985
FXUS65 KGJT 270554 CCA
AFDGJT

Area Forecast Discussion...CORRECTED
National Weather Service Grand Junction CO
1154 PM MDT Wed Jun 26 2024

.KEY MESSAGES...

- Deep subtropical moisture will increase tonight 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. Localized severe storms are possible.

- Flash flooding is possible Thursday and a Flood Watch is now
  in effect from noon to midnight.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 446 PM MDT Wed Jun 26 2024

Moisture in the 12Z/26 June sounding at GJT had climbed to 0.95"
precipitable water (PW), up almost a quarter of an inch from
last evening`s sounding. At FGZ, the PW was just over an inch
with moisture spread relatively uniformly from the surface to
the upper reaches of the atmosphere. The increased moisture in
conjunction with a weak mid-level short wave trough and diurnal
heating has resulted in scattered to numerous showers and
thunderstorms across the forecast area this afternoon. There
have been a few sustained storms capable of producing rainfall
rates up to 0.5" in an hour, but most cells have been short-
lived with mainly light showers. A look outside at a cell
earlier in the afternoon was noticeably high-based, consistent
with the morning sounding. However, these showers were actively
moistening the subcloud layer which has implications for
Thursday.

For tonight, the short wave trough mentioned previously will
shift east of the forecast area tonight while falling
temperatures result in decreased instability. However, models
hint at a weak disturbance aloft which is likely to keep some
showers going overnight, though this activity should be more
stratiform in nature. As mentioned in the previous discussion,
deepening moisture and lingering clouds should buoy temperatures
and morning lows on Thursday are expected to be unusually mild.

A mid-level trough moving ashore over the Pacific Northwest
during the night will cause the high-center to the south to
shift a bit farther to the east. This shift will allow
subtropical moisture from the eastern Pacific and the Mexican
Plateau to continue pushing northeastward across the region.
Consequently, PW levels will continue upward, peaking from 1 to
1.4" according to the HREF which is 200 to 250% of normal.
Meanwhile, modest mid-level lift indicated in q derived omega
fields topped by fairly impressive divergence at the jet level
is expected to lead to widespread showers and thunderstorms.
Modest bulk shear and CAPE values between 1,000 and 1400 J/Kg is
likely to generate a handful of long-lived storms bringing a
slight chance for severe storms with the main threat large hail
and outflow winds in excess of 60 mph. That said, the primary
concern on Thursday is the potential for excessive rainfall
leading to flash flooding, especially for the slick rock areas
of southeast Utah and areas of steep, unstable terrain in
western Colorado. Steering winds appear moderate, but as recent
history has shown, when PW values rise to the levels mentioned
earlier, excessive rainfall rates are possible. Consequently, in
coordination with WPC and neighboring offices, have issued a
Flood Watch for southeast Utah and all of western Colorado from
noon to midnight Thursday. Due to increased clouds and
widespread showers, temperatures will be near or below normal
Thursday afternoon.

As the Northwest system tracks across the northern Rockies
Thursday night flow shifts to a more westerly direction shunting
the moisture plume to the southeast and allowing drier mid-level
air to filter in from the west. Residual moisture and a weak
disturbance moving over the north brings the best chance for
late night showers north of the I-70 corridor while chances will
be less to the south. Decreasing clouds and showers will
translate into cooler overnight lows with readings dropping
close to 5 degrees compared to tonight`s forecast lows.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Issued at 446 PM MDT Wed Jun 26 2024

Models highlight some showers and a few storms along the central and
northern portions of the Continental Divide as we start out the long
term period. Of note, deterministic models have been consistently
downplaying available moisture and instability after a surge of
convection like we`ll see on Thursday. The end result is convective
coverage being much less than what actually happens. The HRRR,
NAMNEST and latest NBM run, on the other hand, have been doing a
better job with convection coverage and intensity and show an uptick
in convection Friday which looks much more realistic. PWATs and
specific humidities do decrease on Saturday and with no noticeable
triggers, a downturn in convection is expected. Don`t be surprised
to see some showers and a few storms over the higher terrain thanks
to daytime heating but that should be about it.

Enjoy the slight downturn in convection, because if models are to be
believed, a broad area of high pressure will set up over Texas. The
location of said high will allow moisture to stream up from the
south and bring more rounds of precip Sunday and Monday. As this
occurs, an upper level trough will be approaching from the west and
this would usually cause widespread convection. The EC is hinting at
this solution while the GFS is keeping things a bit more lowkey.
Looks like the NBM is giving the nod to the EC as  50 to 60% PoPs
are in the forecast during that same timeframe. Other things to
consider will be how much cloud cover remains from overnight
showers/storms and how that will affect daytime heating. All that
being said, some uncertainty remains in the forecast but confidence
is starting to increase for more unsettled weather.

Temperatures will remain on the warm side for much of the long
term...about 5 degrees above more seasonal values.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 1036 PM MDT Wed Jun 26 2024

Isolated showers and storms look to continue overnight with
minimal impacts. A round of widespread storms is expected in the
afternoon and evening. Most sites have a high chance of impacts.
Wind gusts upwards of 40 kt are possible. Heavy rainfall could
cause brief MVFR/IFR conditions. Pockets of stratiform will
linger into the early morning hours.

&&

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

The influx of deep subtropical moisture continues tonight 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.
Additionally, surface dewpoints are expected to climb into the
50s and 60s. Given this juicy atmosphere, any thunderstorms
that develop on Thursday will be capable of producing heavy rain
and, subsequently, flooding or flash flooding, especially as
soils become saturated and steering flow remains moderate. Given
antecedent moisture, felt it was reasonable to hoist Flood
Watches for Flash Flood on Thursday from noon through midnight.
As always, stay tuned to the latest forecast!

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...Flood Watch from Thursday afternoon through Thursday evening
     for COZ001>014-017>023.
UT...Flood Watch from Thursday afternoon through Thursday evening
     for UTZ022-025-027>029.

&&

$$

SHORT TERM...NL
LONG TERM...TGR
AVIATION...TGJT
HYDROLOGY...TGJT