499
FXUS65 KGJT 200938
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
338 AM MDT Sat Jul 20 2024

.KEY MESSAGES...

- Temperatures are expected to remain a few degrees above normal
  with daily thunderstorm chances slowly decreasing through
  early next week.

- Potential for heavy rain and localized flash flooding is
  possible with storms across southeast Utah and southwest
  Colorado today, with less coverage across the north where
  gusty outflow winds are more of the concern than rain. Storms
  will exhibit more of a gusty outflow wind threat than heavy
  rain as we head into the coming week.

- The combination of high based storms, gusty outflow winds and
  lightning will also keep a threat of new wildfire starts in
  the picture.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 338 AM MDT Sat Jul 20 2024

Isolated shower/storm activity is ongoing particularly across
southeast Utah and southwest Colorado this early morning with
even some stray showers across the Grand Valley of west-central
Colorado. This seems to be driven by the subtle remnants of a
shortwave and a few vort maxes moving through the northwest flow
that the HRRR seems to be picking up on fairly well. Our 00Z
GJT sounding last evening indicated 0.88 inches of precipitable
water with current SPC mesoanalysis indicating PWAT between 0.8
and 0.9 across the Four Corners and west- central to southwest
Colorado along the CO-UT border.

Models indicate that drier air will begin to advect in from the
north across mainly areas north of I-70 today. It is evident
that enough remnant moisture and instability exists for shower
and storm activity to be not only ongoing through the night but
also redevelop throughout the afternoon. The projected change in
air mass is reflected well when looking at mixing ratios as
values of 4 to 6 g/kg remain across southeast Utah and southwest
Colorado with values reduced to 2 to 4 g/kg across northwest
Colorado this afternoon. CAPE is better across the south in the
realm of 200 to 400 J/kg where the better moisture exists with
very little instability north of I-70. The HRRR reflects this
well with the potential storm coverage showing the best
potential south of I-70 this afternoon with more isolated
activity north of I-70. Storms are expected to redevelop over
the high terrain this afternoon with best potential across the
south for the lower elevations to be impacted. Heavy rain and
localized flash flooding still remains a concern across
southeast Utah and southwest Colorado where the better moisture
exists and especially if any storm training happens. Despite
less storm coverage across the north, the HRRR does indicate
some gusty outflow winds from a remnant potential storm, most
likely associated with a shortwave dropping across northwest
Colorado out of Wyoming early this evening, so that could be
something to watch for as gusty outflow wind potential of 40 to
50 mph is possible with storms.

Drier air will continue to push southward on Sunday, resulting
in much less in the way of storm coverage. Locally heavy rain is
still possible over the southern areas as this moisture is
pushed south and the drier air slowly infiltrates from the
north, so we may see more of a transition to gusty outflow winds
being a primary concern rather than heavy rain. Temperatures
will remain roughly the same as they have been around 5 degrees
above normal today with slight cooling to near normal by Sunday
as this drier air pushes in from the north. Skies could also be
a bit hazy this weekend as this northerly flow brings some
elevated wildfire smoke around the high pressure to our west
from the fires over the Pacific Northwest and Intermountain
West, especially as the drier airmass advects in from the north.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Issued at 338 AM MDT Sat Jul 20 2024

Sunday evening through Wednesday, models are in good agreement
with high pressure centered in the Great Basin with ridging
extending north into Alberta, and troughing over the Plains
putting eastern Utah and Western Colorado under a continuing
northerly flow. A deep shortwave rotates around the Gulf of
Alaska low into the Pacific Northwest Monday evening, knocking
the top off the ridge Tuesday, but leaving the high undisturbed
in the Great Basin. A second shortwave rotates through late
Wednesday after which the models diverge. Solutions range from
the Gulf of Alaska low moving quickly inland across Canada with
troughing along the West Coast effectively eroding the high
putting the region under a southwesterly flow by Thursday, to
the Gulf of Alaska low wobbling inland over British Columbia and
the troughing not hitting the West Coast until late in the
weekend. There is low confidence in any of these solutions
beyond mid week, so we`ll just have to wait and see how this
plays out. Until then, a lot of monsoonal moisture has wrapped
up into the high in the Great Basin and the circulation brings
periods of drier and moist air over the region for enhanced or
diminished convective activity, but the current pattern of
showers and thunderstorms building up over the mountains each
afternoon lasting into the evening will continue through the
coming week.

Guidance indicates Monday will see drier air push in from the
north shutting down most convection in the Uinta Basin area and
northwestern Colorado and isolated showers and thunderstorm in
the northern and central Colorado mountains, becoming more
scattered farther south in the San Juans. A bit more moist air
rotates into the region Tuesday and Wednesday producing widely
scattered showers and thunderstorms across all the higher
terrain of eastern Utah and Western Colorado. Thursday looks to
dry out again with isolated to scattered convection over the
southern mountains. With the influence of drier air early to mid
week, storms should exhibit more of a gusty outflow wind threat
than wetting rain. Temperatures will be near normal across the
region Monday, but a warming trend will heat things about five
degrees by Thursday as the high pressure to the west expands to
the east into the region.

The forecast guidance is hinting in a significant shift in the
weather pattern with the trough moving in over the Intermountain
West Friday and Saturday initiating widespread showers and
thunderstorms across the region. Again, there is low confidence
in this forecast at this time. Stay tuned to the forecast into
early next week when we should have a better view of next
weekend.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1116 PM MDT Fri Jul 19 2024

A few showers linger across the region, mostly over higher
terrain. Most will end in the next few hours, but a few may
hang on into the morning. Otherwise, VFR conditions with light
terrain driven winds will prevail. Another round of isolated to
scattered showers and thunderstorms starts around 18Z and will
continue into the evening hours with most of the activity over
the southern mountains and along and east of the Divide in the
central Colorado mountains.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

SHORT TERM...MDA
LONG TERM...DB
AVIATION...DB