Home > Storm Timing Causes
Difference in April Precipitation
On April 30, a Pacific storm system
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
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)
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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