Overall, this was a very difficult case to forecast because of the changing precipitation
type. The NWS models had a problem with the timing of the precipitation forecasts but
the mesoscale models did help the forecasts as the event approached. The cloud cover
forecasts were not very good for this case, which was likely to have had a negative
impact on the air and road temperature forecasts.
10.2.5 Heavy Snow Case 15 March 2004
The 15 March 2004 storm was one for the record books in portions of Iowa. A swath of
snow from western to central Iowa dumped nearly 18 inches of snow at Sioux City in
western Iowa. In central Iowa, Des Moines got over 15 inches and Ames received 10
inches. For Des Moines this was the third-highest single-day snowfall in the city record.
The snow began at Ames around 13 UTC on 15 March and lasted through 0730 UTC on
16 March 2004. According to the METAR observations the snowfall was moderate to
heavy from the onset to about 21 UTC. There was a brief period around 21-22 UTC
when only light snow was indicated in the METAR observations, followed by more
moderate to heavy snow through 01 UTC. Additional light snow fell between 0100 and
0730 UTC, then the snow ended.
Synoptically, this event featured an elongated low pressure system that was positioned
over western Nebraska and eastern Colorado at 06 UTC on 15 March (see Fig. 10.51).
By 12 UTC, approximately 1 hour prior to the onset of snow in Ames, the low had
moved to south-central Nebraska and western Kansas. Snow began to form over Iowa
due to strong overrunning ahead of the low. By 18 UTC, the low was centered over
northeastern Kansas with a trough extending across eastern Iowa. Heavy snow was
falling at Ames during this period. The low continued to move slowly east-
southeastward into Missouri, and the trough slid southeastward and out of the state of
Iowa just after 06 UTC March 16. With the loss of dynamic forcing the snow pulled off
to the southeast and out of the Ames area by 08 UTC.
Fig. 10.52 shows radar images corresponding to the times of the surface maps in Fig.
10.51. It is seen here that the precipitation moves in from the west and then off to the
southeast. Areas in southwestern Iowa received very little snow as most of the
precipitation in that area fell as rain because of slightly higher temperatures. High
reflectivity (for snow) is seen over the Ames area in both the 18 UTC and 00 UTC image.
This is consistent with the moderate and heavy reports of snow at the METAR site at
these times.
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