May 20 and 21 - Severe Weather Across Forecast Area

An unstable environment has been present across the Riverton forecast area for the past couple of days requiring the National Weather Service office in Riverton to issue 7 Severe Thunderstorm Warnings since 8pm MDT on Tuesday May 20th. Tuesday’s thunderstorms consisted mainly of strong winds, with several locations seeing winds in excess of 65 mph (remember, a Severe Thunderstorm Warning means that winds in excess of 58 mph are possible). A strong cold front moved into western Wyoming. Thunderstorms were first noted over the Upper Green River basin, impacting the cities of Big Piney and Pinedale before moving over the Wind River mountains. Strong downslope winds accompanied the movement of these strong thunderstorms, creating winds of 74 mph at the Riverton Regional Airport. Below are the Local Storm Reports (LSRs) from Tuesday May 20th:

 ..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....

0534 PM     NON-
TSTM WND GST ROCK SPRINGS            41.60N 109.22W
05/20/2008  M58 MPH          SWEETWATER         WY  

            SHOWERS IN THE AREA.
0830 PM    
TSTM WND GST     BIG PINEY               42.54N 110.12W
05/20/2008  M68 MPH          SUBLETTE           WY  

1012 PM     TSTM WND GST     LANDER                  42.83N 108.73W
05/20/2008  M63 MPH          FREMONT            WY  

1047 PM     TSTM WND GST     RIVERTON                43.04N 108.42W
05/20/2008  M74 MPH          FREMONT            WY  

These storms were ahead of the advancement of a strong cold front which moved into Western Wyoming during the overnight  hours of Tuesday. The atmosphere destabilized again during the afternoon hours today (May 21st), allowing numerous thunderstorms to develop across the area. Instability index values demonstrated a strongly primed environment for severe thunderstorms, especially across the eastern and north central counties of Johnson, Natrona, and Washakie counties. The first Severe Thunderstorm Warning issued by the Riverton forecast office was at 141pm, and was focused around the area of central Fremont county, impacting Lander and Fort Washakie. Reports came in of Nickel sized hail in Fort Washakie at 203pm.  The Storm Prediction Center in Norman OK issued a Severe Thunderstorm Watch for these counties at around 245 pm this afternoon. Several strong thunderstorms were noted after this time frame, with warnings issued for Fremont county, Natrona county, and Johnson county. Reports of nickel to quarter-sized hail were made to the National Weather Service office in the afternoon. Below are LSRs for storm activity on Wednesday:


..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....

0203 PM     HAIL             FORT WASHAKIE           43.01N 108.93W
05/21/2008  M0.88 INCH       FREMONT            WY   POST OFFICE

0250 PM     HAIL             PAVILLION               43.24N 108.69W
05/21/2008  M1.00 INCH       FREMONT            WY   


0310 PM     HAIL             KAYCEE                  43.71N 106.64W
05/21/2008  M0.88 INCH       JOHNSON            WY   PUBLIC

0345 PM     HAIL             6 NE EVANSVILLE         42.93N 106.18W
05/21/2008  M1.00 INCH       NATRONA            WY   TRAINED SPOTTER

Severe thunderstorm activity has tapered down this evening, though thunderstorms continue over the forecast area. 



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