707
FXUS65 KGJT 182059
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
259 PM MDT Sun Aug 18 2024

.KEY MESSAGES...

- Scattered to numerous showers and storms will continue today
  and tomorrow. Storms will develop each afternoon with showers
  likely to persist into the overnight hours.

- Storms will be capable of producing gusty winds, frequent
  lightning, small hail, and periods of moderate to heavy rain.
  Localized flash flooding remains a concern.

- Brief decrease in storm coverage on Tuesday before better
  monsoon moisture arrives from Wednesday onwards, resulting in
  widespread showers and storms through at least Friday.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 118 PM MDT Sun Aug 18 2024

Moisture advection was in full force yesterday under southerly flow
aloft. PWAT values this morning are 140-180 percent of normal with
dew points in the 40s and 50s for most locations. Currently there is
an MCV left over from convection in AZ last night. This MVC is near
the UT/AZ state line now with clouds spreading over the Four Corners
region. This cloud cover will hinder surface heating and therefore
instability in that region. Not sure it will be able to recover. The
northern two thirds of the forecast area has been clear all morning
so instability has been allowed to build. Convection has already
taken place over the mountains and that activity will track
northeastward causing additional convection downstream. Storms
are expected to form over the higher terrain through the
afternoon. Given the abundant moisture and CAPE on the order of
500 J/kg heavy rainfall is possible and storm training could
lead to flash flooding. This flooding potential is highest in
the southern mountains based on precip amounts from the latest
high-res, but a nonzero chance just about everywhere. The
strongest cells may produce small hail, and wind gusts upwards
of 45 mph are possible especially around storm clusters. The
CAPE is expected to decrease overnight leaving mostly
stratiform, but perhaps there is enough CAPE to support a few
pockets of lighting. The left over MCV could track over portions
of northeast Utah and northwest Colorado tonight and support
persistent showers there. The MCV looks to strengthen into a
weak shortwave tonight and lift out over the Northern Plains
tomorrow. This will cause the flow to become more southwesterly
tomorrow. Moisture does not appear to change much tomorrow with
this flow shift. Convection will develop over the higher terrain
around mid day before drifting over the valleys in the
afternoon. Heavy rain and some flash flooding remains a concern
once again. By the evening the storms will weaken and coverage
becomes more isolated.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Issued at 118 PM MDT Sun Aug 18 2024

An incoming wave in the PACNW will push the blocking high around on
Tuesday. This will shunt the monsoonal tap back southward, thereby
quieting the afternoon shower/thunderstorm production of recent
days. Ensemble forecasts have trended PWAT`s downward on recent
runs, with moisture values 120 percent of normal across western
Colorado and slightly higher in eastern Utah. Southwesterly flow
ahead of the incoming PACNW wave will likely spark a few showers on
the favored aspects of the terrain Tuesday afternoon. Drier surface
conditions will likely spell out some gusty outflows for these
storms. The blocking high never really gives up and a series of
waves drop into the PACNW against it from Wednesday onward. The
monsoonal plume fills back in Wednesday and storm coverage picks
back up. Some of the richest moisture feeds in north of the Colorado
River Wednesday. By Thursday, much of the CWA will see PWAT`s in
excess of 170 percent of normal. This will arrive along a region of
forcing near the jet streak stretching from the Great Basin into
Wyoming. Thursday looks like a good bet for one of the wettest days
in this extended period. Storms will be widespread under this
regime, should all hold on as expected. Things get interesting
Friday as models bring a closed low ashore in NORCAL and punch it
into the Great Basin by Saturday. This will keep an unsettled
pattern in place into next weekend. With the blocking high out of
the way, the monsoonal tap effectively closes. Recycled moisture,
along with anything entrained in the passing low will keep some
showers and storms in the forecast Saturday and, perhaps, Sunday.
With such a drastic pattern shift, models are displaying some
inconsistencies. Temperatures don`t ever get a chance to surge
upward much this week. Tuesday will likely turn out to be the
warmest day of the week. Cloud cover and showers will take the bite
out of highs the remainder of the week. Thursday looks like another
day, where area highs struggle to reach mid 80`s in our warmest
valleys.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1145 AM MDT Sun Aug 18 2024

Another round of thunderstorms and showers will work north and
eastward from the Four Corners across the region today. This
will likely produce TEMPO ILS breakpoint and IFR conditions at
mountain terminals and adjacent valley terminals. Gusty outflow
winds to around 50 mph are expected near storms too, along with
some small hail. Targeted areas of concern will be the San Juans
and southeast Utah this afternoon, with areas north of I-70
developing late afternoon and into the overnight hours. Showers
and storms will diminish south to north later this evening.
Most terminals will return to VFR conditions by daybreak, with
some mountain areas west of the Continental Divide hanging on
to clouds and light rain into early Monday morning. A brief
reprieve in storms and showers is expected to close out this TAF
period.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

SHORT TERM...KJS
LONG TERM...LTB
AVIATION...TGJT