660
FXUS65 KGJT 090001
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
601 PM MDT Wed Oct 8 2025

.KEY MESSAGES...

- Isolated showers and thunderstorms develop over the southern
  San Juans this afternoon.

- A surge of tropical moisture is expected to arrive tomorrow
  and peak Friday and Saturday.

- Excessive rainfall may result in flooding of slot canyons,
  normally dry washes, slickrock areas, recent burn scars, urban
  areas, and other low-lying locations.

- Rockslides and mudslides are possible along the highway
  corridors in the mountains.

&&

.SHORT TERM /THROUGH THURSDAY NIGHT/...
Issued at 216 PM MDT Wed Oct 8 2025

With high pressure located over the Southern Plains and a low
pressure system over the PacNW southwesterly flow as begun across
the region. Moisture in that flow has already reached the San Juan
mountains, so an isolated shower is possible through this evening.
Moisture advection continues tomorrow so expect scattered showers
and storms generally in the Four Corners region and favoring the
high terrain. The moisture we are dealing with has tropical origins,
so PWATs and mixing ratios will be approaching record values for
this time of year (300-350% of normal). In fact, some of this
moisture is connected to Hurricane Priscilla in the eastern Pacific
off the coast of the Baja. Given this is a more tropical air mass
midlevel lapse rates will not be particularly steep therefore
limiting instability. So heavy rain from thunderstorms might not be
as prevalent as a typical monsoon event. Heavy rain may come from
warm rain processes, which can be efficient and reach 1 inches per
hour. By tomorrow afternoon and evening the moisture begins to reach
those peak values.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Issued at 216 PM MDT Wed Oct 8 2025

On Friday morning widespread precipitation from the rainfall
event will be well underway. Southwest flow situated between
high pressure centered over Texas, and low pressure off of the
coast of the Pacific Northwest, will continue to pull in
moisture from Tropical Storm Priscilla. The strong moisture
advection will support anomalous PWAT values in excess of 200%
of normal for all, and even above 300% for areas in the southern
half of our CWA. Widespread rainfall will continue through
early Sunday, including during overnight hours. Rainfall looks
to generally be more stratiform. However, stronger showers and
thunderstorms may support periods of heavier rainfall in areas
where lift and forcing is increased. Considering the QPF, and
possibility for periods of heavier rainfall, a Flash Flood Watch
has been issued for parts of southeastern Utah and southwestern
Colorado from Thursday evening through Saturday afternoon.
Flooding in low-lying and flood-prone areas, and rockslides and
mudslides in areas of higher terrain are concerns.

Rain will continue beyond Saturday afternoon, but at this point
moisture will begin moving out as drier air returns. Overnight
Saturday into Sunday is when the strongest upper-level support
moves through the region, which will allow for precipitation to
continue from the remaining atmospheric moisture. Rain during
this timeframe is expected to be mostly north of I-70, and
expected amounts are relatively less, but the increased forcing
may support periods of localized heavier rainfall. High
temperatures on Friday and Saturday will be just below normal,
but a cold front passing early Sunday will drop temperatures
more near our northern border. The precipitation forecast
beyond Sunday is a bit uncertain, but another push of moisture
may be possible as the high pressure centered over Texas shifts
eastward.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 547 PM MDT Wed Oct 8 2025

Today has been quiet and mostly clear, with prevailing VFR
conditions. However, tonight moisture beginning to move in from
the south will bring increased shower and thunderstorm chances.
As such, PROB30 has been included at many TAF sites. Chances
increase from south to north, with KDRO being the first site to
have a potential impacts. Most of the precipitation chances
begin after 18Z. Winds will generally be light overnight.

&&

.HYDROLOGY...
Issued at 216 PM MDT Wed Oct 8 2025

Tropical moisture will stream northward into the region on
Thursday, bringing widespread heavy rain and increasing the
threat of flash flooding across portions of southeastern Utah
and southwest Colorado. An increased risk of flash flooding will
continue through at least Saturday evening across the watch
area. Excessive rainfall may result in flooding of slot canyons,
normally dry washes, slickrock areas, recent burn scars, urban
areas, and other low-lying and flood-prone locations. Rockslides
and mudslides are possible in the mountains.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...Flood Watch from Thursday evening through Saturday afternoon
     for COZ017>023.
UT...Flood Watch from Thursday evening through Saturday afternoon
     for UTZ022-025-027>029.

&&

$$

SHORT TERM...KJS
LONG TERM...GF
AVIATION...TGJT
HYDROLOGY...TGJT