Thunderstorms developed over the eastern Wind River Range on the evening of August 5th and became quite strong as they moved into the Lander Foothills. A COCORAHS observer reported penny size hail 7 miles west-northwest of Lander.
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The episode as NWS RIW wrote it up.
Thunderstorms developed over the eastern Wind River Range on the evening of August 5th and became quite strong as they moved into the Lander Foothills. A COCORAHS observer reported penny size hail 7 miles west-northwest of Lander.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Riverton, WY wrote its last area forecast discussion before the first event at Aug 5, 1:23 PM MST, 5 hours 22 minutes earlier.
Area Forecast Discussion National Weather Service Riverton WY 223 PM MDT Sat Aug 5 2017 .SHORT TERM...(This evening through Tuesday) Issued at 158 PM MDT Sat Aug 5 2017 Temperatures are running 10 to 15 degrees cooler today east of the divide than yesterday after last nights fropa. The weak vort lobe that brought an early round of thunderstorm activity to Northeast Johnson County is exiting across the Nebraska border with the next vort max positioned over Northeast Montana with a trailing weak shortwave that will swing through the CWA later this afternoon and evening. This shortwave will produce isold to scattered thunderstorms, mainly east of the divide, late this afternoon and evening. The Euro depicts this feature the best. SPC has a good chunk of our CWA east of the divide under marginal risk where the best shear is progged. Southwest Wyoming on the other hand, closer to the high center over the Southwestern United States will remain dry. So far, red flag conditions are not being met there. Currently dew points are in the 40s and 50s east of the divide and 20s and 30s west. No storms are expected west of the divide for the rest of the day today. Then a more prominent, stronger shortwave will be tracking southeast around the front side of the aforementioned high pressure center in the nw flow Sunday but will produce fewer late day thunderstorms east of the divide with less instability and moisture to work with. Areas west of the divide will remain precip free Sunday with increasing dew points and less wind. Conversely, on Monday and Tuesday, a weak cut off low and asct moisture will be streaming in from the west southwest, bisecting a continuous stream of weak disturbances in the NW flow within the northern stream. This will result in a better chance of late day thunderstorms west of the divide Monday, spreading into areas east of the divide Tuesday as the long wave ridge within the 4 wave pattern migrates closer to the center of the country. Temperatures will remain relatively cool for this time of the year resulting from nw flow and added precip/mstr. .LONG TERM...(Tuesday night through Saturday) Issued at 158 PM MDT Sat Aug 5 2017 West-northwest flow aloft will characterize the long-term portion of the forecast period. As a result, cooler air will filter south from time-to-time with temperatures hovering around 5-10F below normal through at least Friday. Convection will be isolated to scattered each afternoon and evening as sufficient mean layer moisture filters through the west-northwest flow off the Pacific. Model differences along with run-to-run differences illustrate the difficulty of trying to pinpoint precipitation lulls and peaks Wednesday through Friday. Add to this mix the uncertainty of embedded shortwaves in this flow pattern and we end up with a general broad brush of precipitation chances through the period. Temperatures should trend warmer and conditions perhaps a tad drier next Saturday. This will be in response to a digging trough in the Northern Plains/western Great Lakes with at least a weak ridge correspondingly building in the Great Basin. Lacking southwest flow aloft or a strong northerly push, winds through the period appear to be quiet by Wyoming standards. && .AVIATION...(For the 00Z TAFS through 00Z Sunday afternoon) Issued at 158 PM MDT Sat Aug 5 2017 All terminals to be VFR through Sunday with the possible exception of localized MVFR with the strongest convection late this afternoon and particularly this evening. Showers have remained across the far north into the early afternoon with visible imagery and radar showing building cells over the Absaroka Range. A little bit later show today with the best chance for convection coming between 22Z/Sat and 04Z/Sun. Showers/storms will be across mainly the north, impacting KCOD and KWRL, early in this period. Expect additional convection to then develop along the southern fringes of this area across Fremont County early this evening. It then looks as though convection will become more organized across southern Johnson and Natrona counties this evening. Favorable shear profile and higher dew points should provide additional impetus to strengthen a few of the storms. Overall, main hazards will be gusty wind to around 40kts along with small hail. KCPR should have the best chance of seeing larger hail and brief heavy rain leading to localized MVFR between 01Z- 04Z/Sun. Right-entrance region of a jet streak across northeast Wyoming could also aid in convective development across this area this evening. Convection will largely diminish or move ESE out of the forecast area by 08Z/Sun leaving a drier period through 18Z/Sun. Mountain top obscurations will be found around the higher mountains along and east of the Divide this evening and overnight. There will be enough lingering moisture to generate showers and isolated storms Sunday afternoon over these same areas, although stronger storms will be less likely. South and west of the front, it will be dry with gusty WNW wind of 15-25kts until around 02Z/Sun. These areas are likely to remain dry Sunday with WNW wind of 10-15kts. && .FIRE WEATHER... Issued AT 158 PM MDT Sat Aug 5 2017 ...ELEVATED FIRE DANGER IN SOUTHERN WYOMING THIS AFTERNOON... A cold front has dropped across areas East of the Divide and brought in cooler temperatures and higher relative humidity, decreasing fire danger for the rest of today. However, warm and breezy conditions persist in southern Wyoming. Critical fire weather conditions are occurring in zone 279 at the moment but fuels are not critical there. In Zone 277, conditions are borderline for critical fire weather at the moment but we will hold off on any warnings for now. There will be another round of showers and thunderstorms across areas East of the Divide for the rest of this afternoon and at night. Another round of late day thunderstorms is expected east of the divide Sunday. No precipitation is expected west of the divide Sunday, but the relative humidity is expected to increase along with lighter winds. Mixing and smoke dispersal will range from good in some of the northern Basins to excellent across Sweetwater County. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && SHORT TERM...Lipson LONG TERM...Jones AVIATION...Jones FIRE WEATHER...Lipson
Hazardous Weather Outlook National Weather Service Riverton WY 224 AM MDT Sat Aug 5 2017 WYZ001>020-022>030-061200- 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- 224 AM MDT Sat Aug 5 2017 This Hazardous Weather Outlook is for Western and Central Wyoming. .DAY ONE...Today and Tonight Elevated fire danger across southern Wyoming this afternoon. Isolated to widely scattered thunderstorms this afternoon and tonight, mainly along and east of the Continental Divide. A few strong thunderstorms possible east of the Divide this afternoon and evening with strong winds and hail. .DAYS TWO THROUGH SEVEN...Sunday through Friday Sunday...Isolated thunderstorms in Central Wyoming Monday through Friday...Isolated to locally scattered thunderstorms. .SPOTTER INFORMATION STATEMENT... Spotter are urged to report wind gusts over 45 mph and hail. Thank you.
The Riverton/Lander radar (KRIW), 39 km away, was scanning. The volume nearest the first event began at 01:43 UTC.
1 hail. Local standard time, as filed.
A COCORAHS observer reported penny size hail 7 miles west-northwest of Lander.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Wyoming in 2017Every state in 2017This day in weather historyWarnings in Wyoming, August 2017
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 708508.