"High Plains Snow"
13-14 March 1996

This case involves a high plains snow event from southeast Wyoming to northeast Colorado. It is a typical example of a Front Range upslope snow event with a southeast Colorado low, upper-level support and cold air support. These setups are generally best for the Denver area and north; with areas to the south, such as Colorado Springs, receiving lighter amounts.

Synoptic Overview

Upper Levels

By 1800Z on 13 March, a closed low had settled over the Great Basin. It would remain in the area for at least the next 30 hours. These closed lows often become vertically "stacked" due to their slow movement, as there is little vorticity and temperature advection associated with them.

At 300mb, by 0000Z, the generation of vorticity by the jet max put the front range of Colorado and Wyoming in an area of weak PVA with diffluent flow in western Colorado. This would intensify by 0600Z, with stronger PVA and diffluence, and would continue through 1500Z. Divergence in mid-levels was strong at times also, as seen at 500mb at 0600Z 14 March.

Surface

At the surface, as happens often in these situations, cyclogenesis was occurring in the lee of the Rockies. High pressure builds in behind the frontal passage. Here you can see this at 0000Z 14 March. As the low strengthens and the high sinks south, a pressure gradient forms along the Front Range, causing upslope flow, which would be flow anywhere from north to east. Alone, upslope flow will produce only low cloudiness and light precipitation, but with support aloft, can produce significant weather. Wind direction, speed and depth of upslope are determining factors of strength, and are different with each event. At 0900Z 14 March, flow was NE at 20kts along the Front Range at 700mb. During this storm, upslope flow reached as high as 300mb.

Some snow totals:

Downtown Denver: 4"
Metro Denver area: 5-10"
Cheyenne, WY Municipal Airport: 4.5"
Colorado Springs: 3.5"
Front Range High Foothills: 10-15"

Case Study 07