Special Weather Statement, NWS Riverton, WY

Area Forecast Discussion
National Weather Service Riverton WY
431 PM MDT Thu Jul 29 2021

.SHORT TERM...This evening through Friday night
Issued AT 348 PM MDT Thu Jul 29 2021

Numerous thunderstorms are expected this evening and several of
these could be heavy rain producers due to abundant monsoonal
moisture. Precipitable water values are in excess of 1 inch for
much of the region, as a ridge over the Southern Plains directs
monsoonal moisture northward over the Mountain West. These PW
values are about twice (and in some places more than twice the
climatological normal). In addition to the threat for localized
heavy rainfall, a few strong to severe storms are possible mainly
east of the Divide in Johnson and Natrona county this evening,
where deep sheer combines with ample CAPE in excess of 500 to 700
J/kg. Storms may linger longer into the early overnight than
normal, given the plentiful moisture, but should decrease
significantly in coverage after midnight.

The overall pattern changes little for Friday and the primary
weather concern is again heavy rain producing thunderstorms that
could lead to localized flash flooding. The Green River Basin
(Sublette, Lincoln, and Sweetwater counties) look to be the area
where ingredients come together for widespread thunderstorms. CAPE
again looks to be in excess of 500 J/kg, allowing for decent
vertical motion once storms begin. Storms are also going to be
slow- moving, as a shortwave creeps its way northward on the
western side of an elongated ridge. This situation provides very
little horizontal motion throughout the atmospheric column. If
future model runs continue to show similar conditions, a Flash
Flood Watch for Friday afternoon and evening may be needed for
southwestern Wyoming. Temperatures during the day remain about 5
degrees below average for this time of year, partially due to
abundant cloud cover.

.LONG TERM...Saturday through Thursday
Issued AT 348 PM MDT Thu Jul 29 2021

Cluster analysis showing good agreement among ensemble members
regarding the synoptic pattern evolution in the medium-range. 500mb
ridge axis will be amplified just to our west, which may allow for
slightly drier air to work into the north Saturday in drier
northerly flow aloft. Meanwhile, monsoon moisture continues to push
north into southwest Wyoming. Therefore, widespread showers and
thunderstorms will persist Saturday along and west of the
Continental Divide with gradually decreasing chances moving toward
Johnson County and the northern Bighorn Basin. Wind profiles show a
lack of much support and steering winds will be light. This is
likely to limit thunderstorm development, but the plentiful moisture
will still enable heavy rain showers. Saturday will be cooler owing
to cloud cover and cooler 700mb temperatures ranging from 10C to 12C.

The drier air in northerly flow aloft allows for further drying east
of the Continental Divide Sunday. Surface high pressure slides into
the Northern Plains hindering convective development and shunting
the drier air back along the Divide. Little change in available
moisture west of the Divide, so scattered to even widespread
convection is likely to continue. Not surprisingly, the warmest
temperatures will be in the east where there will be less cloud
cover and convection, and mid-level temperatures warm slightly. The
flow aloft becomes more southwesterly Monday, which enables the
moist flow to persist. The best chance for convection remains west
of the Divide Monday afternoon and evening. However, better coverage
can be expected east of the Divide as the drier air slides east and
is replaced by deeper moisture from the west.

GFS is rather bullish on shortwave energy reaching northwest Wyoming
Tuesday afternoon, while the ECMWF is less excited. It would appear
Tuesday may be drier in the west, as the better moisture begins
to track eastward across the forecast area. Overall, the ridge
shows some deamplification Tuesday into Wednesday as the
aforementioned shortwave energy rides through the flow. This
generates a more zonal pattern Wednesday into Thursday with less
widespread convection and gradually warming temperatures.

&&

.AVIATION...For the 00Z TAFS through 00Z Saturday

Shortwave set to traverse the north half of the forecast area this
evening and provide the impetus for continuing convection across
this region. All terminals to remain VFR with the best chance for
IFR/MVFR visibilities in heavier rain/storms coming at KCOD and KWRL
through about 04Z. Wind gusts around 30 kts will be common with
stronger storms. Convective coverage decreases late this evening
with the loss of solar heating and as the focus shifts eastward.
Light showers will persist overnight in this moist monsoonal flow,
but no impact to prevailing VFR. Occasional mountain top obscurations will continue
through 06Z Friday, particularly over the northwest and northern
mountains. Focus for convection Friday afternoon will be
initially over the southwest and the Wind River Range/Basin.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued AT 348 PM MDT Thu Jul 29 2021

