Special Weather Statement, NWS Grand Junction, CO

Area Forecast Discussion
National Weather Service Grand Junction CO
225 PM MDT Thu Aug 19 2021

.SHORT TERM...(Tonight through Friday night)
Issued at 225 PM MDT Thu Aug 19 2021

Anomalous upper level low pressure centered over northern UT early
this afternoon will move northeast over WY tonight with its major
trough axis crossing eastern UT and western CO between midnight
and early Friday morning. The atmosphere ahead of the trough is
moderately unstable south of I-70...especially toward the Four
Corners where breaks in the clouds have led to greater surface
heating. That's where we expect the greatest potential for strong
to marginally severe thunderstorms through early evening. There
is plenty of bulk shear to organize and sustain storms, and the
unidirectional speed shear profile supports short bowing segment
convection. We've already seen that on display early this
afternoon, and received reports of pea to dime size hail. In
addition, higher than normal precipitable water values around 1
inch will lead to the threat for heavy rainfall and localized
flash flooding. A flash flood watch will remain in effect through
central portions of the county warning area, including recent burn
scars and their corresponding potential for damaging debris
flows.

Short term high resolution models have handled the convection so
far today pretty well and they show continued bands of
showers/storms rolling northeast over the area through mid
evening, before consolidating into one last band along the leading
edge of the upper level trough and moving southeast of the area by
Friday morning. Intensity of the showers/storms will wane after 9
pm with loss of daytime heating.

Friday will be drier as precipitable water values drop to one half
inch or less behind the trough. Temperatures will remain below
normal under northwest flow aloft, but warmer than today with the
abundant sunshine.

.LONG TERM...(Saturday through Thursday)
Issued at 225 PM MDT Thu Aug 19 2021

Another upper level trough moving onshore over the Pac NW Friday
night will move across the northern Rockies Saturday night. There
will be a modest increase in moisture late Saturday with
precipitable water values between one half and three quarters of
an inch. This will lead to isolated to scattered
showers/storms...especially across the north closer to the
dymamics of the passing upper level trough. A minor shortwave will
also come up from the southwest around a westward building upper
level ridge Monday into Tuesday and result in increased chances of
convection over the San Juan Mountains. Otherwise, model mean
ensembles are in good agreement with a dry foreacst and an upper
level ridge building over the region Tuesday through Thursday,
bringing dry weather and temperatures several degrees above normal
for the middle and end of next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1132 AM MDT Thu Aug 19 2021

An approaching closed low to the northwest and surface cold front
will play havoc for TAF sites today as widespread showers and
thunderstorms are expected for much of the day. Given the break
after this morning's showers, convection will become more
widespread this afternoon. Heavy rain will drop visibilities and
ceilings to MVFR conditions from time to time while also dropping
aerodromes below ILS breakpoints. Hail and gusty winds are also
expected as storms evolve. Conditions will start to improve after
the front moves through after 03Z tonight with VFR conditions
expected for Friday.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Flash Flood Watch until midnight MDT tonight for COZ003-006>011-
     013-017-020.

UT...Flash Flood Watch until midnight MDT tonight for UTZ025-027>029.

&&

SHORT TERM...BM
LONG TERM...BM
AVIATION...TGJT

Hazardous Weather Outlook
National Weather Service Grand Junction CO
455 AM MDT Thu Aug 19 2021

COZ001>014-017>023-UTZ022>025-027>029-201200-
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-
455 AM MDT Thu Aug 19 2021

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

.DAY ONE...Today and Tonight

Another round of storms and showers will occur today with hail,
heavy rain, and gusty winds all being a concern. Monsoonal
moisture will allow heavy rainfall rates over portions of eastern
Utah and western Colorado so a flash flood watch remains in
effect. Burn scars, especially the Grizzly Creek, will be
susceptible to flash flooding and debris flows from the heavier
rain. Most precipitation will dwindle overnight as the cold front
and upper level system responsible for the convection slides
eastward.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

Clearing conditions will kick in Friday and heading into the
weekend. Temperatures will start to increase day by day.
Widespread precipitation does not look to occur through Wednesday
though there is a slight chance for some showers along the Wyoming
border late Saturday night.

.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
110 PM MDT Thu Aug 19 2021

COZ021-UTZ022-028-191945-
Four Corners/Upper Dolores River CO-La Sal and Abajo Mountains UT-
Southeast Utah UT-
110 PM MDT Thu Aug 19 2021

...A strong thunderstorm will impact portions of west central Dolores
and east central San Juan Counties through 145 PM MDT...

At 110 PM MDT, Doppler radar was tracking a strong thunderstorm 11
miles east of Blanding, moving northeast at 40 mph.

HAZARD...Winds in excess of 40 mph and pea size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

This storm will remain over mainly rural areas of west central
Dolores and east central San Juan Counties.

This includes U.S. Highway 491 in Utah near mile marker 15.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 3748 10930 3763 10941 3784 10908 3762 10893
TIME...MOT...LOC 1910Z 238DEG 33KT 3761 10926

MAX HAIL SIZE...0.25 IN
MAX WIND GUST...40 MPH

KJS

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Special Weather Statement
National Weather Service Grand Junction CO
237 PM MDT Thu Aug 19 2021

COZ011-017-192100-
Uncompahgre Plateau/Dallas Divide CO-
Central Gunnison and Uncompahgre River Basin CO-
237 PM MDT Thu Aug 19 2021

...Strong thunderstorms will impact portions of northwestern Ouray
and eastern Montrose Counties through 300 PM MDT...

At 236 PM MDT, Doppler radar was tracking strong thunderstorms along
a line extending from 14 miles west of Olathe to 12 miles northeast
of Norwood. Movement was east at 45 mph.

HAZARD...Winds in excess of 40 mph and pea size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
Montrose and Olathe.

This includes the following highways...
 U.S. Highway 550 in Colorado between mile markers 118 and 130.
 U.S. Highway 50 in Colorado between mile markers 78 and 96.
 Colorado 90 between mile markers 82 and 89.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 3872 10788 3833 10778 3825 10821 3856 10829
TIME...MOT...LOC 2036Z 248DEG 38KT 3854 10824 3829 10818

MAX HAIL SIZE...0.25 IN
MAX WIND GUST...40 MPH

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