It is evident from this rather difficult case that cloud cover forecasts were, again, a key
parameter in order to get the air and road temperature predictions closer to reality. The
precipitation forecast for this light event was missed because the QPF was just too low.
This is an issue with very light snow events, which are inherently difficult for both
models and human forecasters to predict. RWFS wind speed forecasts look good during
the time of the blowing and drifting snow, but may have underestimated locally higher
wind speeds where drifting was prevalent (e.g. to the west of Ames). Reruns of the
blowing snow algorithm produced low-level alerts for the Ames area during this event,
primarily due to forecasts of relatively meager wind speeds.
10.2.4 Mixed Precipitation Case 20 February 2004
The February 20, 2004 case was a complex system to forecast because of the changing
phase of the precipitation during the event. The event started as rain around 06 UTC on
20 February. According to observers, the precipitation changed fairly quickly from rain
to sleet and then to snow at ~1130 UTC. It then snowed through 13 UTC, after which
time the precipitation ended. The NWS models predicted a fairly lengthy period of snow
to follow the rain in early forecasts prior to the start of the event. In later RWFS runs,
where the end of the event was within the FSL models' 15-hr run time, their inclusion
resulted in a more accurate prediction of event end time.
The synoptic setup at 00 UTC on 20 February featured a low pressure system over the
northeastern section of Kansas with an inverted trough along the western portion of Iowa
(Fig. 10.36). The low moved toward the east-northeast with time and the trough was near
Ames by 06 UTC. The low continued to move in this fashion and a fairly tight pressure
gradient formed on its west side, causing increasing winds over the state of Iowa. By 12
UTC the low was in the southeastern portion of Iowa and the synoptic-scale lift quickly
pulled out of the demonstration domain soon thereafter.
NEXRAD radar images (Fig. 10.37) indicate that showery precipitation initially moved
into Ames area from the southwest. Rain was observed at the Ames METAR site just
after 06 UTC. Light precipitation dominated the event, but a few heavier bands were
present over the area as well. After the precipitation turned to snow at ~11 UTC, the
reflectivity decreased quickly and shifted off to the southeast.