434
FXUS65 KGJT 250930
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
330 AM MDT Tue Jun 25 2024

.KEY MESSAGES...

- Residual moisture will continue to fire isolated afternoon
  showers and thunderstorms this afternoon.

- A push of deep subtropical moisture moves in by Wednesday,
  returning widespread showers and thunderstorms through
  Thursday.

- Mid-week moisture content, exceeding 200% of normal returns
  the threat of flash flooding due to heavy rain showers and/or
  training storms.

&&

.SHORT TERM /THROUGH WEDNESDAY/...
Issued at 324 AM MDT Tue Jun 25 2024

Satellite imagery shows the models have a grasp on the general
weather pattern with the polar jet along the Canadian Border, an
upper level high extending from Texas west along the Mexican Border
with a low off Baja, and these features extending down into the mid-
levels with the high from Louisiana west to the Desert Southwest and
a weak low off Baja with these features slightly north of the upper-
level features. This is pretty much the limit of the models as going
into the lower levels, there are many meso-scale features in the
satellite imagery that the models can`t resolve. even the high
resolution models miss these features likely due to the initial
conditions and to a lesser extent, smoothing in the ensembles.
imagery shows a low-level high over the southwestern Utah-Arizona
Border with a disturbance rotating around the northwestern corner
supporting a few showers in the Four Corners area, a second high
circulation over the White Mountains of Arizona and a trough over
central New Mexico. Though these low-level features seem minor, they
are driving the current weather across much of the Southwest
allowing tropical moisture to sneak north under the high pressure
circulation aloft. The models want to dry out the region, but
dewpoints continue to run in the 40`s to low 50`s across eastern
Utah and Western Colorado which combined with recycled moisture is
plenty to support isolated to scattered orographic showers and
thunderstorms through the afternoon across the higher terrain. As
the high pressure builds north over the region, subsidence will try
to cap the convection, but diurnal heating will likely break the
inversions.

Temperatures today will run five to ten degrees above normal for
late June under the high pressure with isolated to scattered showers
and thunderstorms across the higher terrain much like yesterday, but
there may be a slight down tick with the increased subsidence under
the high. The primary threats from these storms will be gusty
outflow winds to 50 mph, small hail and lightning. Deep moisture
from the Desert Southwest starts to pour back into the region
Wednesday with clouds increasing from the southwest. Temperatures
will be a little cooler Wednesday due to the increased cloud cover
and with the increased shower and thunderstorm coverage. With pwats
topping one inch again through the afternoon, concerns for heavy
rain from the thunderstorms leading to localized flash flooding
return, especially in eastern Utah and the far western Colorado
along the Utah Border. Confidence on the timing of the moist surge
is too low to issue a flood watch at this time, so stay tuned for
forecast updates over the next 24 hours. This threat increases
Wednesday night into Thursday, but more on that below in the long
term discussion.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Issued at 324 AM MDT Tue Jun 25 2024

Nocturnal showers and embedded thunderstorms can be expected on
Wednesday night as a trough of low pressure begins to dig into the
Pacific Northwest. This will see the ridge flatten with flow aloft
shifting to the southwest into Thursday. Impressive moisture
advection will bring precipitable water values up to at least 200 to
220 percent of normal across the Western Slope for late June. As a
result of this juicy atmosphere (with surface dewpoints staying in
the mid 50s), look for widespread shower and thunderstorm activity
on Thursday. The Pacific trough will quickly push east into Idaho /
Montana by Thursday afternoon, tightening the gradient overhead and
bringing embedded shortwaves across the forecast area. Modest lift
paired with the abundance of moisture will lead to some stronger
thunderstorms throughout the day, those capable of producing hail
and gusty winds in addition to periods of heavy rain. In fact, the
Weather Prediction Center already has a widespread marginal risk for
excessive rainfall on Thursday with almost the entirety of western
Colorado under a slight risk. Therefore, flash flooding will be a
concern with storms, especially depending on how much moisture falls
with activity on Wednesday. Stay tuned to any potential flood
highlights in the coming shifts.

The trough takes on a positive tilt Thursday night before lifting
overhead on Friday. Showers will gradually taper off once the trough
passes through but it certainly won`t be a clean transition. PWATs
will stay elevated and daytime heating and orographics will keep
showers and thunderstorms in the forecast through the end of the
week. Areas along the Divide in particular will see the best
coverage of convection as low level moisture persists. On Saturday a
transitory ridge of high pressure builds overhead, though as usual
diurnally-driven showers and storms will develop over the higher
terrain, favoring the Divide. Sunday will see another, broader
trough of low pressure push onshore across the western CONUS. Flow
aloft will shift to the southwest which will re-introduce
subtropical moisture into the region as an elongated ridge extends
across the southeast states. The trough will push farther inland on
Monday which will further enhance the clockwise flow around the
high. So, after a brief downturn in convection on Friday but more-so
on Saturday, unsettled weather looks to return from Sunday onwards.

Temperatures will be a bit of a grab-bag through the long term
period given the fluctuating shower and thunderstorm coverage.
Overall, Thursday will be the coolest day of the period due to the
uptick in clouds and showers with Friday also being below normal for
most areas behind the trough. Temperatures will rebound slightly on
Saturday under the weak ridge before values likely trend back down
with the next round of moisture. Overnight lows will remain on the
mild side through the period.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1030 PM MDT Mon Jun 24 2024

VFR conditions will develop overnight and continue for most
terminals through the period. Isolated showers and
thunderstorms will remain a threat on the high terrain tomorrow
afternoon, with less coverage than today. A few showers will
drift off their terrain and decay over valleys, leading to gusty
outflow winds and the potential for light hail. Winds will hold
westerly, with a few terrain influenced gusts tomorrow
afternoon. Cloud cover will remain scattered at the mid levels,
with a few TEMPO ceilings in the vicinity of showers and
storms. Shower activity will quickly diminish towards sunset.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...None.
UT...None.

&&

$$

SHORT TERM...DB
LONG TERM...MMS
AVIATION...TGJT