374
FXUS65 KGJT 171709
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1109 AM MDT Sat Aug 17 2024

.KEY MESSAGES...

- Monsoonal moisture makes its return today, with increasing
  shower and storm coverage favoring the higher terrain of
  eastern Utah and northwestern Colorado.

- Initial storm threats will be gusty winds and small hail
  today, transitioning to a heavy rain and flash flooding
  concern for Sunday and Monday. Frequent lightning remains a
  threat throughout.

- Tuesday and Wednesday are looking drier, but deeper moisture
  looks to return late week.

- Temperatures start out near normal, warming to around 5
  degrees above normal by midweek, and then cooling to below
  normal values late in the work week.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 224 AM MDT Sat Aug 17 2024

While the dry air and quiet weather of the last few days has been a
pleasant change of pace, changes are already on the way. Satellite
imagery indicates the pool of deeper monsoonal moisture over Arizona
and starting to sneak into southern Utah and the Four Corners
region. This moisture, being fed by the warm waters of the Gulf of
California and subtropical Pacific, is being drawn northward by the
clockwise flow around the subtropical high centered over eastern New
Mexico. This high, which has been closer to the Four Corners the
last few days, is being nudged eastward by a deep Pacific trough off
the West Coast, resulting in the moisture plume shifting eastward as
well. Model guidance still places the deepest moisture over western
portions of Utah, but above normal moisture still makes its way into
eastern Utah and western Colorado, beginning this afternoon.
Isolated to scattered showers and thunderstorms are expected to fire
off the higher terrain of eastern Utah this afternoon and gradually
track eastward into western Colorado through the evening hours. With
dry surface levels still in place today, the main threats with any
showers or thunderstorms will be gusty outflow winds, frequent
lightning, and small hail. The threat for heavy rain today is
relatively small, but still present, as the Weather Prediction
Center has included eastern Utah and far western Colorado in a
marginal risk of excessive rainfall. Of perhaps a greater concern is
inclusion of northeastern Utah and far northwestern Colorado in a
marginal risk of severe weather this afternoon, per the Storm
Prediction Center. This is thanks to the increasing moisture
combined with decent wind shear thanks to a weak jet passing
overhead, and some enhanced lift with a quick moving shortwave
tracking through Utah and into Wyoming. Essentially, what the
marginal risk means in terms of today`s forecast is that storms in
this corner of the CWA will be more organized, perhaps longer
lasting, and most likely to produce severe threats. In this case,
the main concern will be severe wind gusts. As the shortwave
continues tracking east through the evening hours, CAM guidance
favors a trailing line of showers and thunderstorms to track all the
way to the Divide by the small hours of tomorrow morning. This
activity should finally quiet down by daybreak tomorrow, although
lingering clouds could be an issue for afternoon convection on
Sunday. The moisture will be there, as models indicate the high, and
therefore the monsoonal moisture plume, shifting slightly further
east through the day Sunday. CAM guidance does seem a bit less
enthused about convective coverage on Sunday, with the isolated to
scattered showers and storms mainly north of the I-70 corridor.
However, another passing shortwave around the periphery of the high
could help with that. Additionally, as the deep monsoonal moisture
continues to saturate the atmosphere across the region, the threat
for heavy rainfall and potential flash flooding will be on the
increase for Sunday. WPC maintains a marginal risk for excessive
rainfall for eastern Utah and western Colorado not just for Sunday
but all the way into early next week. More about that in the Long
Term Discussion.

Temperatures remain slightly above normal today as we transition out
of the warmer and drier pattern to a cooler and wetter one. Tomorrow
will see highs drop 3-5 degrees, topping out near normal for most
locations. Lows remain mild and a few degrees above normal.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Issued at 224 AM MDT Sat Aug 17 2024

We`ll kick off the long term period with a similar synoptic pattern,
as an elongated trough of low pressure spins off of the West Coast
while a strong ridge amplifies north along the spine of the Rockies.
Sunday`s shortwave rounding the periphery of the high will exit the
region by Sunday night, allowing the majority of showers and storms
to diminish in response. Party to mostly cloudy skies will persist
into Monday as unsettled southwest flow prevails overhead. Another,
more notable, shortwave undercutting the high will brush eastern
Utah and western Colorado on Monday. This will lead to another day
of scattered to numerous showers and storms. PWATs will remain
elevated at 120 to 140 percent of normal across the southern tier of
the forecast area while locations farther north sit at 160 to 180
percent of normal PWATs. The potential for heavy rain with storms
will increase as a result, and therefore we`ll have to keep an eye
out for flash flooding. Not overly concerned with training storms
attm as the trough will lead to slightly quicker steering flow
aloft, but we`ll continue to monitor. Outside of convection we can
expect to see mostly to partly sunny skies. Another quiet night will
set up for Monday night as the trough lifts into the Plains while
the high`s center remains over east-central and southeast Arizona.
Tuesday looks to be a down day in comparison with no shortwaves
projected to track across the area, though residual moisture and
daytime heating will be sufficient to fuel scattered storms over the
higher terrain in particular.

The upstream trough begins to push inland into the Pacific Northwest
on Tuesday evening. However, it will split with the northern wave
lifting into southern British Columbia while the drier, southerly
feature slides into northern California. As this weaker wave pushes
into the Great Basin on Wednesday, it will fizzle out, with broad
troughing dominating the western states. In response, the high will
begin to shift farther east with its center nudging towards the
Southern Plains by Thursday morning. During this time, embedded
waves in the unsettled southwesterly flow will see shower and storm
coverage increase on Wednesday as additional moisture is advected
into the area. This trend continues on Thursday and likely into
Friday as well as the door to the monsoon is opened, with yet
another series of waves on track to push into the region. These
daily rounds of convection will lead to a continually saturating
profile, with the potential for heavy rain outweighing odds for
gusty winds with storms. As a result, flash flooding concerns will
increase from midweek onwards so, similar to last week, be sure to
stay tuned to the latest forecast before heading out or making plans
to recreate.

Temperatures through the period will be a bit of a rollercoaster. As
the new work week begins daytime highs will be sitting a few degrees
above normal before climbing to 4 to 8 degrees above normal for
Tuesday. If the monsoon surge arrives as anticipated, Wednesday`s
highs will moderate by a couple of degrees before dropping back to
below normal from Thursday onwards. Overnight lows through the long
term will be unseasonably warm for mid to late August.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1105 AM MDT Sat Aug 17 2024

Scattered storms are moving through eastern Utah now and that
trend will continue through the evening. Additional develop over
the high terrain is expected over portions of Colorado as well.
These storms will be capable of strong winds and brief heavy
rainfall. VFR conditions will prevail, but period of MVFR are
possible. Showers linger tonight with minimal impacts.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

SHORT TERM...BGB
LONG TERM...MMS
AVIATION...TGJT