-- Discussion containing changed information from previous version are highlighted. --
363
FXUS65 KGJT 201131
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
531 AM MDT Thu Jun 20 2024

.KEY MESSAGES...

- Scattered to widespread showers and storms are expected 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.

- More general thunderstorms are possible Saturday afternoon
  with drier conditions for Sunday.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 218 AM MDT Thu Jun 20 2024

The next 48 hours are going to be fairly active across the forecast
area. The pattern will be driven by an expansive ridge over the
central US and a trough over the Great Basin. The ridge to our
east is already advecting moisture into the region and that
continues through tomorrow. This moisture is rich with PWAT
values 200-300 percent of normal, mixing ratios around 10 g/kg
and surface dew points above 45F more typical of late July. That
moisture is colliding with steep lapse rates due to the trough
out west. This combo will produce instability on the order of
500-1000 j/kg with daytime heating. Convection is expected to
develop around noon and will be mainly driven by the terrain.
After that outflows will dictate where storms propagate, which
is just anywhere in the forecast area. Given this very anamolous
moisture there is a chance for high rainfall rates. Due to the
trough...steering flow should keep storms moving toward the
northeast at a modest clip. This may limit the heavy rain
potential. Although high-res models show that storm cells could
train over the same areas. Given the instability hail is
certainly possible with the stronger storms. Even though
moisture is working into the area surface RH values across the
southwest half of the area will be in the 20-30 percent range,
which means DCAPE will be high. This will support efficient cold
pools and wind gusts upwards of 60 mph. As the evening progresses
a mix of stratiform and scattered convection is expected.
Models do show a decreases in cape especially for the higher
elevations. This should lead to an overall decrease in storm
intensity and rainfall rates. None the less chances for
scattered showers and some storms continue through the night
for a good portion of the area given the moisture advection does
not stop.

Tomorrow morning there could still be pockets of clouds and
showers across the area. The moisture values will peak tomorrow,
which increases the chances for heavy rainfall. Despite the
potential for clouds in the late morning the models are still
indicating cape values around 500-1000 j/kg. One uncertainty is
will this cape be realized if clouds do not clear out. As of
now models show weaker instability closer to the Divide and the
highest cape across the western half. In addition to all of this
a weak shortwave is projected to break away from the base of
the trough and pass just to our north. This wave should provide
some lift to help initiate convection during the afternoon.
Either way development on the higher terrain is still likely.
The wave will also supply decent flow and therefore the chance
for more organized convection. The HREF does show a slight
chance for rotating storms capable of producing large hail
across the western half of the area in the afternoon. Gusty
winds remain a threat especially with any storm clusters or
complexes. The models show that as the afternoon progresses
storms could congeal into these complexes or lines and move into
the eastern half of the forecast area in the evening. How much
they maintain intensity is uncertain. In general everything
should transition to more stratiform in the late evening and
eventually clear the area after midnight.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 218 AM MDT Thu Jun 20 2024

Residual moisture will likely lead to additional storms Saturday,
especially in the high terrain. Mixed layer CAPE upwards of 1000
J/kg is currently present in the models early Saturday afternoon,
but with dew points gradually decreasing throughout the day so will
the CAPE. Because of this the heavy rain threat is not expected to
continue into Saturday. Mositure continues its downward trend on
Sunday and into the new week with an upper-level ridge building in
overhead. This will generally lead to warm and dry conditions with
periodic afternoon precipitation developing in the higher
elevations. High temperatures will likely be 5 to 10 degrees
above normal Tuesday through Thursday before the ridge slides
off to the east for the latter half of the week.

&&

.AVIATION /12Z TAFS THROUGH 12Z FRIDAY/...
-- Changed Discussion --
Issued at 531 AM MDT Thu Jun 20 2024

Scattered to widespread showers and storms will develop around
mid day across the higher terrain. These showers will be
capable of producing strong winds upwards of 60 mph and brief
heavy rainfall. Just about all of the sites have a chance of
being impacted by these showers. Convection will linger into the
overnight, but most likely weaken overall.
-- End Changed Discussion --

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

SHORT TERM...KJS
LONG TERM...KAA
AVIATION...TGJT