One of the more difficult forecasting challenges for Northwest Missouri is the arrival of heavy snowfalls, which can occur frequently during the cold season. These heavy snowfalls are events that occur in association with synoptic-scale transients. However, heavy snowfalls typically occur on time and space scales more consistent with mesoscale phenomena. Thus, our goal is to develop a 50-year snowfall climatology using a dense station observation network which covers northwest and central Missouri. This region encompasses the county warning area for which the National Weather Service Forecast Office (WSFO) in Pleasant Hill, MO, is responsible. The study was also done using station data that is available through the Missouri Climate Center. Heavy snowfall events have been categorized by snowfall amounts as follows: moderate (3 - 6 inches), heavy (6 - 10 inches), and extreme (greater than 10 inches). Our objectives are: 1) to compile a database which can be used by the WSFO in Pleasant Hill, 2) to examine the statistics, such as frequency of occurrence, and to look for long and short-term trends, 3) to examine interannual variability in snowfall occurrence and relate these results to phenomena such as the El Nino and Southern Oscillation, and 4) to examine the success and false-alarm rate of the forecasts generated by the WSFO in Pleasant Hill. We will also perform a companion study in order to examine the dynamic characteristics of the synoptic scale flow regime associated with each snowfall category.
Initial results demonstrate that northwest Missouri can expect approximately three heavy and/or extreme snowfall events during the winter season. While no significant trend in heavy snowfall occurrence has been noted, there has been a significant reduction in the number of extreme (and the sum of the heavy and extreme) events over the region. Also, fewer snowfall events were noted during El Nino years, while more heavy snowfall events tended to occur during years classified as "neutral". Finally, the heavy snowfall events primarily occurred in the northwest quadrant of an occluded cyclone that propagated from the southwestern US and then across southern Missouri. Deepening rates associated with each cyclone in the 24-h previous to the snowfall occurrence were generally modest (0.25 Bergerons), however there was a significant fraction of these cyclones that deepened rapidly (0.90 Bergerons).