820
FXUS65 KGJT 101206
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
606 AM MDT Sat Aug 10 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.

- This uptick in available moisture will bring an enhanced
  threat of flash flooding and debris flows through the weekend
  and into early next week.

- Temperatures will be near normal through the weekend cooling
  to below normal going into early next week with the increase
  in precipitation and cloud cover.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 422 AM MDT Sat Aug 10 2024

High pressure over Texas and low pressure along the Baja Coast keeps
the door open to monsoonal moisture to flow up through Arizona and
Southern California into eastern Utah and Western Colorado.
Troughing/low pressure over the Pacific Northwest and southern
Ontario keep the jet just to the north over Wyoming with increased
shear and helicity across the northern areas, especially overnight
tonight. Deep moisture with pwats at or above an inch continues
across the region and will continue well into the extended forecast
period (more on that below). There is currently weak convection,
mostly light to moderate showers, over the San Juans that will
linger into the morning hours before diurnal heating will initiate
the development of thunderstorms in the late morning in the southern
mountains, spreading into the central mountains and the Uintas by
noon and becoming widespread over the higher terrain across the
whole region in the early afternoon. Threats today will be much the
same as yesterday with training storms leading to localized flash
flooding, especially in the areas prone to training storms and
flooding. With the lower atmosphere mostly saturated, strong winds
are less of a concern, but gusts over 40 mph are still likely with
the heavier rain, especially north of the I-70 corridor where the
rain shafts can transport the momentum of the winds aloft down to
the surface.

Tonight, dynamics that can support stronger nocturnal thunderstorms
moves over the northern areas generally along and north of the I-70
corridor due to a weak disturbance moving through the jet to the
north. The NAM based models latch onto these dynamics, forecasting a
strong nocturnal storm system moving through the northern areas
overnight, but most of the other CAMS push wide spread convection
through the northern region through the late afternoon and early
evening, expending the energy built up form the daytime heating
before the dynamics set up. This will be something to watch; should
convection across the north be more widely scattered through the
afternoon, then plenty of energy will be available overnight to
support strong, possibly severe thunderstorm. Unfortunately, we`ll
need to wait until late this afternoon to know if this will be the
case. Currently leaning to the majority of the CAMS with the region
getting worked over by evening and the system stratifying out to
mostly showers with a few embedded thunderstorms through the
overnight, much like the southern mountains had last night.

The surge of monsoonal moisture pushes farther north Sunday with
pwats of an inch or more through the Uinta Basin and east across
northwestern Colorado. We`ll see pretty much a repeat of Saturday
with thunderstorms developing in the late morning and afternoon,
lasting well into the evening, with the heavier precipitation seen
across the southern and central areas today spreading north to the
Wyoming border. This will include the risks for training storms and
localized flash flooding.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Issued at 422 AM MDT Sat Aug 10 2024

The active weather pattern continues through early next week as
monsoonal moisture continues to funnel into the area with the high
pressure ridge positioned to the southeast over Texas. PWAT
anomalies remain right in that sweet spot of 150 to 180 percent of
normal on average. Sunday night looks to continue to be active as a
shortwave moves through the flow across the south, keeping
thunderstorm activity ongoing for mainly the central and southern
areas, as evidenced by the latest HRRR output. PWAT anomalies are
even shown to increase more heading into Monday and Tuesday as
values increase up to 200 percent of normal. This is likely due to a
shift from a westerly to more of a southwesterly or southerly flow,
funneling the monsoonal moisture through eastern Utah and western
Colorado from a deepening low pressure trough over the Pacific
Northwest and the high pressure ridge still positioned over Texas.
Additional shortwaves continue to move through thanks to an active
northern stream, allowing for this forcing to act on the abundant
moisture. Storms will be capable of heavy rain, frequent lightning
and potential for flash flooding especially over more vulnerable
areas like steep terrain, burn scars and already saturated soils.
Given the shortwave disturbances, nocturnal activity seems like a
sure bet at least through Tuesday night. The Pacific Northwest
trough looks to finally push across the area by Wednesday after
hanging out for a few days prior.

Changes look possible late in the coming week though as high
pressure builds in across the Desert Southwest following the passage
of the Pacific Northwest low on Wednesday. This allows drier air to
push across the Great Basin and Western Slope, with the flow
shifting to a northwesterly direction, effectively shunting the
monsoonal moisture tap to our south and east. PWAT anomalies drop to
60 to 80 percent of normal by Thursday into next weekend. Time will
tell if this actually pans out cause models do have a tendency to
dry things out too quickly. Enough remnant moisture could remain for
isolated to scattered storms each afternoon mainly over the higher
terrain, but the threat of excessive rainfall looks to gradually
lessen late in the coming week.

Given the increased clouds and showers through mid week, expect high
temperatures to remain 5 or so degrees below normal with overnight
lows a bit milder at 5 or so degrees above normal. Temperatures look
to moderate towards near normal values by late week as drier air
begins to advect in from the west and northwest.

&&

.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 538 AM MDT Sat Aug 10 2024

Showers and isolated thunderstorms across the southern mountains
will continue for the next few hours before redeveloping into
widespread showers thunderstorms after about 16Z. These storms
will spread north to the Wyoming border by 20Z with periods of
MVFR at times in heavier showers. Look for thunderstorms to
continue through the evening, diminishing to showers after about
06Z Sunday. Winds will be gusty out of the west across the
northern areas and generally light elsewhere.


&&

.HYDROLOGY...
Issued at 538 AM MDT Sat Aug 10 2024

Monsoonal moisture will keep showers and thunderstorms over the
region through at least mid next week. Most of the higher
terrain across the region, especially the southern and central
mountains will see potential rainfall rates in excess of 1-2"
per hour through this period. This will lead to increasing
threats for flash flooding and debris flows across the majority
of the area. Stay weather aware over the coming days, especially
if recreating outdoors and in locations prone to flash flooding
and debris flows, and stay tuned for forecast updates, as well
as any Watches, Warnings, and Advisories over the coming days.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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