Special Weather Statement, NWS Grand Junction, CO

Area Forecast Discussion
National Weather Service Grand Junction CO
306 PM MDT Fri Jul 23 2021

.SHORT TERM...(Tonight through Saturday night)
Issued at 129 PM MDT Fri Jul 23 2021

Like clockwork, storms began firing off higher terrain shortly
preceding the noon hour and coverage has flourished across the
Western Slope since then. As hammered in previous forecast
discussions, storms are feeding off an abundance of monsoonal
moisture this afternoon with PWATs exceeding 1.0 to 1.2 inches.
Current meso analysis shows pockets of SBCAPE in excess of 2500
J/kg over the southwest San Juans, with 1500 - 2000 J/kg draped
across regions south of I-70 and along the Divide. Early morning
cloud cover has quickly eroded within the last hour across these
areas, allowing diurnally driven convection to thrive in this
unstable environment. In addition, a skinny - yet deep - CAPE
profile also indicates the returned threat of heavy rainfall with
these storms. Have no intention in changing current Flash Flood
highlights, as a forecast line of convection (stretching from
northeast Colorado to the Four Corners) has favored these zones
through late this evening. CAM guidance has routinely shown this
diagonal line across the state, drifting westward later this
evening. There's a subtle moisture gradient along this line of
showers, driven by the low pressure system retrograding to our
southwest. As the low nudges across central New Mexico overnight,
showers will be carried westward along with. Elevated instability
and extra forcing along the mositure gradient may maintain heavy
rain rates across the Four Corners region beyond midnight. In
addition, outflow boundaries colliding late this evening will
likely generate new cells over some valley floors. As mentioned
previously, this moisture is well received in some places plagued
by exceptional drought conditions. However, heavy rain still be
quite hazardous for our steeper terrain, slot canyons, arroyos,
and of course...burn scars. The daily potential for heavy rain
directly over the Griz remains and will continue to threaten the
I-70 corridor with possible debris flows.

Nocturnal showers should weak/diminish around sunrise Saturday.
The low's westward trajectory and passing shortwave over the
Northern Rockies will further truncate ridging overhead. As a
result, the strong pull of monsoonal moisture will be suppressed
and PWATs decrease across northeast Utah and northwest Colorado.
The diagonal line of afternoon storms becomes more zonal in nature
and will generally keep moderate to heavy precipitation rates
along and south of I-70. Faster moving storms are expected
tomorrow afternoon across the San Juans, as they'll be in closer
proximity to the Desert Southwest Low and its associated energy.
Meanwhile, storms will have less steering winds to work with
across the central ranges and I-70 corridor due to a difluent
zone aloft. So, the caveat for this region: weaker rain rates are
expected...however, slow-moving cells still pose the threat of
localized flooding. Thus, overall confidence in flooding/debris
flow hazards lowers on Saturday; however, can not rule out the
threat completely due to above average moisture content lingering
across the southern half of the CWA. Storm coverage generally
wanes after sunset, though a few isolated cells will roll through
the midnight hour and linger across the region Saturday night.

.LONG TERM...(Sunday through Friday)
Issued at 129 PM MDT Fri Jul 23 2021

Deterministic guidance and ensemble clusters favor the ridge of
high pressure building back over the central Rockies through the
long term. Though the strongest pull of subtropical moisture may
be slightly displaced as the center of the high rotates over the
Great Plains, residual mositure will still maintain good coverage
of daily showers across the Western Slope through next week (especially
over the San Juans). Case in point, the monsoonal tap remains with
some daily variability in moisture content and placement.

High temperatures will continue to hover around normal through
this period, while muggy conditions will maintain warmer overnight
lows.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1142 AM MDT Fri Jul 23 2021

Scattered to numerous showers and thundestorms will increase in
coverage and strength this afternoon...mainly over the mountains
along and south of I-70, and the Four Corners region. Heavy
rainfall and gusty outflow winds remain threats to airfields from
early afternoon through the late evening hours. For the most part
VFR conditions will be in place. The exception will be when a
storm temporarily impacts a terminal with heavy rainfall producing
MVFR visibility. Improvement comes through the overnight
hours...though if some of the valleys receive heavy rainfall
there is a low probability of lower clouds and fog Saturday
morning.

