Page created and maintained by Roger Edwards, Storm Prediction Center
On December 3-5, 1999, an intense low pressure area moved northeastward out of the southern Rockies, causing severe thunderstorms and tornadoes in its warm sector from the southern plains across Arkansas, and locally deep snow over its cold sector. The cyclone left a continuous band of heavy snow for over 800 miles from eastern New Mexico to northern Illinois. Some of the heaviest snow in the belt from eastern New Mexico to Missouri fell from thunderstorms elevated over the cold air layer near the ground. The heavy snow also closed down Interstate 40 across the Texas Panhandle and Interstate 29 in northwest Missouri.
During the day on December 5, cloud cover moved out to reveal about 700 miles of the band between the upper Pecos valley of New Mexico and the Iowa-Missouri border. Many places in the band received from six inches to a foot of snow; see this roughly 300K GIF for some specific storm totals. Over southern Kansas, the heavy wet snow pulled tree limbs down across electric lines, causing numerous power outages.
[If you have a fast connection or several minutes to wait for a download on a modem, check out this Javascript visible satellite loop of the snow band as it erodes along the edges, from just after sunrise to near sunset.]
During the day on December 5, as the snow-free ground heated under the sun's rays, the snow fields stayed cold (~300K GIF). Why the temperature difference? Snow, being bright white, reflects most sunlight right back into space. Elsewhere, darker vegetation and soil absorbs the sun's heat, which then warms the layer of air near ground level. Also, in a process called sensible heat transfer, the snow itself directly absorbs heat from any warmer air which occurs just above it, cooling the air down toward the temperature of the top of the snow, and warming the snow. This is how the snow melted along the edge of the band, shrinking it throughout the day, while keeping the air above it relatively cold.
There are also areas of snow in eastern Colorado, where upslope flow helped deep snows accumulate as the storm system moved across New Mexico. Upslope flow is simply wind blowing from lower ground to higher ground. In upslope flow, the air must rise; the land forces it to go higher! Here, the northeast winds in eastern Colorado rose across the Great Plains into the high country, rich moisture condensed into clouds, which dumped the moisture on the ground as snow. This is a classic setup for heavy snows on the north side of the Palmer Divide, western Park Plateau, and the east- and northeast-facing slopes along the Front Range and Sangre de Cristos. Moist upslope flow is the most efficient precipitation machine known, and has caused most of the heaviest snow falls around the world. The same goes for this event, too; this storm buried areas around Cuchara, CO, in the Sangre de Cristos, in up to 61 inches of snow!
Numerous geologic features show up well in the snow fields. For the geography of the snow band, including locations mentioned here, see this large GIF image (about 300K).
