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Late Afternoon Thunderstorms Create Outflow Boundaries That Result in More Thunderstorms Over Fremont County

• SUMMARY OF WEATHER EVENT

  On the afternoon of Saturday May 24, 2003 scattered thunderstorms developed over the higher terrain surrounding Fremont County.  The main areas of convection were to the northwest over Dubois and the southern end of the Absaroka Mountains; to the southwest over South Pass; to the southeast over Beaver Rim and the Green Mountains; and to the northeast over the Bridger Mountains.

  The general surface wind flow was from the southeast at 5 to 10 mph and the mid level wind flow was out of the west northwest at 10 to 15 mph.   With this pattern, the storms remained nearly stationary at first, and then began to move from the northwest to the southeast at 5 to 10 mph.

  As the thunderstorm from the northwest moved over Crowheart and approached Riverton near dinner time, an outflow boundary was evident on Doppler RADAR.  Outflow boundaries were also evident from the north, northeast, and the southwest.

• Composite Reflectivity RADAR image that shows four outflow boundaries in Fremont County.

  

• Visible Satellite Imagery showing the convection along the outflow boundaries.

  When the boundaries collided to the south and east of Riverton, additional thunderstorms developed and were very impressive, looking at the storms visibly from the National Weather Service office in Riverton and with the RADAR imagery. One hour lightning plots at 7 PM and 8 PM show where the main activity was from 6-7 PM and from 7-8 PM, respectively.

  With the storms to the south and east and clearing to the west, the sun managed to peek through and provide Riverton with a nice rainbow.

  

  The Storm Total Precipitation Estimate ending at 8:20 PM indicates several areas of heavy precipitation. The initial storms that formed along the boundary collision were located to the southeast of Riverton near Sand Draw and Highway 135. RADAR estimates for that area are in the 1.5 to 2 inch range. To the north of Lysite northeast of Riverton), RADAR estimates reached a maximum of 3 to 3.5 inches. Mr. Ralph Bradock, who lives 10 miles north of Lysite at the base of Lysite Mountain, reported a precipitation total of 1.32 inches and 2-3" of large pea size hail.

  If you have any comments or questions about this news story, please contact Brett McDonald, Science and Operations Officer, or another member of the NWS Riverton WY WFO staff at 307-857-3898.

  This page was last updated on 10/16/04.

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