559
FXUS65 KGJT 280814
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
214 AM MDT Fri Jun 28 2024

.KEY MESSAGES...

- One more round of widespread afternoon convection is expected
  today, with strong to severe storms possible across the north.

- Drier air aloft and ridging will lead to a brief downturn in shower
  and storm activity on Saturday, along temperatures running
  around 5 degrees above normal.

- Deep subtropical moisture returns for Sunday onward, with a
  return to widespread convection and an increased concern for
  flash flooding.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 209 AM MDT Fri Jun 28 2024

As anticipated, the convection that has been tracking across Utah
earlier today has reached western Colorado in the last few hours.
Much of this activity stratiformed out as it reached the border due
to a lack of remnant instability, although a few sporty cells in
Northeast Utah managed to find and tap into a lingering pool of
instability has have prompted a few Special Weather Statements. This
activity, tied to a little wiggle in the zonal flow aloft, will
continue tracking eastward over the next several hours, weakening as
it continues moving into an unfavorable environment. Embedded cells
within the stratiform precipitation will be capable of producing
heavy rain, gusty winds, and frequent lightning.

As the upper level trough tracking across the northern Rockies
finally moves east of the Divide this morning, the high pressure to
the south will be shunted back eastward. This means that the deep
tropical moisture tap we`ve had the last several days will be pushed
east as well, cutting off the Desert Southwest. Plenty of surface
moisture is forecast to stick around, however, as dew points in the
50s to low 60s across the lower elevations, and in the 40s at higher
elevations, will be in place today. In addition, clearing behind the
early morning convection will act to produce another day with 1000-
1500 J/kg of CAPE, mainly along and north of I-70. Across the
southern half of the area, values are forecast in the 500-1000 J/kg
range. That`s two of the three ingredients needed for another round
of afternoon storms, leaving us just in need of a lifting mechanism.
This will come in the form of a combination of differential daytime
heating on the terrain, and modest jet dynamics across the north.
The jet along the CO/WY border will also add an addition ingredient
to storms north of the I-70 corridor - wind shear. All this is a
long way to say that while storms will be possible across much of
the area this afternoon, convection along and south of I-70 will be
on the weaker side and generally tied to the terrain. North of I-70
will be where the stronger convection will be, and this convection
will have a better chance of surviving if it moves off the terrain
compared to that across the south. This falls in line with the
current convective outlook provided by the Storm Prediction Center,
where the northern half of the area is painted under a marginal risk
of severe, while areas south of I-70 fall under General Thunder. The
main threats with the stronger convection across the north will be
strong, gusty winds, hail, frequent lighting, and heavy downpours.
With the moisture tap cut off, as the lingering surface moisture is
used up the threat for heavy rain and flash flooding will diminish
as the day wears on. The main threats with the southern convection
will be similar but on the lower end of the impact spectrum.

Convection will hang on through sunset before beginning to gradually
diminish. Without as much upper level support, this round of showers
and storms is expected to taper off quicker compared to previous
nights, with activity shutting down by midnight. Drier air begins
moving in aloft late Friday night into Saturday morning as ridging
builds in from the west. This will start to nibble away as the
pooled surface moisture, leaving us with lower forecasted dew points
for tomorrow and a general decrease in convective coverage. That
said, enough moisture will still be around Saturday for us to see a
more typical round of afternoon convection firing off the terrain,
with the Colorado Divide most favored. With the drier air moving in,
look for the threat of gusty outflow winds to increase at the
expense of the threat of heavy rains. Small hail and frequent
lightning will also be possible.

The increased moisture and expected convection this afternoon will
keep today`s high near normal values, with low to mid 90s across the
desert valleys, upper 70s to low 80s for the higher elevation
valleys, and upper 50s to mid 60s for the mountain towns.
Temperatures will run higher tomorrow thanks to the drying trend and
higher pressure moving in aloft. Overnight lows remain about 5
degrees above normal.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Issued at 209 AM MDT Fri Jun 28 2024

Nocturnal showers and perhaps a few embedded storms will continue on
Saturday night across southwest Colorado as a weak wave rounds the
periphery of the high. On Sunday the ridge will remain stretched
across the Southern Plains, allowing additional subtropical moisture
to stream into southeast Utah and southwest Colorado. We can expect
at least scattered showers and thunderstorms throughout the day,
favoring the southern two-thirds of the forecast area. By this point
precipitable water values will increase to 200 to 220 percent of
normal so, with storms, we can expect to see periods of moderate to
even heavy rain though the usual threats of gusty outflow winds and
small hail will be likely. Sunday night and into Monday the next
trough of low pressure will dig into the Pacific Northwest, dragging
farther south than previous systems. This will result in a more
notable shift to southwest flow across eastern Utah and western
Colorado as well as the shunting of the ridge farther east. This
will only enhance moisture advection across the area with PWATs
maxing out around 250 percent of normal for some locations. On
Monday afternoon the trough will dig into the Northern Rockies which
will provide some generous lift and, therefore, more widespread
shower and thunderstorm activity given anomalously high moisture
content. Depending on how active Sunday is Monday could see
increased potential for flash flooding as soils would be better
saturated. Either way, heavy rain will once again be possible with
storms in addition to those stronger winds and hail.

The trough will exit Monday evening but lower heights will continue
to dominate the western CONUS going into Tuesday as additional
systems push across the Intermountain West. These systems don`t
appear to dive as far south as the Monday one but, regardless,
unsettled southwest flow will keep anomalously high moisture in the
region into the midweek point. Daily showers and storms can be
expected as there will be plenty of residual moisture to work with,
but we`ll have to wait a bit longer to nail down the overall
coverage. Guidance is attempting to build a ridge of high pressure
into the West Coast as we head into the latter half of the week
which would hint at a shift to drier and warmer weather. Time will
tell.

Temperatures will return to slightly warmer than normal levels
Saturday and Sunday, particularly over the northern zones. Below
normal temperatures are expected on Monday with the uptick in
clouds, showers and thunderstorms as well as Tuesday behind the
trough.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1022 PM MDT Thu Jun 27 2024

Another round of scattered showers and a few storms will move
across the area tonight with minimal impact. Afternoon
thunderstorms are possible at some of the sites once again.
Gusty winds around 45 mph and brief heavy rain is possible with
these storms. Given the coverage only included vicinity for this
issuance. Mainly VFR conditions are expected through the taf
period.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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