Special Weather Statement, NWS Grand Junction, CO

Area Forecast Discussion
National Weather Service Grand Junction CO
611 PM MDT Sat Sep 18 2021

.SHORT TERM...(Tonight through Sunday night)
Issued at 320 PM MDT Sat Sep 18 2021

Light showers with embedded thunderstorms have developed as
advertised across eastern Utah this morning, spreading east into
western Colorado this early afternoon. As expected, gusty outflow
winds appear the primary concerns as the low levels remain very
dry despite increase in precipitable water levels. Winds have
gusted in the 40s across portions of northwest Colorado as these
virga showers have moved through, enhanced by the tighter gradient
winds ahead of an approaching Pacific Northwest trough. Critical
fire weather conditions were localized across northwest Colorado
and brief but despite the gusty winds, relative humidity is
already rising this afternoon across the region, so fire weather
concerns are lessening. The 12Z GJT morning sounding indicated a
strong inverted V profile as also seen in model soundings with
around 1000 J/kg of DCAPE, so these strong gusty outflow winds are
being realized. On the other hand, mixed layer and surface based
CAPE remains very low, so not expecting widespread thunderstorm
activity but rather isolated to scattered storms with some of the
showers. Hi res guidance shows the bulk of shower activity coming
in from the west late this afternoon and tracking overhead western
Colorado this evening with a leading shortwave ahead of the main
PacNW low. This is accompanied by a 90 kt jet streak so enough
forcing is available for sustained convective activity through
late this evening with some showers lingering into early Sunday
morning. Some of these storms could be strong with gusty winds to
40 kts or greater being the primary concern and little in the way
of rainfall.

The main upper level trough will approach the CWA Sunday morning
and move across the Intermountain West and northern Rockies Sunday
evening. Some storms may fire off the higher terrain Sunday
afternoon but anticipate coverage to be more isolated until Sunday
evening when the main trough moves across the north with the best
upper level support over northwest Colorado. Showers will increase
in northwest flow as cold air advection accompanies this trough,
as H7 temperatures drop to the 0C to -4C range for areas north of
I-70, especially northwest Colorado. This would favor the
northwest Colorado mountains with snow although latest model
trends indicate up to an inch at the higher peaks, though wouldn't
shock me if some higher peaks receive a little more. Below
freezing temperatures are possible in the Upper Yampa River Basin
Sunday night, which includes the Yampa and Steamboat Springs
areas, so issued a Freeze Watch for COZ005 for that potential
being a potential season ending freeze. Elsewhere, while
temperatures will remain cooler than normal, it looks like most
areas should remain above freezing. Freeze potential looks more
expansive and more likely Monday night into Tuesday morning as
skies clear out behind this trough. More details on this in the
Long Term section. Temperatures still remain above normal this
weekend ahead of the cold front Sunday evening.

.LONG TERM...(Monday through Saturday)
Issued at 320 PM MDT Sat Sep 18 2021

The well-advertised trough will have already moved through our
region by the start of the long term (Monday morning). This
leaves the forecast area under northwest flow Monday. Expect some
light, lingering precipitation during the day on Monday as this
energy works out of the region. A weak transitory ridge will move
in Tuesday and Wednesday as another wave of energy moves toward
the Pacific Northwest. Models are showing this trough diving south
on Wednesday night / Thursday...resting over the southwest
Thursday evening. According to the most recent runs, there is some
disagreement (given 100+ hours) in the track of this system. The
bigger uncertainty comes more from the variable strength of what
looks like a cutoff low forming over the southwest. At any rate,
forecast certainty really tanks after that system throws a wrench
starting mid next week.

Temperatures start out cooler than normal across much of the
forecast area on Monday.  In the wake of the aformentioned cold
front, highs will struggle to reach 5-10 degrees below normals
everywhere except south of the San Juans, where near normal temps
prevail.  Northern areas like Vernal and Craig could be as much as
15 degrees below normal highs on Monday.  As these (northern) areas
lose cloud cover Monday night and Tuesday morning, freezing
conditions become even more widespread than Monday morning.  Much of
Moffat, Routt, Eagle, and portions of Garfield and Rio Blanco
Counties stand the best chance to see low elevation freeze concerns
Tuesday morning. As high pressure approaches from the west,
temperatures warm slightly Tuesday (still remaining below normal).
With the high overhead Wednesday, we'll see highs near or just above
normal and remain there through the week.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 608 PM MDT Sat Sep 18 2021

Scattered showers with embedded thunderstorms will continue to
move quickly northeast across the area this evening. Gusts of 45
to 55 mph have been common on the leading edge of the activity
moving along I-70. This should weaken through the evening with
winds becoming less of a threat after sunset. Outside of
convection, VFR conditions will prevail through the period.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Freeze Watch from late Sunday night through Monday morning for
     COZ005.

UT...None.
&&

SHORT TERM...MDA
LONG TERM...SS
AVIATION...TGJT

Hazardous Weather Outlook
National Weather Service Grand Junction CO
146 AM MDT Sat Sep 18 2021

COZ001>014-017>023-UTZ022>025-027>029-191300-
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-
146 AM MDT Sat Sep 18 2021

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

.DAY ONE...Today and Tonight

Clouds and the chance for showers and thunderstorms will increase
from west to east today as a disturbance moves through the region.
Gusty winds will accompany this system though increased humidities
will limit fire weather concerns. Unsettled conditions will
continue overnight.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

Scattered showers and thunderstorms will continue on Sunday,
focused along the Continental Divide, and again on Monday with
activity shifting north of I-70. Light snow will be possible at
the highest northern and central mountain peaks. Gusty winds will
be the main threat with storms. A significant cool down is
expected Monday into Tuesday in the wake of a cold front before
temperatures rebound mid to late week as high pressure builds
overhead.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will not be needed.

