A SYNOPTIC CLIMATOLOGY OF SIGNIFICANT RAINFALL EVENTS OVER THE MID-MISSISSIPPI RIVER VALLEY


Stephen R. Considine*, James T. Moore*, and Fred H. Glass+


*Saint Louis University
Department of Earth and Atmospheric Science
3507 Laclede Ave.
St. Louis, MO 63103
Tel: (314)977-3126
Fax: (314)977-3117
e-mail: moore@eas.slu.edu

+National Weather Service Forecast Office
12 Research Park Drive
St. Charles, MO 63304
Tel: (314)447-1876
Fax: (314)447-1769
e-mail: Fred.Glass@noaa.gov


Significant rainfall events were gathered for the entire state of Missouri and the southern portion of Illinois (south of 41.2 deg N). Rainfall data from 1 January 1990 to 31 December 1996 were collected using 120 first order surface stations scattered throughout the region. An "event" is defined if a total of 2 inches or more of rain within 24 hours was reported for at least 4 stations within an area of approximately 100 sq. km. A total of 80 events, including warm and cold season cases, were classified for the seven-year period using this scheme. Surface, 850, 500, and 250 mb maps were obtained for the time of initialization for each heavy rainfall event. The 80 cases were then segregated into five categories based on common surface features. They consisted of the following synoptic-scale surface signatures: 1) a west-east frontal boundary with the heavy rain in the cool sector, 2) a mesoscale outflow boundary with the heavy rain in the cool sector, 3) a northeast-southwest frontal boundary with the heavy rain in the warm sector, 4) a northeast southwest frontal boundary with the heavy rain in the cool sector, and 5) heavy rain in the warm sector of an extratropical cyclone. A statistical analysis of the 80 case dataset was performed to determine the geographical distribution, initiation time, and seasonal tendencies for each synoptic grouping.

For each group, composite charts were constructed for upper air parameters to identify synoptic signatures associated with these events. First, all fields were computed on a 31 x 23 grid having a 190.5 km grid mesh covering the United States. Next, a 9 x 9 subgrid was centered around the grid point closest to the initiation point of the precipitation activity. The composited fields are geographically and temporally independent as both 00 and 12 UTC upper air fields were used and convective initiation points from each case were spatially distributed within the small grid. In the paper, we will discuss the composited fields which uniquely identify the synoptic patterns of thermodynamic and kinematic features for each group. We believe this synoptic climatology is an important first step in recognizing the potential threat of significant precipitation events.