Special Weather Statement, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
336 PM MDT Fri Sep 10 2021

.SYNOPSIS...The ridge of high pressure over central New Mexico
will continue to weaken as a low pressure trough and associated
cold front moves through northern Utah tonight and Saturday. These
features will cause a transition to zonal flow across the area
through next week. A ridge of high pressure may attempt to build
back into southern Utah by next weekend.

&&

.SHORT TERM (Through 12Z/6AM MDT Monday)...
A shearing mid-level trough axis is forecast to translate
northeastward across the Northern Rockies through Saturday. This
will drag an attendant cold front across northern Utah and SW
Wyoming late tonight into early Saturday morning. These features
will aid in the continued development of showers and thunderstorms
across portions of mainly high terrain areas of southern Utah,
central and northern Utah and SW Wyoming from this afternoon
through Saturday night. An analysis of the environment reveals
limited buoyancy and wind shear, although effective bulk shear is
expected to increase into the 25-30kt range late this afternoon
and early evening. PWAT values are expected to increase to near 1"
(or ~2x greater than normal for this time of year) from late this
afternoon through this evening. The main threat from any activity
from this afternoon through this evening will be gusty outflow
winds, owing to forecast and observed DCAPE values into the
1,000-1,500J/kg range, inverted-V soundings and unidirectional SW
flow through the boundary layer. High resolution convective-
allowing models (CAMs) generally show isolated to scattered
convection particularly across northern Utah, SW Wyoming and high
terrain areas of central and southern Utah. There is some
potential for a consolidated outflow boundary to develop across
the west deserts and track westward into the Wasatch Front this
evening, show effective/shear cold pool balance be achieved. High
LCLs around 10kft and relatively weak wind shear may make this
balance a difficult proposition to achieve. Regardless, any cell
will have the potential to produce winds into the 50-65 mph range.
The gusty wind threat will continue into this evening. Small hail
and brief heavy downpours cannot be ruled out, although the
primary hazard will be gusty outflow winds.

Showers and the occasional thunderstorm is expected to continue
across mainly northern Utah and SW Wyoming tonight into Saturday
evening. LCLs and DCAPE will be slightly lower in the post-
frontal environment on Saturday, but with the aid of daytime
heating on Saturday afternoon and evening, gusty outflow winds
will once again be a hazard with any shower and thunderstorm
activity as relatively dry low-levels will remain in place. HREF
wind probabilities indicate a 70% chance neighborhood probability
of wind gusts exceeding 30kts Saturday afternoon/early evening,
with a lack of 50kt contours indicated. This is in contrast to a
30% contour of 50kt winds indicated across northern Utah this
afternoon and evening.

Temperatures will cool from near-record values this afternoon to
near seasonal normals by Saturday and Sunday. There is some
potential for Saturday high temperatures to underachieve by a
couple of degrees depending upon how extensive the cloud canopy
remains.

Zonal mid to upper level flow is forecast to develop across the
region on Sunday. A trailing mid-level trough is now indicated in
the latest round of deterministic and ensemble runs -- and this
trough is expected to move into the PacNW Sunday afternoon. This
will maintain weak broad-scale lift, and may aid in the
development of an isolated shower or thunderstorm across mainly
high-terrain areas of northern Utah Sunday afternoon/evening.

.LONG TERM (After 12Z/6AM MDT Monday)...
The previously-mentioned upper-level trough is expected to slide
east through the Northern Rockies on Monday while gradually
gaining amplitude. As a result, models are now indicating an
attendant increase in potential shower and thunderstorm activity
across northern Utah and SW Wyoming Monday afternoon and evening.
Some of this activity may extend southward into central Utah as
well.

Zonal flow is expected to re-emerge by Tuesday and Wednesday of
next week with a return to dry conditions and seasonable
temperatures. There remains some disagreement from next Wednesday
and beyond regarding the pattern evolution across North America.
The biggest sources of uncertainty revolves around two potential
scenarios in this timeframe. One potential scenario would involve
the development of a trough across the Northern Rockies, and this
would favor cooler temperatures and potential precipitation across
northern Utah and SW Wyoming. An alternate scenario would favor
the re-development of a four-corners or southern Great Basin high
pressure ridge. This scenario would result in warming temperatures
and dry conditions for the region. Cluster analysis reveals that
both scenarios are very much in play, with just a slight nod in
favor of the ridge scenario.

&&

.AVIATION...
KSLC...VFR conditions are expected through the valid TAF
period with brief reductions to MVFR possible with passing
thunderstorms. Winds will generally remain out of the southeast with
gusts in excess of 26 kt possible through the evening and overnight
with passing showers and thunderstorms. Thunderstorm outflows could
bring brief periods of north and east winds. A lull in activity is
possible between 04-08Z, with showers redeveloping in the early
morning Saturday. There is a 20% chance winds will shift out of the
north overnight (between 05Z and 09Z) before returning to southeast.

REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions are expected
through the valid TAF period. Northern sites could see reductions to
MVFR or IFR for brief periods with passing thunderstorms producing
heavy rain or strong winds lofting dust. Less impactful isolated
convection is possible for the southern half of the state, with
storms largely tied to the terrain. Showers and thunderstorms will
continue into Saturday, where thunderstorm activity is expected to
pick up again late morning and continue throughout the day.

