458
FXUS65 KGJT 211921
AFDGJT

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

.KEY MESSAGES...

- SPC has issued Severe Thunderstorm Watch #441 for hail
  possibly reaching 2 inches and wind gusts up to 70 mph through
  7PM this evening.

- A Flash Flood Watch remains in effect for much of eastern Utah
  and southwest Colorado for the potential of very heavy rain
  and flash flooding.

- 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 1211 PM MDT Fri Jun 21 2024

The Storm Prediction Center has issued Severe Thunderstorm Watch
#441 that encompasses all of eastern Utah and much of western
Colorado. Anomalous moisture remains across the forecast area
this afternoon with forecast CAPE values reaching between 2500
to 3000 J/kg. Along with this exceptionally high CAPE,
substantial speed shear is also present which may allow some
supercells to form this afternoon. There is some concern that
cloud cover from this morning`s showers and storms will limit
destabilization this afternoon. Having said that, models have
remained consistent with the severe weather threat. The Severe
Thunderstorm Watch will remain in place until 7PM this evening.
Main threats will be scattered large hail and isolated very
large hail up to 2 inches in diameter. Wind gusts up to 70 mph
will also be possible.

&&

.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 THURSDAY/...
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 /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1128 AM MDT Fri Jun 21 2024

Widespread showers and thunderstorms are once again expected
this afternoon and evening. Large hail in excess of 2 inches is
possible in the Four Corners Region, along with gusty winds of
40-50 knots, and torrential rain. There is a small (2%) chance
of tornadoes in the Four Corners Region as well. Frequent
lightning will also be likely. Low ceilings are likely through
the day, and periodic reductions in visibility with passing
storms. With all the passing convection, winds will be erratic
until late this evening, when typical terrain patterns kick back
in. VFR conditions will prevail outside of shower activity,
within showers and storms, look for reductions below ILS
breakpoints and MVFR to IFR conditions with heavy rain.

&&

.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