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878
FXUS65 KGJT 211147
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
547 AM MDT Fri Jun 21 2024

.KEY MESSAGES...

- Significant moisture supports the potential for heavy rain and
  flash flooding favoring west-central and southwest Colorado
  and southeast Utah.

- There is a marginal threat for severe thunderstorms across the
  entire forecast area today, with increased risk for the Four
  Corners region. The main threat will be large hail, but
  localized tornadic activity can`t be ruled out.

- Drier conditions with more general afternoon thunderstorms are
  expected daily beginning Saturday.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 435 AM MDT Fri Jun 21 2024

Showers and thunderstorms continued to sweep northeastward
across the forecast area during the overnight hours along with
with a few stronger storms. Cells were clipping right along at
around 30 miles per hour moving to the northeast, so little
threat of flooding at this point.

Precipitable water levels are expected to remain high today,
with HREF guidance indicating levels ranging from 1 to 1.2"
inches with locally higher amounts in the Four Corners region.
Meanwhile, a short wave trough ejected from the main trough
over the West is expected to result in increased storms this
morning with daytime heating bringing more widespread tstorm
activity this afternoon. Yesterday`s activity raised the
subcloud moisture level as evidenced by dew point readings in
the upper 40s and lower 50s across the region early this
morning. Consequently, developing storms will be efficient
rain makers. Cape values between 1000 to 1500 J/Kg combined with
jet divergence and moderate bulk shear of 40 to 45 knots raises
the potential for some severe thunderstorms, especially over
the Four Corners region. The main threat will be large hail with
the potential for supercells along with a lesser chance for
localized tornadic activity. Meanwhile, though average storm
motion is projected to be out of the southwest at 20 to 25 mph,
stronger cells will be capable of generating excessive rainfall
bringing the potential for localized flash floods. Given forcing
aloft, training storms are possible which only exacerbates
flash flood potential. Given the previous, felt the current
Flood Watch needed to be expanded, so in coordination with the
ABQ and PUB forecast offices, added the remainder of southwest
Colorado including the San Juan Mountains and the Uncompahgre
River Basin to the watch.

Upper level forcing wanes this evening while diurnal cooling
helps stabilize the atmosphere. As a result, showers and
thunderstorms will decrease during the latter part of the
evening, largely dissipating by sunrise Saturday. Drier air
begins to move into the area on Saturday in zonal flow beneath a
shallow ridge of high pressure over the Great Basin. This will
begin to erode the moisture over the area, but enough will
remain for scattered activity, mainly over the higher terrain
Saturday afternoon/evening, though the threat of flooding and
severe weather becomes minimal.

Increased clouds and showers today will cause afternoon highs to
fall below normal today while overnight lows tonight should
remain mild. Drying on Saturday will allow for more solar
insolation during the morning and as a result, forecast highs
are expected to be near normal.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Issued at 435 AM MDT Fri Jun 21 2024

High pressure overhead will generally lead to warmer and drier
conditions through much of the extended forecast. However, the lack
of strong forcing to fully evacuate this anomalously moist air mass
allows residual moisture to hang around into the new week.
Precipitable water values remain at least one standard deviation
above normal for this time of year through the forecast period. This
will likely result in diurnal showers and storms every afternoon in
the high terrain. Lightning is the primary threat of any storms that
form. Early guidance also points to the potential for higher
coverage precipitation on Thursday afternoon as the crest of the
ridge begins to move off to the east and ripples of shortwave energy
move in. Temperatures warm to 5-10 degrees above normal by midweek.
Valley locations look to flirt with 100+ degree temps, especially on
Wednesday.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
-- Changed Discussion --
Issued at 547 AM MDT Fri Jun 21 2024

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

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 before
largely dissipating overnight.
-- End Changed Discussion --

&&

.HYDROLOGY...
Issued at 435 AM MDT Fri Jun 21 2024

Deep subtropical moisture remains over eastern Utah and Western
Colorado. This combined with an upper-level disturbance moving
up from the southwest this morning brings the threat of
training thunderstorms and/or thunderstorms with heavy rain
producing possible flash flooding across the region throughout
the day and into the evening. Therefore, a Flood Watch for flash
flooding remains in effect from 8 AM this morning to midnight
tonight across far western Colorado and southeast Utah,
generally along and south of the I-70 corridor. Continue to
monitor for forecast updates and be prepared to take action
should Flash Flood Warnings be issued.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

-- Changed Discussion --
CO...Flood Watch through this evening for COZ003-006-007-011-
     017>023.
UT...Flood Watch through this evening for UTZ022-025-027>029.
-- End Changed Discussion --

&&

$$

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