Vertical Motion Evaluation of a Colorado Snowstorm from a Synoptician's Perspective

Abstract

The causes of vertical motions associated with a September snowstorm which resulted in a heavy snowband in northeastern Colorado have been examined from a synoptician's perspective. The aim of the examination is to look at the various causes of vertical motion as a forecaster might in a real-time situation. The three ways in which the vertical motions were estimated were by 1) a ?conventional? approach, including 500 mb positive vorticity advection (PVA) and pattern recognition techniques; 2) a strictly quasi-geostrophic approach, including advection of vorticity by the thermal wind and a Q-vector analysis; and 3) a look at possible ageostrophic forcing in excess of, or operating an smaller scales than those expected from a purely quasi-geostrophic (QG) framework. Additionally, an examination of the atmosphere's potential response to forcing was attempted via a conditional symmetric instability analysis. The results show a failure of both the conventional approach and the purely QG forcing analysis in explaining the heavy snowband, although Q-vectors did much better than 500 mb PVA on the general area and timing of precipitation. Ageostrophic forcings operating on smaller scales than those resolved by QG analyses seem a likely reason for the vertical motions associated with the heavy snowband. The atmosphere was conditionally symmetrically unstable and thus likely to produce banded precipitation in response to forcing.