Special Weather Statement, NWS Grand Junction, CO

Area Forecast Discussion
National Weather Service Grand Junction CO
130 PM MDT Wed Aug 18 2021

.SHORT TERM...(Tonight through Thursday night)
Issued at 122 PM MDT Wed Aug 18 2021

Everything is on track for active weather over the next few days. A
low pressure has begun to move over the northern Great Basin, which
is causing an increase in the southerly flow. PWATS have increased
overnight and will support the potential for widespread showers and
storms. Most of the initial develop will be driven by diurnal
heating, but as things evolve that may change. Closer to the low
pressure forcing is much stronger in central Utah, and those storms
will move into our area. Lift on the nose of a subtropical jet is
moving through central AZ and that will continue northeastward into
our area that will force more convection. The highest instability
will be across the southwestern third of the CWA and the best shear
is over the northwest third. Where this overlaps will be the highest
potential for strong to severe storms. This is supported by several
CAMs, which show rotating updrafts in around that area. Large hail
and damaging winds are threats through the evening. In addition to
that heavy rainfall will bring the potential for flash flooding.
Storm motion is quicker than the normal summer pattern, but rainfall
rates will exceed 1"/hr and training storms looks possible. Clusters
and or lines of storms could form as well. Over the course of the
evening things could evolve into more stratiform with embedded
convection that continues overnight. Northeast Utah and northwest
Colorado are highlighted by models to receive the most rainfall.

Tomorrow morning the main piece of energy associated with the trough
trigger additional convection in central Utah. Soundings show that
this convection could become strong to severe in the morning hours.
The models show that moving eastward across our area during the day,
which will be supported by the forcing. Shear will actually be a bit
stronger although instability might be some what hindered. The lift
may be able to compensate for that. The more robust convection may
stay confined to the southeast half of the area. Hail, strong winds
and heavy rainfall remain a threat through tomorrow. The Grizzly
Creek will be one big concern during this event. There will be
several opportunities for impact on the burn scar with storms
lifting northward across the region this afternoon through tomorrow
evening. That energy exits the area tomorrow evening so most these
threats come to an end. One last shortwave could rotate over the
area on Friday spawning more precipitation in the central and
southern mountains. PWATs will be decreasing during this time frame
so heavy rainfall potential should decrease.

.LONG TERM...(Friday through Wednesday)
Issued at 122 PM MDT Wed Aug 18 2021

The upper level trough will lift out of the region by midday Friday
as the axis shifts east of the Continental Divide. Drier
northwesterly flow will be advected into the region in its wake.
However, PWATs will not take much of a hit during this period so
another round of scattered showers and thunderstorms can be expected
during the afternoon and early evening hours. After a brief
transition to zonal flow aloft on Friday night, southwest flow will
return for the weekend as the next Pacific trough digs into the
northwestern CONUS. This system will quickly trek across the
Intermountain West on Saturday with its base brushing the northern
tier of the forecast area for Sunday. The shallower nature of this
system paired with a broad area of high pressure situated to the
southeast will minimize the influx of subtropical moisture from the
south. Regardless, this aided lift aloft will be enough to result in
an uptick in convection across at least the higher terrain through
the weekend. Another trough is quick to dig into the Pacific
Northwest at the start of the new week as the previously mentioned
center of the upper level ridge drifts slowly westward. For now the
extended forecast guidance is relatively consistent, though this may
change as the week progresses.

Temperatures will run below normal through the weekend in the wake
of the trough and as the next system moves through before slowly
rebounding early next week thanks to the ridge gradually shifting
westward. However, this is subject to change, especially if any of
the quick-moving shortwaves trend farther south.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1151 AM MDT Wed Aug 18 2021

Widespread showers and thunderstorms are expected today,
overnight, and into tomorrow. VCTS and TS on station is expected
with hail, gusty winds, and heavy rainfall possible for TAF sites.
Convection has just started and will continue through the rest of
the day. MVFR conditions will be possible under and near the
stronger cells that form for the next 24 hours. ILS breakpoints
will likely be met from time to time.

&&

.HYDROLOGY...
Issued at 122 PM MDT Wed Aug 18 2021

The influx of monsoon moisture and lift from a system is likely
to produce thunderstorms with heavy rainfall and larger areas of
persistent moderate rainfall rates. Areas closer the UT and CO
border will have the greatest threat of excess runoff over the
next 36 hours. However our burn scars including the high impact
Grizzly Creek and other favored areas in steep terrain will also
be susceptible to debris flows. The current Flash Flood Watches
highlight the favored areas for flooding threats through tomorrow
night.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Flash Flood Watch through Thursday evening for COZ001-003-
     006>011-013-017-020.

UT...Flash Flood Watch through Thursday evening for UTZ023>025-
     027>029.

&&

SHORT TERM...KJS
LONG TERM...MMS
AVIATION...TGR
HYDROLOGY...TGJT

Hazardous Weather Outlook
National Weather Service Grand Junction CO
139 PM MDT Wed Aug 18 2021

COZ001>014-017>023-UTZ022>025-027>029-191945-
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-
139 PM MDT Wed Aug 18 2021

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

.DAY ONE...This Afternoon and Tonight

Thunderstorm coverage will increase significantly today and many
of the storms will have the threat of heavy rainfall rates and
gusty winds. The development of stronger storms in eastern Utah
will also be capable of producing large hail and severe winds to
70 mph. Heavy rainfall rates or prolonged moderate precipitation
could impact burn scars and lead to other runoff issues over the
next 24 hours. The Grizzly Creek burn scar will be especially
susceptible to flash flooding which will impact travel if this
occurs. Nocturnal showers and storms will continue overnight.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

Thunderstorms with the threat of heavy rainfall will continue to
be a threat to eastern Utah and Western Colorado on Thursday.
There is low probability that a few storms could produce large
hail over eastern Utah Thursday afternoon as well. Burn scars and
steep terrain will remain under the highest threat for flooding
and debris flows into Thursday evening. These threats decrease on
Friday as the storm lifts eastward. Daily showers and storms will
favor the higher elevations through the weekend with generally dry
conditions elsewhere.

.SPOTTER INFORMATION STATEMENT...

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

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