Severe Thunderstorm Warning 87, NWS Riverton, WY

Area Forecast Discussion
National Weather Service Riverton WY
222 PM MDT Wed Aug 21 2019

.SHORT TERM...(This evening through Thursday)
Issued at 220 PM MDT Wed Aug 21 2019

Today the west and southwest continue to be hot and dry with RH
values in the teens to even a few single digits. Highlights are
not out for today as winds will still be too light. The bigger
story today is instead the threat for strong to possibly severe
storms across central and northern WY. RH and dewpoints are both
higher on the eastern side of the Divide mainly thanks to the
boundary that previously moved into those areas yesterday. Skies
were clear this morning and there is plenty of time for heating
and instability to grow. With this set up of moister air and
growing instability the weak shortwave moving into the area fired
off a few storms even sooner than previously anticipated. Storms
have been mainly sticking to the mountains and have decreased in
strength when moving into lower elevations. Based on mesoscale
analysis, NAM was indeed a bit too aggressive with CAPEs and GFS
has done a better job with predicting 2000 J/kg. Both the NAM and
HRRR have showers and storms lingering in eastern areas the
longest with all activity ending by 06Z. The main threat from any
storm will be strong winds and some smaller hail, but even weaker
showers could produce strong gusts when they collapse and die.

Thursday morning, the upper level trough axis should be over Idaho.
This trough is bringing a cold front, increased moisture and
precipitation chances, and also will tighten the pressure
gradient. As just the tail end of the system will impact WY, the
northern third of the state has the greatest chance of showers and
thunderstorms. Southwestern WY will remain dry. Breezy winds will
develop across the state as the pressure gradient tightens
through the day with the approach of the system. Wind gusts will
be strongest across southern portions; gusts 30 mph to 35 mph are
currently expected. With the moisture from this system confined to
the north and a mostly west flow prevailing in the south, RH
values will once again be in the teens to single digits across the
south. The low RH and gusty winds will combine to create critical
fire weather during the afternoon and evening hours.

.LONG TERM...(Thursday night through Wednesday)
Issued at 202 PM MDT Wed Aug 21 2019

On Thursday night, the strong shortwave trough axis will track
across the CWA. The models are still indicating -7 lifted indices
over Johnson County by 00Z Friday along with elevated capes.
Strong thunderstorms are still possible well into the evening in
Johnson County Thursday evening. The NAM is the fastest bringing
the trough axis through Thursday night with the other models
bringing it through Friday, along with a mid and upper level low
trying to cut off over Northern Wyoming Friday, but should be
inconsequential with lack of moisture. High temperatures will be
several degrees cooler Friday east of the divide behind the trough
axis and associated weak cool front due to come through Thursday
night. Winds with this strong shortwave will likely extend the
critical fire weather conditions into Friday along with the low RH
behind the trough axis.

By Saturday, we will be in a well established zonal flow as a fast
west northwest flow establishes over the Northwest quarter of the US
including Wyoming. A fast moving shortwave within the flow could be
enough to kick of some thunderstorm activity in our northeast zones
Saturday evening and to a lesser extent late Sunday. Otherwise, the
winds will be the most prominent feature with this next shortwave as
winds surface under the right front quad of the associated jet max.
These winds will usher in drier air so Sunday is shaping up to be
another potentially critical fire weather day. Temperatures will
increase Sunday as well, with highs in the 90s east and 80s west.
Similar conditions are expected Monday with continued breezy
conditions as another ripple in the flow keeps the gradient rather
tight, but with slightly higher RH and decreased temperatures.

The models are struggling with the timing and placement of the next
shortwave feature for Tue/Wed. The GFS keeps the jet well to our
northeast and without a shortwave feature to speak of for our area.
The Euro tracks this next shortwave through us Tuesday and the
Canadian Wednesday. With the uncertainty of the models, kept very
low pops in the north and northeast for Tuesday and Tuesday night.
The steering winds will be from the north northwest behind this
trough coincident with an associated cold front,  so at some point
during the middle of next week, we should see a trend of noticeably
cooler weather as 700mb temps begin to drop to 6C.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 202 PM MDT Wed Aug 21 2019

All is quiet currently and light winds and VFR conditions are
widespread, but showers and thunderstorms will begin to develop
east of the Divide between 18Z and 19Z, clouds are already
starting to build. Thunderstorms will develop around the
Absaroka/Wind River mountains, and shift eastward through the
afternoon and evening. Terminals most likely to be impacted are
KCOD, KLND, KRIW, KWRL and KCPR; VCTS is forecasted for those
terminals. Gusty to strong winds will be the main threat from
storms and brief MVFR/IFR conditions are possible with any
convection. Activity should be over by 06Z/07Z. Thursday morning
will be quiet with light winds, but gusts will develop in the
afternoon.

&&

.FIRE WEATHER...
Issued AT 202 PM MDT Wed Aug 21 2019

...Multiple Fire Weather Watches for Thursday afternoon across
southwest Wyoming...

Conditions today will be similar to Tuesday with isolated to
scattered thunderstorms returning mainly over the mountains during
the remainder of this afternoon. Some storms may be strong with
gusty winds being the main threat. West of the Divide will remain
hot and dry this afternoon, with poor RH recovery tonight. Winds
will be stronger today than Tuesday but not to the point of causing
fire concerns. Thursday will see a cold front push across Wyoming
east of the Divide. This will bring more afternoon showers and
thunderstorms east of the Divide, mainly in zones 281 and 285, and
increase winds in the still very dry areas west of the Divide. A
Red Flag Warning has been issued for many fire zones along and
west of the Divide for Thursday afternoon and evening. These
critical fire weather conditions will extend into Friday across
the southern zones.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from noon Thursday to 8 PM MDT Friday for
WYZ277>279.

Red Flag Warning from noon to 8 PM MDT Thursday for WYZ288-289.

&&

SHORT TERM...Bourque
LONG TERM...Lipson
AVIATION...Bourque
FIRE WEATHER...Lipson

Hazardous Weather Outlook
National Weather Service Riverton WY
337 PM MDT Wed Aug 21 2019

WYZ001>020-022>030-221200-
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-
337 PM MDT Wed Aug 21 2019

...RED FLAG WARNING for Southwest Wyoming West of the Divide Valid
from 12 Noon Thursday through 8 PM Friday...

...RED FLAG WARNING for Portions of Central Wyoming East of the
Divide Valid from 12 Noon Thursday through 8 PM Thursday...

This Hazardous Weather Outlook is for Western and Central
Wyoming.

.DAY ONE...This Afternoon and Tonight.

Isolated to widely scattered thunderstorms are possible this
afternoon in north and central Wyoming. Some storms could become
strong with the main threat being gusty winds.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Thursday...Isolated to widely scattered thunderstorms in the
north. Strong gusty winds and low relative humidities in southwest
Wyoming as well as across portions of Central Wyoming east of the
Divide is expected. A Red Flag Warning has been issued for much
of the areas west of the Continental Divide, as well as portions
of Central Wyoming east of the Divide.

Friday...Strong gusty winds and low relative humidities in southwest
Wyoming. A Red Flag Warning has been issued for much of the areas
west of the Continental Divide.

.SPOTTER INFORMATION STATEMENT...

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

TEAM RIVERTON

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