P4.6 UPPER AIR CONSTANT PRESSURE COMPOSITES OF MIDWESTERN THUNDERSNOW EVENTS Patrick S. Market *,1 , Angela M. Oravetz 1 , David Gaede 2 , Evan Bookbinder 2 , Rebecca Ebert 1 , and Christopher Melick 1 1 Department of Atmospheric Science University of Missouri-Columbia Columbia, MO 2 National Weather Service Forecast Office Springfield, MO 1. INTRODUCTION Thundersnow events from 1961 through 1990 were examined. Events chosen for further scrutiny occurred in the Midwest on synoptic hours and featured thunder and snow only, a process that led to 97 cases of thundersnow. The thundersnow events were classified into categories based upon the setting in which each event occurred; examples include: with a cyclone, with a front, in a lake effect situation, etc. Events that occurred with a cyclone (N=80) were the focus of this study, and further classified based upon where the thundersnow occurred in relation to the cyclone (northwest of it, northeast, etc.) in keeping with previous work (Market et al. 2002). These event classes were then subject to a composite process, which is described below. 2. METHOD Mean fields of standard atmospheric variables at several levels (900 mb, 850 mb, 700 mb, 500 mb, and 300 mb) were generated with the compositing software of Moore et al. (2003) for the time of initiation as well as 12, 24, 36, and 48 hour prior to thundersnow initiation, hereafter referred to as T-00, T-12, T-24, T-36 and T-48, respectively. This was done in hopes of determining which dynamic and thermodynamic features are present, not only when the thundersnow is occurring, but also before the event; the intention was to produce more accurate forecasts of thundersnow in the future. All composites were centered on the location of thundersnow initiation. Given the relatively small sample size, the maximum number of events was sought; thundersnow reports from hours 0800 Universal Time Coordinated (UTC) to 1900 UTC were evaluated with 1200 UTC rawinsonde data while thundersnow reports for the 2000 UTC to 0700 UTC period were represented by 0000 UTC balloon data. ______________ Corresponding author: Patrick S. Market, University of Missouri-Columbia, 387 McReynolds Hall, Columbia, MO 65211. E-mail: market@bergeron.snr.missouri.edu 3. COMPOSITE RESULTS Such convective snow events associated with cyclones occur in a dynamic environment, with ample moisture and lift. The composite thundersnow event occurring northwest of a cyclone center (N=33) tends to occur with deeper, better-developed, negatively-tilted cyclones as a result of upright convection. Conversely, the composite thundersnow event occurring to the northeast of a cyclone (N=19) tends to be associated with weaker, less-developed, neutrally-tilted systems and more likely the result of slantwise motions. A majority of the thundersnow events occurred to the northwest of the cyclone in question. The composites depicted distinct differences between events northwest of the cyclone center (NWC) and those northeast of the cyclone center (NEC). The rationale for this approach is based on the work of Market et al. (2002), and their scatter diagram suggesting that these two areas are preferred for thundersnow development in a given cyclone (see their Fig. 6). Although thundersnow to the north of a cyclone was composited, further investigation into this group was not pursued for this project. Finally, we note that these composites are intended as much for pattern recognition as they are for dynamic understanding. Obtained from averaged grids, the fields shown herein lack many of the important features present in each individual case. However, those regions where thundersnow exists are, on average, ones of copious moisture and robust forcing for ascent. 3.1 T-00 - Nor hwest of cyclone t With these cases, thundersnow is in progress to the northwest of a surface cyclone, and we begin our examination close to the surface. We note, too, that discussions of parameter values are valid at the thundersnow initiation site unless otherwise noted. At 900 mb, heights over the thundersnow initiation site are at their lowest (Fig. 1a), at 910 gpm. The mean heights fall at 850 mb (not shown) to their lowest levels over the course of the previous 48 hours as well, to 1368