590
FXUS65 KGJT 301128
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
528 AM MDT Sun Jun 30 2024

.KEY MESSAGES...

- A Flood Watch for southeast Utah and southwest Colorado is in
  place through Monday afternoon. Abundant monsoonal moisture
  will bring a heavy rainfall rate threat to these areas today
  through tomorrow.

- Besides heavy rainfall some storms will be capable of
  producing gusty outflow winds and hail the next few
  afternoons as well.

- Monsoonal moisture gets shunted east on Tuesday, and
  afternoon showers will be far more isolated in nature.

- This drying trend could lead to elevated fire weather concerns
  as we head into the late week period as afternoon humidity
  decreases and afternoon winds increase.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 305 AM MDT Sun Jun 30 2024

The early morning hours are decidedly more quiet than 24 hours
ago but that is about to change over the next 48 hours or so.
The H500 hand analysis map shows a high amplitude pattern over
the high latitudes with a ridge over W.Canada squeezed between
two strong low pressure systems. Over the mid latitudes the jet
stream is meandering over the northern States with the
SubTropical High centered over the Red River Valley of TX/OK
dominating the pattern over the S.CONUS. Being on the periphery
of this high...subtropical moisture continues to advect
northward across the Desert SW into Central Rockies and the 00Z
KGJT RAOB shows PWAT has swung up above an inch. This is just an
number...but what is more important is the anomalous trend in
moisture that will continue to feed through and Northeast of the
4 Corners over the next 24 hours. Models suggest we hit
250-300% or 2 to 4 Std Dev above normal of PWAT by this
evening...normal being near 2/3 of an inch by July 1 and the
99th percentile near 1.2 inches. This moisture will lead to the
same issues we have seen the last week or so...strong afternoon
thunderstorms...many capable of producing extreme rainfall rates
and a few producing larger hail and gusty outflow winds.
Afternoon soundings suggest moderate CAPE(1000-2000j/kg) and
moderate shear with fairly straight hodographs over the
southeast 2/3 of the CWA this afternoon. This would suggest
splitting cells...some of which may drop some bigger hail before
the cooler outflow becomes more dominant. We have seen this
outflow pattern the past few nights as well with early
convection throwing out boundaries/gravity waves which continue
force more storms late into the evening and overnight thanks to
mositure/instability aloft. A bigger threat arises due to the
amount of moisture in the profile...fairly slow storm motion and
the threat of training storms allowing heavy rainfall rates
to produce excess runoff. This threat looks to continue well
into the night as the lower atmosphere cools and soundings take
on that long-skinny CAPE profile. Warm rain processes will
dominate with the warm cloud depths well over 8Kft in some
areas. Favorable jet dynamics aloft both from the SubTrop high
and approaching upstream trough help provide the large scale
ascent to keep things firing as well. Not as confident on
exactly where the heaviest rainfall will be as that will be up
to storm scale meteorology. Model QPF seems to be somewhat
infused with convective feedback which sometimes points to right
area but often not. This may be why National guidance and model
solutions continue to favor SW Colorado mountains and adjacent
valleys for the heaviest rainfall totals through tomorrow and
overall this is encompassed in our Flood Watch area. GEFS
M-Climate QPF percentiles also back up this area to some degree
as well. The periphery of this area will still be under a threat
due to the chaos of the outflow boundary interactions. Farther
N and W in our CWA conditions will be slightly drier as the
upstream trough becomes more influential. There will still be
some storms but gusty outflow winds and moderate rainfall rates
appear to be more of a threat than heavy rainfall. On
Monday...troughing deepens over the Northern Rockies into the
Great Basin which will suppress the moisture ever so slightly
southward. Depending on where the heaviest storm tracks are
Sunday into Sunday Night...Monday could be the day to watch more
closely for the flooding threats in the South. Though drier
conditions will be working in with the trough...shear will be
increasing aloft and jet dynamics still playing a role in
adding large scale ascent. So organized storm formation remains
a threat for both hail and gusty winds in most areas with a
continued heavy rainfall rate threat in the South. So a few more
days of busy weather before things calm down a bit with drier
air moving in. Unfortunately that likely means a switch from
flooding issues to fire weather issues.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Issued at 305 AM MDT Sun Jun 30 2024

With the high still over the Southern Plains and helping to pump
deep subtropical moisture into the Desert southwest, and the trough
axis passing overhead accompanied by a shortwave in the base, look
for evening convection to continue well into the overnight. Some of
these storms could be on the stronger side as well, producing gusty
winds, heavy rain, and frequent lightning. This activity should taper
off somewhere around daybreak on Tuesday morning.

As the upper level trough continues tracking east, it will shunt the
subtropical high and the moisture plume eastward as well...away from
eastern Utah and western Colorado. Now, models are notorious for
scouring out surface moisture far too quickly in situations like
this, especially when we`re talking 3-4 days from now. So, while
model guidance right now has barely any chances of afternoon
convection on Tuesday, isolated to scattered afternoon convection
over the higher terrain is still in the cards. Deeper troughing
returns for Wednesday, and some hint is there in the model guidance
that the moisture plume tries to reassert itself, but the dry
northwesterly flow aloft will work to limit its impact on eastern
Utah and western Colorado, keeping much of the plume to our south
and east. All the same, a slight increase in storm coverage on
Wednesday is expected, mainly over the southern mountains. High
pressure builds in from the west beginning on Thursday, with dry
northerly flow finally scouring out that pooled surface moisture for
the end of the week. This promises a mostly dry Fourth of July.
Temperatures will start out near to a few degrees below normal
before gradually warming to around 5 degrees above normal by Friday.

Models begin to diverge from Friday night onward, with some
guidance wanting to continue pushing the West Coast ridge eastward
into the Great Basin, while other solutions bring a strong trough
down through the Intermountain West along with a push of moisture.
Cluster guidance is showing a close to 50-50 split between these two
solutions, so jury is still out as to which will materialize, and
whether we`ll see dry and warm conditions, or a return to cooler and
unsettled.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 527 AM MDT Sun Jun 30 2024

Plenty of moisture is in place which enhances instability and
this keeps at least a threat of isolated showers and storms
this morning. As heating commences expect the storms to become
scattered to widespread over all but the far northwest CWA this
afternoon with plenty of left over convection surviving through
the night. Outflow winds the main threat to most TAF sites over
the next 24 hours. Rainfall rates will be high enough to bring
MVFR to IFR conditions if the right shower moves over an
airfield...but this remains a low probability forecast attm.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...Flood Watch from noon MDT today through Monday afternoon for
     COZ009-011-012-014-017>023.
UT...Flood Watch from noon MDT today through Monday afternoon for
     UTZ022-028-029.

&&

$$

SHORT TERM...15
LONG TERM...BGB
AVIATION...TGJT