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
528 PM MDT SAT SEP 18 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0504 PM     TSTM WND GST     4 NE CAMEO              39.20N 108.27W
09/18/2021  M55 MPH          MESA               CO   MESONET

            MESONET STATION CO118 070E051 BEAVER TAIL
            TUNNEL.

&&

Special Weather Statement
National Weather Service Grand Junction CO
529 PM MDT Sat Sep 18 2021

COZ003-007>010-013-190015-
Roan and Tavaputs Plateaus CO-Central Colorado River Basin CO-
Flat Tops CO-Grand and Battlement Mesas CO-
Debeque to Silt Corridor CO-
Gore and Elk Mountains/Central Mountain Valleys CO-
529 PM MDT Sat Sep 18 2021

...Strong thunderstorms will impact portions of north central Mesa
and southeastern Garfield Counties through 615 PM MDT...

At 529 PM MDT, Doppler radar was tracking strong thunderstorms along
a line extending from 15 miles northeast of De Beque to 6 miles
southeast of Parachute to near Collbran. Movement was northeast at 40
mph.

HAZARD...Wind gusts of 50 to 55 mph.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects.

Locations impacted include...
Rifle, New Castle, Silt, Parachute, Collbran, Battlement Mesa and
Plateau City.

This includes the following highways...
 Interstate 70 in Colorado between mile markers 74 and 111.
 Colorado 13 between mile markers 1 and 14.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 3983 10767 3974 10738 3940 10743 3915 10800
      3935 10801 3957 10813
TIME...MOT...LOC 2329Z 230DEG 34KT 3954 10809 3939 10796 3919 10798

MAX HAIL SIZE...0.00 IN
MAX WIND GUST...55 MPH

15

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
544 PM MDT SAT SEP 18 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0534 PM     TSTM WND GST     7 NNW CAMEO             39.24N 108.38W
09/18/2021  M51 MPH          MESA               CO   MESONET

            MESONET STATION PNRC2 PINE RIDGE, CAMEO 6NW.

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
602 PM MDT SAT SEP 18 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0553 PM     TSTM WND GST     2 E RIFLE               39.53N 107.73W
09/18/2021  M63 MPH          GARFIELD           CO   ASOS

            ASOS STATION KRIL GARFIELD COUNTY RGNL AP.

&&

Special Weather Statement
National Weather Service Grand Junction CO
616 PM MDT Sat Sep 18 2021

COZ007-008-010-013-190100-
Central Colorado River Basin CO-Flat Tops CO-
Debeque to Silt Corridor CO-
Gore and Elk Mountains/Central Mountain Valleys CO-
616 PM MDT Sat Sep 18 2021

...Strong thunderstorms will impact portions of northwestern Eagle,
eastern Garfield and east central Rio Blanco Counties through 700 PM
MDT...

At 616 PM MDT, Doppler radar was tracking strong thunderstorms along
a line extending from 15 miles southeast of Meeker to New Castle to 7
miles south of Silt. Movement was east at 40 mph.

HAZARD...Wind gusts up to 50 mph.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects.

Locations impacted include...
Glenwood Springs, New Castle, Shoshone, Cattle Creek and Dotsero.

This includes the following highways...
 Interstate 70 in Colorado between mile markers 103 and 134.
 Colorado 82 between mile markers 1 and 8.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 4012 10712 3976 10701 3946 10712 3941 10765
      3958 10755 3994 10769
TIME...MOT...LOC 0016Z 248DEG 37KT 3991 10767 3959 10755 3943 10764

MAX HAIL SIZE...0.00 IN
MAX WIND GUST...50 MPH

15

Special Weather Statement
National Weather Service Grand Junction CO
621 PM MDT Sat Sep 18 2021

COZ011-014-017-190100-
Upper Gunnison River Valley CO-Uncompahgre Plateau/Dallas Divide CO-
Central Gunnison and Uncompahgre River Basin CO-
621 PM MDT Sat Sep 18 2021

...Strong thunderstorms will impact portions of northwestern Ouray,
southwestern Delta and eastern Montrose Counties through 700 PM
MDT...

At 620 PM MDT, Doppler radar was tracking thunderstorms along a line
extending from 20 miles west of Delta to 13 miles southwest of
Olathe to 14 miles northeast of Norwood. Movement was northeast at
40 mph.

HAZARD...Wind gusts 40 to 50 mph.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects.

Locations impacted include...
Montrose, Delta, Orchard City, Olathe, Lazear, Cory, Eckert and
Austin.

This includes the following highways...
 U.S. Highway 550 in Colorado between mile markers 120 and 130.
 U.S. Highway 50 in Colorado between mile markers 60 and 108.
 Colorado 65 between mile markers 1 and 8.
 Colorado 90 between mile markers 82 and 89.
 Colorado 92 between mile markers 1 and 15, and between mile markers
27 and 29.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 3889 10798 3872 10764 3842 10762 3824 10813
      3849 10823 3867 10853
TIME...MOT...LOC 0020Z 244DEG 34KT 3865 10842 3851 10819 3829 10812

MAX HAIL SIZE...0.00 IN
MAX WIND GUST...50 MPH

15