524
FXUS65 KGJT 220220
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
820 PM MDT Fri Jun 21 2024

.KEY MESSAGES...

- Showers and thunderstorms will come to an end later this
  evening. A few light showers, or a quick storm, may fire along
  the CO/NM border.

- Drier conditions return for Saturday onward, with a slow
  warming trend into next week. Chances for daily afternoon
  showers and thunderstorms remain over the higher terrain.

&&

.UPDATE...
Issued at 812 PM MDT Fri Jun 21 2024

The severe thunderstorm watch has been allowed to expire as the
threat has decreased across the area. Light to moderate rain
will continue along the I-70 corridor from Debeque Canyon to
Vail Pass decreasing from west to east as the evening continues.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 120 PM MDT Fri Jun 21 2024

With deep tropical moisture in place today, the threat for heavy
rain and flash flooding is maximized. The PWAT on this morning`s 12z
GJT sounding was 0.96 inches, which is well above normal and is
approaching the record value for the time and date, 0.98 inches.
Projected CAPE values across the region today are in the 1000-1500
J/kg range, with some models even suggesting 2000 J/kg across SE
Utah, which is unusually juicy for the Western Slope. The abundant
moisture and high forecast instability will combine with jet
divergence aloft and dynamic lift provided by a passing shortwave to
keep widespread strong thunderstorms going through the afternoon and
evening. In addition, there is substantial wind shear in place
across much of eastern Utah and western Utah, again maximized near
the Four Corners. A widespread area of 40 knots of effective bulk
shear, with an embedded area of 50 knots of shear over southern San
Juan County in southeast Utah. In addition to the deep layer shear,
helicity levels are maximized in this same region, with nearly 300
m2/s2 of storm relative helicity. All that is to say that not only
is this atmosphere unusually juicy, but it`s capable of producing
tilted, rotating updrafts. This setup is extremely conducive to
producing large hail, damaging winds, and even tornadoes, although
the chance of that is very low. The Storm Prediction Center has
taken a look at this highly potent and highly unusual environment and
decided to issue a Severe Thunderstorm Watch for all of eastern Utah
and almost all of Western Colorado, bar the farthest eastern
counties along the Continental Divide. Per the wording of this
watch, and what environmental parameters suggest, threats with these
storms include large hail up to 2 inches, gusty winds of 55-70 mph,
and an extremely low but present chance of tornadoes (2%).

This environment will persist into the evening, with discrete
supercells eventually merging into more linear convective systems.
As this change in storm type occurs, look for a shift away from the
larger hail toward stronger winds. One final shortwave will track
through late this evening, dragging one final line of thunderstorms
before things begin to settle down. Behind this wave, drier air and
subsidence will build in, especially across northeast Utah and
northwest Colorado, leading to clearing skies. Flow aloft will
weaken through the day tomorrow as high pressure begins to build in,
although moisture trapped at the surface will continue to feed
chances for showers and thunderstorms over the higher terrain
tomorrow afternoon. This will be a much more normal afternoon
thunderstorm setup, with convection driven first by differential
heating over the terrain, and then by propagating outflows through
the late afternoon and evening. With lower levels of the atmosphere
still quite moist, heavy rain remains a possibility, but the
potential for flash flooding will be low tomorrow. Lightning, gusty
outflow winds, and small hail will also be threats. The usual summer
afternoon mix. As skies will be clearer and moisture will be on the
decrease, temperatures will be able to climb up to near normal
values. Quiet weather will be in control for tomorrow night, with
mild lows near to a few degrees above normal.

&&

.LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
Issued at 120 PM MDT Fri Jun 21 2024

With the subtropical high to our south and a ridge of high pressure
overhead, drying and warming are expected through the long term
period. Rich tropical moisture trapped under the high will fuel
afternoon showers and thunderstorms forming daily off the terrain,
with threats slowly transitioning away from heavy rain and toward
gusty outflow winds. Frequent lightning and small hail will also be
possible. Passing waves rounding the high will lead to expansion in
storm coverage for Wednesday and Thursday. A draw of subtropical
moisture also looks to set up during the late week period, which may
see a return of the heavy rain threat to the Western Slope.
Temperatures will run around 5 degrees above normal to begin the
long term period, and climb to around 10 degrees above normal by mid-
next week. This means the potential return of triple digit heat to
the lower desert valleys.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 815 PM MDT Fri Jun 21 2024

Showers and storms will come to an end this evening though KASE
and KEGE will continue to see showers and possibly an embedded
storm through 06Z. Some gusty winds up to 40 mph will continue
for KRIL and KGJT for the next few hours before decreasing. Brief
MVFR will be possible for those TAF sites but anticipate
widespread VFR. Midlevel ceilings across the area are also
expected to continue lifting through the evening and overnight
hours. A stray shower or quick storm will be possible along the
CO/NM border through 06Z so KDRO might see some precip over the
next few hours.

&&

.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...

CO...Flood Watch until midnight MDT tonight for COZ003-006-007-011-
     017>023.
UT...Flood Watch until midnight MDT tonight for UTZ022-025-027>029.

&&

$$

UPDATE...TGR
SHORT TERM...BGB
LONG TERM...BGB/KAA
AVIATION...TGJT
HYDROLOGY...DB