Home > Storm Timing Causes Difference in April Precipitation Amounts for Worland

On April 30,  a Pacific storm system moved across the western United States and stalled over the northern High Plains. This storm system brought much needed precipitation to the Big Horn Basin and to the Big Horn Mountains.  However, the timing of this storm system created a unique situation in the monthly precipitation values for Worland.

In Worland, the National Weather Service collects data from two different sources. The first source is an automated surface observing system (ASOS) located at the Worland Airport. The second source is a cooperative observer (COOP) located in town. ASOS records the information based on the calendar day (midnight to midnight), while the COOP records the information from 7 AM to 7 AM. For April, ASOS recorded 1.49 inches of precipitation (177% of normal).  The COOP measured 0.23 inches of precipitation (27% of normal). Since the sensors record the data at different time intervals, the monthly precipitation figures for the two sites can be different if precipitation falls after 7 AM on the last day of the month. This was the case on April 30, 2003. 

The Worland Airport recorded 1.31 inches of precipitation on April 30; 1.12 inches was reported after 7 AM. The COOP site reported 0.16 inches of rain ending at 7 AM on April 30 and 1.21 inches of rain ending at 7 AM on May 1, 2003. Since the ending time of the report is 7 AM on May 1, the 1.21 inches of precipitation is counted in the May monthly values instead of April. This is why the two monthly precipitation values for Worland are significantly different in April and will again be different in May.

In addition to the monthly precipitation values, the radar data recorded from this storm was also unique.  The following is a radar image from the Riverton radar at 1 PM on April 30, 2003. At this same time, moderate rain was occurring in Worland. However, since the precipitation from low stratiform clouds was located below 6500 ft above ground level, the radar did not capture the precipitation well.  Also, the radar was unable to capture an accurate storm total precipitation amount, due to the bulk of the precipitation being so close to the ground. The picture on the right below is the storm total precipitation captured by the Riverton radar from April 29 - April 30.  (Click on the images to enlarge)

RIW Composite Reflectivity - Click on to Enlarge                          RIW Storm Total Precipitation - Click on to Enlarge

This situation is just one of the unique challenges that forecasters face everyday. If you would like more information on this situation, or anything that you see on our web page, please contact the Riverton National Weather Service. We are always glad to help!

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