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

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  • 1 NOAA/NWS Forecast Office, Denver, Colorado
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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.

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.

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