-- Discussion containing changed information from previous version are highlighted. --
102
FXUS65 KGJT 261106
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
506 AM MDT Wed Jun 26 2024

.KEY MESSAGES...

- Deep subtropical moisture will arrive today and remain in
  place through Thursday. As a result, widespread shower and
  thunderstorm activity is expected.

- Stronger storms will be capable of producing heavy rain, gusty
  winds, and hail.

- Flooding and flash flooding will be possible both days, but
  more-so on Thursday after soils become saturated today. Stay
  tuned to potential flood highlights!

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 330 AM MDT Wed Jun 26 2024

The center of the upper level ridge continues to drift east
towards western Texas early this morning as a trough of low
pressure slowly elongates off the coast of the Pacific NW. This
will open the door for strong moisture advection across eastern
Utah and western beginning today and continuing through the
short term period. The prominent dry slot found in the mid and
lower levels of the 00Z sounding will gradually erode as a
result, as the shift to southwest flow ushers in precipitable
water (PWAT) values that are anomalously high for late June.
Anomalously high meaning...values that are over an inch or well
over 200 percent of normal. This will translate to surface
dewpoints in the upper 40s to low 50s this afternoon with a
notable jump to the low 60s on Thursday. But, we`re getting
ahead of ourselves.

It will be a quiet start to the morning for the northern two-
thirds of the forecast area while plenty of clouds linger across
the Four Corners. CAM guidance remains consistent in an area of
showers lifting across southeast Utah around 12Z before pushing
into southwest Colorado a few hours later. This presents the
main forecast challenge for the day as the scope and duration of
this shower activity will help determine how much instability we
have to work with this afternoon. The main driver for showers
and thunderstorms elsewhere across the Western Slope today will
be a shortwave trough rounding the periphery of the ridge,
arriving this afternoon before sliding across the Divide this
evening. Scattered to numerous showers and thunderstorms are
expected under this regime but, as mentioned previously, how
strong storms become will be dependent on this morning`s
activity. And, subsequently, how much heavy rain we see from
storms as well as the potential for gusty outflow winds and
hail. This conundrum is well represented in SBCAPE projections
for this afternoon. The NBM`s probability of exceeding 500 J/kg
SBCAPE from the 2 to 6 PM timeframe is between 30 and 50% with
far southeast Utah maxing out around 60%. Given the uncertainty,
have once again opted to not issue any flood highlights with
this morning`s package, since less instability would lead to
weaker storms or stratiform showers not conducive to flooding.
Either way, today`s activity will prime the forecast area for
another active day on Thursday.

But first, tonight. After the departure of today`s shortwave we
will see unsettled zonal flow aloft persist into early Thursday
morning. This will be in response to the main Pacific trough
slowly digging into northern Washington overnight. Nocturnal
showers and scattered storms will continue, most notably across
the southern and central portions of the forecast area.
Elsewhere will see plenty of clouds persist so tonight`s low
temperatures will be several degrees above normal for this time
of year. The upper level trough will continue to push across the
far Northern Rockies on Thursday, dragging additional embedded
shortwaves through eastern Utah and western Colorado as it does.
PWATs will remain elevated under this regime and, given the
likelihood for some saturated soils going into the day and
continued weak steering flow aloft, the potential for flash
flooding will increase. In fact, the Weather Prediction Center
continues to show a slight (level 2 of 4) risk for excessive
rain across western Colorado and a marginal (level 1 of 4) risk
for eastern Utah on Thursday. Will wait to see how today plays
out before considering flood highlights for tomorrow but, either
way, stay tuned. These passing waves will fuel widespread shower
and thunderstorm activity as well as modest instability through
the day. The probability of exceeding 800 J/kg of SBCAPE on
Thursday afternoon is expected to reach 80 to 90% for much of
the region. So, the potential for stronger thunderstorms will
be there, with the Storm Prediction Center already putting the
CWA under a marginal risk (or a level 2 on a scale of 5) for
severe storms. That being said, once again please stay tuned to
the latest forecast for any updates over the coming shifts.
Given the uptick in clouds, showers and storms on Thursday,
temperatures will drop back to below normal area-wide.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Issued at 330 AM MDT Wed Jun 26 2024

