to our west, over the Great Basin, finally washes out on
Saturday as the subtropical high slides west across the southern
half of the country. This will bring increasingly dry air aloft,
leading to steepening lapse rates on Saturday afternoon. With
lingering surface moisture and projected 500-1000 J/kg of CAPE, look
for one more round of stronger convection Saturday afternoon. With
the drier air at mid and upper levels, the threat of heavy rain and
flash flooding with be on the downturn, though localized heavy rain
and the potential for slow or training storms does mean the threat
won`t be completely gone. Otherwise, convection Saturday will fire
off the terrain in the early afternoon, and outflows from this
activity will be the driving force for further storms through the
afternoon and evening. Look for gusty outflow winds, lightning,
small hail, and brief heavy rain to be the main threats.

As the subtropical high settles over the Desert Southwest Sunday,
ridging pushes north along the Divide, bringing a gradual drying and
warming trend through mid next week. The surface moisture trapped
under the high will be slow to erode, and will be recycled daily
leading to continued threats of afternoon showers and storms over
the higher terrain, with gusty winds and lightning being the main
threats. Highs will increase from near normal values on Saturday to
around 10 degrees above normal by mid-next week. As the forecast
stands right now, this will mean a return to triple digit heat for
the desert valleys. Overnight lows will remain mild and show the
same warming trend through the long term period.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1142 PM MDT Thu Jun 20 2024

A broad area of showers and embedded thunderstorms will continue
to lift northeastward across southeast Utah and southwest
Colorado in response to an upper level disturbance. This
activity will bring light to moderate rain and wind gusts to 35
mph. However, this activity won`t lower flight rules below
visual.

As the night wears on, shower/thunderstorm activity will weaken
as it spreads to the northeast. Another disturbance combined
with deep moisture will get showers and thunderstorms
reinvigorated shortly after 12Z with southeast Utah and west-
central Colorado favored through noon. Daytime warming will
bring increased coverage and intensity of thunderstorms during
the afternoon and evening potentially resulting in brief MVFR
ceilings and visibility with stronger storm cells. Gusts to 55
mph are possible. Some shower activity will linger through
midnight across the south according to latest model data.

&&

.HYDROLOGY...
Issued at 308 PM MDT Thu Jun 20 2024

Deep tropical moisture has moved up into eastern Utah and
Western Colorado. This combined with an upper-level disturbance
moving up from the southwest Friday morning is leading to a
threat of training thunderstorm and/or thunderstorms with heavy
rain producing possible flash flooding across the region through
the day and evening. A Flood Watch is in effect from 8 AM to
midnight across far Western Colorado and southeastern Utah
generally along and south of the I-70 corridor. Please monitor
later forecasts and be prepared to take action should Flash
Flood Warnings be issued.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...Flood Watch from 8 AM MDT Friday through Friday evening for
     COZ003-006-007-017-020-021.
UT...Flood Watch from 8 AM MDT Friday through Friday evening for
     UTZ022-025-027>029.

&&

$$

SHORT TERM...DB
LONG TERM...BGB
AVIATION...NL
HYDROLOGY...DB