The primary fire weather concern this evening, Friday, and into
the weekend will be lightning associated with thunderstorms.
Monsoonal moisture in the region will cause thunderstorms to be
quite wet over the next several days. This may mitigate new fire
starts; however, heavy rain could also be a concern for crews in
drainages where flash flooding could occur. It may be wise to
watch radar closely over the next several days, and watch out for
drainages that are receiving 30 minutes or more of heavy rain
with slow moving storms. Winds will be light (save for in the
immediate vicinity of thunderstorms), and relative humidities
well above critical thresholds throughout the period. Conditions
begin to dry somewhat next week, after drier air pushes in from
the northeast.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

SHORT TERM...VandenBoogart
LONG TERM...Jones
AVIATION...Jones
FIRE WEATHER...VandenBoogart

Hazardous Weather Outlook
National Weather Service Riverton WY
324 AM MDT Thu Jul 29 2021

WYZ001>020-022>030-300930-
Yellowstone National Park-Absaroka Mountains-Cody Foothills-
North Big Horn Basin-Southwest Big Horn Basin-
Southeast Big Horn Basin-Owl Creek and Bridger Mountains-
Bighorn Mountains West-Bighorn Mountains Southeast-
Northeast Johnson County-Southeast Johnson County-
Teton and Gros Ventre Mountains-Jackson Hole-
Wind River Mountains West-Wind River Mountains East-
Upper Wind River Basin-Wind River Basin-Lander Foothills-
Green Mountains and Rattlesnake Range-
Natrona County Lower Elevations-Casper Mountain-Star Valley-
Salt River and Wyoming Ranges-Upper Green River Basin Foothills-
Upper Green River Basin-South Lincoln County-
Rock Springs and Green River-Flaming Gorge-East Sweetwater County-
324 AM MDT Thu Jul 29 2021

This Hazardous Weather Outlook is for Western and Central Wyoming.

.DAY ONE...Today and Tonight.

Scattered showers and thunderstorms into tonight. The main threat
will be heavy rain that could cause some flash flooding.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Friday through Monday...Showers and thunderstorms likely each
day. The main hazard will be locally very heavy rainfall which
could cause some flash flooding.

.SPOTTER INFORMATION STATEMENT...

Weather spotters are encouraged to report significant weather
conditions according to standard operating procedures.

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Special Weather Statement
National Weather Service Riverton WY
1010 PM MDT Thu Jul 29 2021

WYZ020-300445-
Natrona County Lower Elevations-
1010 PM MDT Thu Jul 29 2021

...SIGNIFICANT WEATHER ADVISORY FOR NORTHEASTERN NATRONA COUNTY UNTIL
1045 PM MDT...

At 1008 PM MDT, Doppler radar was tracking a strong thunderstorm
near Teapot Rock, or about 15 miles south of Midwest, moving
northeast at 20 mph.

Penny size hail and winds in excess of 30 mph will be possible with
this storm, along with torrential rainfall.

This storm will remain over mainly rural areas of northeastern
Natrona County.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

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

&&

LAT...LON 4311 10627 4319 10634 4349 10616 4349 10608
      4326 10607
TIME...MOT...LOC 0408Z 210DEG 23KT 4321 10624

21

Special Weather Statement
National Weather Service Riverton WY
1024 PM MDT Thu Jul 29 2021

WYZ020-300500-
Natrona County Lower Elevations-
1024 PM MDT Thu Jul 29 2021

...SIGNIFICANT WEATHER ADVISORY FOR WEST CENTRAL NATRONA COUNTY UNTIL
1100 PM MDT...

At 1022 PM MDT, Doppler radar was tracking a strong thunderstorm near
Hiland, or 35 miles east of Shoshoni, moving north at 10 mph.

Pea size hail and winds in excess of 30 mph will be possible with
this storm...along with torrential rainfall. Significant ponding of
water is expected. Please slow down.

This storm will remain over mainly rural areas of west central
Natrona County, including the following locations... Arminto...and
areas from about 5 miles east of Hiland, westward to the Fremont
County line.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

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

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 4302 10731 4304 10750 4327 10750 4320 10720
TIME...MOT...LOC 0422Z 190DEG 10KT 4311 10742

21