-- Discussion containing changed information from previous version are highlighted. --
805
FXUS65 KGJT 200536
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1136 PM MDT Wed Jun 19 2024

.KEY MESSAGES...

- Scattered to widespread showers and storms are expected
  tomorrow and Friday. Significant moisture supports the
  potential for heavy rain during this period.

- Conditions dry out for this weekend, but there is still a
  chance for afternoon storms daily going into next week.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 300 PM MDT Wed Jun 19 2024

With the trough and high anchored to the West/East Coasts
respectively, eastern Utah and Western Colorado remain under a dry
southwesterly flow, but the pattern is changing as the high pressure
builds to the west into Texas overnight into Thursday shifting the
flow in the lower levels more southerly pulling tropical moisture
off the Gulf into the region. Dewpoints along the Front Range are
already in the 50`s with dewpoints in the 40`s creeping west into
the mountains. We`re seeing some of that moisture spill over the
Divide that will likely produce some of the first isolated showers
and thunderstorms along the Divide this afternoon and evening.
Overnight the deeper moisture will start moving in from the south
with scattered showers and thunderstorms developing on the southern
face of the San Juans spreading east in tho the Four Corners area
through the morning hours. Some of these storms may become more
organizes with significant CAPE and directional shear across
southeastern Utah and southwestern Colorado to produce larger hail
and strong winds, definitely something to watch.

Moisture continues to pour into the region from the south through
the day becoming numerous through the southern and central areas
with widely scattered showers and thunderstorms spreading north to
Wyoming by evening. Overnight rain becomes more stratified with
embedded showers and isolated thunderstorms across the whole region
bringing wetting rain to much of the area along and south of the I-
70 corridor. The southern face of the San Juan Mountains is an area
of concern with the strong upslope flow bringing over an inch of
precipitation through the overnight period, yet something else to
watch with flooding possible from training/prolonged showers. The
cloud cover tomorrow will limit diurnal heating through the
afternoon, but H500 heights increase as the high pressure builds in
from the east to warm us five degrees over today.

&&

.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
Issued at 300 PM MDT Wed Jun 19 2024

Deep Gulf moisture will already be in place by Friday morning with
forcing from embedded shortwaves allowing convection to continue
through the morning hours. Friday looks to be a very active day as
moisture rises even further by Friday afternoon with mixing ratios
climbing to between 7 and 9 g/kg. PWAT is also upwards of 200 to 300
percent of normal on average with some pockets near the Four Corners
up to almost 400 percent of normal. Dewpoints are projected to be
well into the 50s across much of the area with dews near 60 across
the Four Corners. Basically, this all means we will have significant
deep moisture advection for efficient heavy rain potential with any
showers and storms that form. CAPE values rise up to 1000 J/kg or
more, so plenty of instability exists for strong storms to form. The
real question though which would limit how much of this instability
we can tap into is the amount of cloud cover and precipitation
lasting through Friday morning. This could limit the convective
potential due to lack of surface heating from the sun to act on the
instability. Regardless, the potential is there for heavy rainfall
and this is typically the type of moisture we would see during late
July. Gusty outflow winds are also possible upwards of 50 mph or
more but how strong these are all depends on how much instability is
left and if we can get any breaks in the cloud cover and surface
heating to help aid stronger convection. If it`s any consolation,
the latest hi-res CAMs including both the NAMNest and HRRR indicate
convection rumbling through western Colorado Friday morning with
enough of a break late Friday morning before storms fire in earnest
early to late Friday afternoon with the best period ahead of and
along the cold front associated with the shortwave trough passage
heading into Friday evening. We could see some stronger storms
during this period, with the idea that we`d see enough breaks in the
cloud cover late Friday morning into early afternoon to get some
surface heating from the sun. WPC has much of eastern Utah and
western Colorado practically along and south of I-70 included in the
slight risk category for excessive rainfall, which makes perfect
sense given the environment.

Drier air pushes in from the west after the passage of this robust
shortwave trough Friday night that will drive much of this deep Gulf
moisture eastward, leading to a drier day on Saturday and more so on
Sunday. However, enough moisture looks to linger behind with a
little bit of forcing from embedded shortwaves moving across the
north to produce some isolated to scattered storms favoring the high
terrain. Dew points are still a bit elevated also, so this is
something to watch as models might be too quick to dry things out,
which would result in more storm coverage. This seems more of a
thing Saturday rather than Sunday though as dew points come down
quite a bit by Sunday into Monday. High pressure builds back in
across the south with westerly flow remaining overhead this weekend
into early next week, before the ridge axis tries to nudge slightly
eastward by middle of next week. Models are struggling with how to
handle this but the general trend for the coming week is a return to
hot and dry conditions. Temperatures Friday into Saturday though
will be much cooler though, especially Friday given the abundant
moisture and resultant clouds and showers/storms with highs about 5
degrees below normal through Saturday. Highs jump to 4 to 8 degrees
above normal Sunday into the coming week.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
-- Changed Discussion --
Issued at 1132 PM MDT Wed Jun 19 2024

Scattered to widespread showers and storms will develop around
mid day across the higher terrain. These showers will be
capable of producing strong winds upwards of 60 mph and brief
heavy rainfall. Just about all of the sites have a chance of
being impacted by these showers. Convection will linger into the
overnight, but most likely weaken overall.
-- End Changed Discussion --

&&

.GJT WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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