September 5 - Strong System Makes a Commanding Entrance

A large area of Low Pressure and its associated trough pattern impacted Wyoming on September 5th, especially Western Wyoming. Models used by NWS Forecasters were demonstrating the great impacts this system would have on the forecast area - which included heavy rains, strong thunderstorms, and much cooler temperatures. During the very early morning hours of September 5th, the strong area of Low Pressure was found over north central Nevada. Forecast models demonstrated this area of low pressure would move north northeast through Eastern Idaho, and then enter Northwestern Wyoming during the evening hours of September 5th. Ahead of the system’s direct arrival into the state, plenty of moisture and storm activity was pushed into Western Wyoming during the afternoon of September 5...of which some storm activity was very dangerous.

Severe Thunderstorms moved through Eastern Idaho during the afternoon hours and even produced a tornado as it approached the Western Wyoming border. This line of strong thunderstorms moved into Western Wyoming between 1 and 2 pm. The NWS office in Riverton issued its first Severe Thunderstorm Warning (SVR) of the day for North-Central Lincoln County at 131 pm MDT for threats of winds to 70 mph. A second SVR was issued at 137pm for Southwestern Lincoln County for hail reaching the size of a Quarter (coin), and also for destructive winds to 70 mph.  As the storms progressed, rotation was noted in the Doppler Radar images, and spotters around the Alpine, WY area began to report seeing Funnel Clouds. The radar’s hint at rotation, spotter reports, and knowledge that the storm had previously produced a tornado prompted the issuance of a Tornado Warning (TOR) at 142 pm for Northwestern Lincoln County. A second TOR was issued at 202pm for Southern Teton County as the storm made its slow, northeast progression through the state. Additional SVR statements were issued for Northern Lincoln, and Southern Teton County until around 300 pm.

Throughout these dangerous thunderstorms, very heavy rains were experienced. At 207pm, the NWS in Riverton issued a Flash Flood Warning (FFW) which covered the area of Hoback Junction, and Snake River Canyon. At 230 pm, reports came into the NWS office that 2 mudslides occurred in the Western end of Snake River Canyon. One impressive Mudslide was reported to be around 190 feet wide, and 5 feet deep. These slides closed US26/89 going through the canyon - which has since been reopened.

Heavy rains continued throughout the afternoon hours, with additional flooding statements issued over the areas including Sylvan Pass in Yellowstone National Park, Central Teton County, Togwotee Pass, and Northwestern Fremont County throughout the afternoon. The stronger storm activity had weakened by the early evening hours, with mainly heavy rain showers remaining after 8 pm. Besides for the report of the mudslide and funnel clouds, no direct severe weather was reported to the National Weather Service in Riverton. If you witnessed any Penny-sized hail, strong winds or wind damage, or flooding over any of the aforementioned areas, please contact the National Weather Service in Riverton at 1-800-211-1448. We appreciate all reports.

Besides for severe weather, a drastic temperature change was noted over the area. The table below summarizes High Temperature values from Tuesday September 4th and from Wednesday September 5th for select locations across Western and Central Wyoming.

High Temperature Comparisons

Location 9/4/2007 9/5/2007
Afton

82

72

Big Piney

81

63

Buffalo

95

75

Casper

91

80

Cody

91

73

Evanston

81

67

Greybull

98

80

Jackson

77

64

Lake Yellowstone

76

61

Lander

93

75

Pinedale

81

66

Riverton

93

75

Rock Springs

86

58

Sheridan

99

80

Worland

96

79




Rainfall Totals
Rainfall Map (High Res)

Rainfall Map (Low Res)

CountyGeneral Station NameRainfall
Big Horn Burlington 4 South 0.08
Big Horn Mill Creek 0.44
Big Horn Hyatt High 0.17
Big Horn Basin 5 Northwest 0.1
Big Horn Greybull Airport 0.08
Fremont Raspberry 0.36
Fremont Elkhorn 0.26
Fremont Anderson Ridge 0.08
Fremont Camp Creek 0.08
Fremont Wind River 0.05
Hot Springs Thermopolis 0.15
Johnson School House 0.11
Johnson Poker Creek 0.09
Johnson Barnum 5 Northwest 0.08
Johnson Kaycee 17 Northwest 0.04
Johnson Buffalo Airport 0.03
Lincoln Kemmerer 7 East 0.16
Lincoln Afton 12 South 0.15
Natrona Casper Mountain 0.02
Park Rattlesnake 0.47
Park Eagle 0.3
Park Cody 0.27
Park Cody 0.25
Park Cody Airport 0.18
Park Crandall 0.15
Park Meeteetse 0.13
Sublette Half Moon 0.38
Sublette Big Piney Airport 0.27
Sublette Hoback 0.15
Sublette Pinedale 13 Southeast 0.13
Sweetwater Rock Springs Airport 0.37
Sweetwater Red Desert 0.31
Sweetwater Rock Springs 0.18
Teton Moose 0.24
Teton Grand Teton 0.16
Teton Burro Hill 0.04
Washakie Ten Sleep 5 North 0.29
Washakie Worland Airport 0.06
Yellowstone Bechler 0.6
Yellowstone Quadrant 0.26



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