&&

.FIRE WEATHER...
A mid-level trough will drag a cold front into northern
Utah tonight, and the front is expected to hang across northern
Utah into Saturday night before weakening. These features will aid
in the development of isolated to locally scattered showers and
thunderstorms from high-terrain areas of southern and central Utah
as well as all of northern Utah from late this afternoon into
Saturday evening. This activity will have the potential to produce
gusty outflow winds, frequent lightning and locally heavy
rainfall, especially during the late afternoon and early evening
hours. Outside of thunderstorms, locally critically fire weather
conditions will develop across mainly the west deserts due to
gusty winds and low relative humidity. On Saturday, minimum
relative humidity values will increase to mainly above 15 percent
for most areas.

A trailing wave will promote isolated shower and thunderstorm
activity across mainly northern Utah Sunday afternoon and early
evening, as well as Monday afternoon and early evening. Slightly
more coverage is expected on Monday when compared to Sunday.
Heading into the middle of next week, dry zonal flow and
seasonable temperatures are forecast. By the end of next week, the
forecast is more uncertain, when one of two possible weather
outcomes include either dry and warming conditions with a building
ridge, or cooler and showery conditions across northern Utah with
a developing Northern Rockies trough.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...None.
WY...None.
&&

ADeSmet/Wessler

For more information from NOAA's National Weather Service visit...
http://weather.gov/saltlakecity

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php

Hazardous Weather Outlook
National Weather Service Salt Lake City UT
417 AM MDT Fri Sep 10 2021

UTZ101>131-WYZ021-111200-
Great Salt Lake Desert and Mountains-Tooele and Rush Valleys-
Eastern Box Elder County-Northern Wasatch Front-Salt Lake Valley-
Utah Valley-Cache Valley/Utah Portion-Wasatch Back-
Bear Lake and Bear River Valley-Wasatch Mountains I-80 North-
Wasatch Mountains South of I-80-Western Uinta Mountains-
Wasatch Plateau/Book Cliffs-Western Uinta Basin-
Western Millard and Juab Counties-Eastern Juab/Millard Counties-
Central Mountains-Sanpete Valley-Sevier Valley-Castle Country-
San Rafael Swell-Southwest Utah-Lower Washington County-
Zion National Park-Southern Mountains-Upper Sevier River Valleys-
Bryce Canyon Country-South Central Utah-
Capitol Reef National Park and Vicinity-Western Canyonlands-
Glen Canyon Recreation Area/Lake Powell-Southwest Wyoming-
417 AM MDT Fri Sep 10 2021

This Hazardous Weather Outlook is for the western two thirds of
Utah and southwest Wyoming.

.DAY ONE...Today and Tonight

Gusty winds from mainly high based showers and thunderstorms will
be the main hazard today. An increase in shower and thunderstorm
activity is forecast to develop tonight over northern Utah in
conjunction with a cold front.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

Isolated showers and thunderstorms will likely continue across
central and northern Utah and SW Wyoming through Saturday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed. Please relay any information
about observed severe weather to the NWS while following all
local, state, and CDC guidelines.

For more information from NOAA/s National Weather Service visit...
http://weather.gov/saltlakecity

For information on potential road travel impacts visit...
http://www.udottraffic.utah.gov/roadweatherforecast.aspx

Special Weather Statement
National Weather Service Salt Lake City UT
248 PM MDT Fri Sep 10 2021

WYZ021-102130-
Southwest Wyoming WY-
248 PM MDT Fri Sep 10 2021

...A strong thunderstorm will impact portions of eastern Uinta County
through 330 PM MDT...

At 248 PM MDT, Doppler radar was tracking a strong thunderstorm 7
miles east of Robertson, or 30 miles west of Manila, moving northeast
at 25 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...
Lyman, Robertson, Mountain View, Lonetree, Millburne, Church Buttes
and Fort Bridger.

This includes Interstate 80 in Wyoming between mile markers 36 and
56.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 4099 11017 4100 11020 4120 11045 4155 11035
      4158 11025 4158 11005 4111 11004
TIME...MOT...LOC 2048Z 220DEG 22KT 4115 11027

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

Webber

Special Weather Statement
National Weather Service Salt Lake City UT
356 PM MDT Fri Sep 10 2021

UTZ109-110-112-WYZ021-102245-
Bear Lake and Bear River Valley UT-Western Uinta Mountains UT-
Wasatch Mountains I-80 North UT-Southwest Wyoming WY-
356 PM MDT Fri Sep 10 2021

...A strong thunderstorm will impact portions of southeastern Rich...
east central Morgan...northwestern Summit and southwestern Uinta
Counties through 445 PM MDT...

At 356 PM MDT, Doppler radar was tracking a strong thunderstorm 11
miles northeast of Coalville, moving northeast at 30 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...
Evanston, Wahsatch, Emory, Castle Rock, Almy, Altamont and Piedmont.

This includes the following highways...
 Interstate 80 in Wyoming between mile markers 1 and 14.
 Interstate 80 in Utah between mile markers 171 and 196.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 4091 11119 4101 11140 4142 11110 4122 11060
      4103 11091
TIME...MOT...LOC 2156Z 225DEG 28KT 4102 11122

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

Webber

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