Widespread showers and thunderstorms continue through Thursday
evening across northern and central areas generally north of the San
Juan Mountains under falling heights as the low passes to the north
along the Canadian Border. The models hint of a frontal boundary
sliding in across the northern areas overnight, possibly reaching as
far south as the I-70 corridor, but there isn`t a lot of definition
to a boundary beyond a weak shortwave trough and a weak thermal
gradient from central Idaho/southern Montana south into northern
Colorado. The dynamics associated with a front would only enhance
the convection across the northern areas, but the rich moisture and
falling heights will likely be enough to sustain strong convection
with possible isolated severe thunderstorms producing strong gusty
winds, large hail and heavy rain leading to the risks for flash
flooding extending through the evening hours. As the shortwave
trough moves off to the east after midnight, convection will
diminish to mostly stratified conditions with isolated to maybe
scattered showers over the northern mountains going into Friday
morning. Behind this trough, models indicate a westerly to
northwesterly flow to dry out the region, but the models have been
doing this for the past few weeks. Without a strong boundary to
sweep the moist air out of the region, expect significant moisture
to hang around as it has this past week. The rising heights will
bring subsidence to eastern Utah and Western Colorado to clear out
the skies overnight and prime the pump through the morning hours
with diurnal heating leading to a return of afternoon showers and
thunderstorms Friday and Saturday favoring the higher terrain, fed
by the lingering moisture, and driven by the diurnal heating and
orographic lift.

From Thursday evening onward, the synoptic pattern will see the high
pressure to the south retreat to the east over eastern Texas by
Sunday and Alabama by Tuesday, pushed by a second low traversing
southern Canada with a deep trough extending south into the Great
Basin and Rocky Mountains. Sunday will see another surge of moisture
move up from the southeast into the southern Colorado driving pwats
back up over an inch. Look widespread showers and thunderstorms over
the southern and central mountains of Colorado Sunday afternoon with
more isolated convection becoming widely scattered through the
evening to the west in southeastern Utah and to the north in the
northern Colorado mountains. This convection will become isolated to
scattered overnight into Monday morning through the central and
southern mountains, picking up again through the afternoon with
diurnal heating. After this last surge, expect a return to afternoon
orographic showers fed by the lingering moisture into mid week.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
-- Changed Discussion --
Issued at 506 AM MDT Wed Jun 26 2024

Showers and thunderstorms are expected through the TAF period.
Isolated showers are possible across southwest Colorado and
southeast Utah late this morning before more widespread
activity develops this afternoon and evening. All TAF sites will
see at least VCSH / VCTS during the period. Storms will be
capable of producing gusty winds in excess of 40 kts as well as
heavy rain with MVFR CIGS/VIS possible in the stronger storms.
Showers and some storms continue overnight and into Thursday
morning. Outside of showers, VFR will prevail at TAF sites.
-- End Changed Discussion --

&&

.HYDROLOGY...
Issued at 330 AM MDT Wed Jun 26 2024

Deep subtropical moisture arrives today and will remain in place
through Thursday. Precipitable Water values are projected to run
in excess of 200% of normal, peaking around 250% of normal for
the southern two-thirds of the forecast area in response.
Additionally, surface dewpoints are expected to climb into the
50s and 60s. Given this juicy atmosphere, any thunderstorm that
develops over the next two days will be capable of producing
heavy rain and, subsequently, flooding or flash flooding,
especially as soils become saturated and steering flow remains
weak. Have decided to hold off on any flood watches at the
moment, given uncertainty regarding how strong storms will be
this afternoon after this morning`s projected shower activity
dissipates. Stronger storms are expected on Thursday but will
wait and see how today plays out before issuing any highlights.
Stay tuned to the latest forecast!

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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