&&

.HYDROLOGY...
Issued at 129 PM MDT Fri Jul 23 2021

Deep monsoonal moisture remains entrenched across much of western
Colorado and eastern Utah this afternoon. Stronger storms across
southwest Colorado will have faster storm motion, with heavy
rain rates and possible hail. Meanwhile, storms along the Divide
and I-70 corridor will be slower moving with some training echoes
anticipated. Several areas have already received moderate
rainfall amounts the past several days including many burn scars.
Flash Flood Watches remain in effect through late this evening and
numerous highlights have been issued to hone in susceptible areas
across eastern Utah. The westward trajectory of a low to the
southwest may enhance storm initiation later this evening and
prolong the typical, diurnal weakening of storms. Heavy rainfall
may persist after midnight today near the Four Corners before
finally exiting into northeast Arizona by 3 AM MDT. The Desert
Southwest Low will also divert the main surge of monsoonal
moisture to our southwest tomorrow which has resulted in lower
confidence of dictating how far north the moderate to heavy
rainfall rates will fall. In coordination with neighboring WFOs
and WPC, have opted to exclude additional Flash Flood highlights
for Saturday and will allow the night shift to take another look.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Flash Flood Watch until midnight MDT tonight for COZ008-010-
     012>014-018-019.

     Flash Flood Watch through late tonight for COZ021>023.

UT...None.
&&

SHORT TERM...ERW
LONG TERM...ERW
AVIATION...TGJT
HYDROLOGY...15/ERW

Hazardous Weather Outlook
National Weather Service Grand Junction CO
539 AM MDT Fri Jul 23 2021

COZ001>014-017>023-UTZ022>025-027>029-241145-
Lower Yampa River Basin-Central Yampa River Basin-
Roan and Tavaputs Plateaus-Elkhead and Park Mountains-
Upper Yampa River Basin-Grand Valley-Debeque to Silt Corridor-
Central Colorado River Basin-Grand and Battlement Mesas-
Gore and Elk Mountains/Central Mountain Valleys-
Central Gunnison and Uncompahgre River Basin-
West Elk and Sawatch Mountains-Flat Tops-
Upper Gunnison River Valley-Uncompahgre Plateau and Dallas Divide-
Northwest San Juan Mountains-Southwest San Juan Mountains-
Paradox Valley/Lower Dolores River-
Four Corners/Upper Dolores River-Animas River Basin-
San Juan River Basin-Southeast Utah-Eastern Uinta Mountains-
Eastern Uinta Basin-Tavaputs Plateau-Arches/Grand Flat-
La Sal and Abajo Mountains-Canyonlands/Natural Bridges-
539 AM MDT Fri Jul 23 2021

This hazardous weather outlook is for eastern Utah and western
Colorado.

.DAY ONE...Today and Tonight

Deep monsoonal moisture will result in thunderstorm development
with the potential for heavy rainfall rates today.  Localized
flash flooding will be possible in response to heavy rains and
saturated soils.The recent burn scars will be especially
vulnerable with debris flows a likely impact. Gusty outflow winds
will accompany the strongest storms and possibly small hail.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

Daily scattered to numerous showers and thunderstorms can be
expected through the weekend. Moderate to heavy rain and gusty
winds will be the primary threats with storms. Localized flash
flooding will be possible in response to heavy rains and
saturated soils. Unsettled weather is expected into the new work
week.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected today or tonight, however
spotters are encouraged to relay rainfall amounts or any observed
flooding to the National Weather Service in Grand Junction.

Special Weather Statement
National Weather Service Grand Junction CO
1218 PM MDT Fri Jul 23 2021

COZ009>012-231845-
Grand and Battlement Mesas CO-
Central Gunnison and Uncompahgre River Basin CO-
Gore and Elk Mountains/Central Mountain Valleys CO-
West Elk and Sawatch Mountains CO-
1218 PM MDT Fri Jul 23 2021

...SPECIAL WEATHER STATEMENT...

At 1217 PM MDT, Doppler radar was tracking a strong thunderstorm 8
miles north of La Sal. This storm was nearly stationary.

Pea size  hail and winds in excess of 30 mph will be possible with
this storm.

Locations impacted include...
Cedaredge, Redstone, Grand Mesa and Ragged Mtn.

This includes the following highways...
 Colorado 65 between mile markers 11 and 34.
 Colorado 133 near mile marker 40, and between mile markers 50 and
53.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Heavy rainfall is also occurring with this storm, and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

&&

LAT...LON 3883 10809 3898 10821 3941 10737 3917 10720
TIME...MOT...LOC 1817Z 047DEG 1KT 3842 10924

AS

Special Weather Statement
National Weather Service Grand Junction CO
1221 PM MDT Fri Jul 23 2021

UTZ028-231900-
La Sal and Abajo Mountains UT-
1221 PM MDT Fri Jul 23 2021

...SPECIAL WEATHER STATEMENT...

At 1220 PM MDT, Doppler radar was tracking a strong thunderstorm 8
miles north of La Sal. This storm was nearly stationary.

Pea size  hail and winds in excess of 30 mph will be possible with
this storm.

Locations impacted include...
La Sal.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Heavy rainfall is also occurring with this storm, and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

&&

LAT...LON 3827 10927 3837 10936 3847 10924 3840 10912
TIME...MOT...LOC 1820Z 047DEG 1KT 3842 10924

AS

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