Overall the weather and road conditions were forecast well for this event. The forecast
temperatures (air, dew point, and road) as well as wind speeds were all quite close to the
observations during the most critical time to the maintenance personnel; during the
snowfall. The precipitation start time 9 hours before the event was nearly perfect when
FSL models were included. However, the forecast of the end of the event was about 10
hours too early. The total snowfall depth forecast was about half the estimated actual
snow depth, but this is primarily attributable to an underestimated snow to liquid ratio for
such a cold event. A key lesson learned from this case is that the snow to liquid ratio
should vary with surface temperatures, at the very least, increasing to 20:1 during very
cold events and decreasing to about 6:1 during very warm events.
10.2.3 Blowing Snow Case 9-11 February 2004
At about 03 UTC on 10 February 2004 there was a short period of very light snowfall
that dusted the demonstration area with around 0.2 inches of snow. The next day strong
westerly winds moved into the area and created dangerous blowing snow conditions
across parts of the domain for much of the day. Several cars ended up in the ditch and
the Iowa DOT had to treat and plow roads because of all the blowing and drifting snow.
According to lab personnel in Ames that week, there were portions of US-69 that had
over 10 inches of drifting snow on the road late in the afternoon and the plow truck could
not keep up with the drifts. Conditions on US-65 also deteriorated, with some locations
having 2 inches of snow on the roadway by early afternoon, however I-35 stayed in fairly
good shape and only needed one spot treatment during this event.
A cold front associated with a low pressure system centered near the Great Lakes pushed
through central Iowa overnight on 9 - 10 February (Fig. 10.26). Along with the passage
of the cold front came a few snow flurries between 03 and 04 UTC on 10 February (Fig.
10.27). After the first low pressure system pushed off to the east another low began
influencing the area with a tight pressure gradient as it moved east-southeast from North
Dakota through Minnesota. This pressure gradient caused westerly winds to pick up in
the Ames area. Significant drifting snow developed in the late morning and lasted
through much of the day on 11 February. The light snow that fell the previous night was
believed to be just enough to really be blown around and create extensive drifts in some