248
FXUS65 KGJT 091008
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
408 AM MDT Fri Aug 9 2024

.KEY MESSAGES...

- Daily afternoon and evening thunderstorms with the threat of
  heavy rain rates, gusty outflow winds, frequent lightning, and
  small hail until further notice.

- An uptick in available moisture starting today will lead to
  increased storm coverage and an enhanced threat of flash
  flooding and debris flows through the weekend and into early
  next week.

- Temperatures will be cooling back toward normal with the
  increase in precipitation and cloud coverage through at least
  the middle of next week.

&&

.SHORT TERM /THROUGH SATURDAY/...
Issued at 408 AM MDT Fri Aug 9 2024

An area of stratiform showers persisted across portions of SE
Utah and SW Colorado early this morning due to lift provided by
a mesoscale convective vortex evident in looped radar imagery.
Showers were trending downward, but there`s a good chance some
of this activity will persist through sunrise. Elsewhere, aside
from northeast Utah and a small portion of northwest Colorado,
skies were mostly cloudy under a lid of mid and high clouds
streaming over the forecast area from the southwest.

Winds in the 850-700mb layer indicated the movement of moisture
both directly from the Gulf of Mexico and the Mexican Plateau
will move into the area today. As a result, precipitable water
levels will be on the rise as has been discussed over the past
several days. According to the GEFS, levels will climb to
around 0.8" in the north and to near 1" or more over the Four
Corners region. Elevated moisture levels combined with daytime
heating and dynamic forcing associated with divergence in the
right entrance region of the jet moving over the Colorado`s
northern border is expected to lead to numerous showers and
thunderstorms from late morning through early evening. Most
storms will initiate over higher terrain before moving over
adjacent valleys. Localized excessive rainfall is possible,
especially across the south where subcloud moisture was deeper
as evidenced by dew points in the lower to mid 50s. Gusty
outflow winds to 45 mph are possible with stronger cells and
areas north of the I-70 corridor where there will be less
subcloud moisture.

Showers diminish from northwest to southeast in response to
diurnal cooling tonight. However, a shallow mid-level short wave
trough combined with jet divergence across the south is expected
to sustain scattered shower/thunderstorm activity overnight.
Moisture levels on Saturday were indicated to remain similar to
those forecast for today. Meanwhile, jet divergence and another
shallow, fast moving short wave trough which, when combined
with diurnal warming, will bring another round of widespread
shower and thunderstorm activity. The subcloud moisture should
be a bit higher than today increasing the potential for
excessive rainfall, especially south of the I-70 corridor.

As discussed previously, afternoon highs are expected to finally
fall to near normal levels today in response to increased clouds
and shower/thunderstorm activity. A similar amount of cooling is
expected overnight with highs on Saturday similar to those
forecast for today.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Issued at 408 AM MDT Fri Aug 9 2024

High pressure sets up over west Texas with low pressure over the
Baja peninsula throughout the extended period. As a result, a
monsoonal flow of mid level moisture from the Gulf of Mexico and
lower level moisture off the Gulf of California will continue
to advect north into the region through at least the middle of
next week. This deep moisture will keep PWAT`s near, or above
an inch across eastern Utah and western Colorado leading to
the potential for excessively heavy rain leading to flash
flooding/debris flows, especially in susceptible areas. Cloud
cover from all this moisture will limit diurnal heating causing
daily high temperatures to run 3 to 7 degrees below normal.
Expect widespread afternoon showers and thunderstorms to
develop daily over the higher terrain across the region with
scattered storms rolling off the terrain and over adjacent
valleys. These storms will continue through the evening before
changing over to mostly stratiform showers as they diminish in
the late evening and overnight. Be aware that some of these
nocturnal storms could be stronger with heavy rain leading to
night-time flash flooding. Stay tuned to the short term forecast
for possible risks in your area.

Models were hinting at a possible change Wednesday into Thursday
as a trough digging south along the West Coast pulls drier air
off the Pacific pushing the monsoonal flow east of the Divide.
Residual moisture with diurnal heating warming the region back
to near normal for mid August will keep afternoon showers and
thunderstorms in the forecast through the second half of the
week, though coverage will likely decrease. One concern with
this pattern is the increased shear from the approaching trough
and the drier airmass aloft may spawn strong thunderstorms
producing large hail (>1 inch) and strong outflow winds (>60
mph). Confidence is low for this severe convective activity at
almost a week out, and should the trough stay farther to the
north, the deep monsoonal flow across the region could just
continue. Stay tuned to see how this plays out.

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1130 PM MDT Thu Aug 8 2024

Currently light stratified showers continue across southeastern
Utah and southwestern Colorado generally south of the line
KCNY-KGUC that will diminish overnight though some will likely
linger into the morning along the south face of the San Juan
Mountains. There is a chance for brief MVFR conditions in these
showers. Look for thunderstorms to develop around noon across
the higher terrain and last into the evening and switching over
to showers after 03Z.

&&

.HYDROLOGY...
Issued at 245 PM MDT Thur Aug 8 2024

Monsoonal moisture will be enhanced by additional moisture
originating from the Gulf of Mexico increasing daily shower
and thunderstorm activity over the area through next week. Areas
south of the I-70 corridor and especially the southern Colorado
and Utah mountains will see the most consistent activity
through this period. The threat of excessive rainfall rates of
1-2" per hour increases as this moisture lingers over the area.
In turn, this will lead to an increasing threat for flash
flooding and debris flows across the majority of the area,
potentially lasting through much of next week. Be sure to stay
weather aware over the coming days, especially if recreating
outdoors and in locations prone to flash flooding and debris
flows. Stay tuned for forecast updates, as well as any Watches,
Warnings, and Advisories in the coming days.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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