After a long warm and dry period, the end of November began to show promise of major changes for Oklahoma. Medium-range forecast models (such as the MRF) predicted several major upper-level storm systems would cross the southern Plains in early December. As the first upper-level storm arrived on the 2nd, severe weather struck parts of central Oklahoma, including some rare December tornadoes. Two more strong upper-level storms were forecast after that, according to the short- and medium-range models. The second storm was responsible for a second, less-intense, outbreak of severe weather on the 3rd. It was apparent that the cold air being drawn southwest behind the departing storm would make the third storm a winter storm in northwest Oklahoma and the Panhandles.

On the morning of the 4th, an upper-level storm was centered over eastern New Mexico. At the surface, a strong, but stationary, front extended across eastern Oklahoma into northern Texas. Warm and humid air (by December standards) covered the eastern part of Oklahoma and Arkansas, leaving that area in a risk of severe thunderstorms. Meanwhile, on the cold side of the front, temperatures were in the 30s (°F). Snow was already falling across the central and western Panhandles.

In winter storms, the heaviest snowfall usually occurs just to the left of the track of an upper-level storm center. The center of this storm traveled from east central New Mexico the morning of the 4th to west-central Oklahoma the evening of the 4th. It continued its generally northeast movement and was centered over east-central Kansas the morning of the 5th. The snow dutifully fell just left (northwest) of this track.

Snow forecasting is a complex task, and methods vary somewhat from forecaster to forecaster. To differentiate rain from snow, some things that most forecasters look for include: 850 mb (about 5,000 feet altitude) temperatures a few degrees below freezing, 500 mb (about 18,000 feet altitude) temperatures at, or slightly colder than, -25 °C (-13 °F), and sufficient moisture and lift. All of these features were in place the morning of the 4th and these conditions shifted toward the northeast as the storm progressed in that direction. The large amounts of lift and moisture suggested that heavy snow was likely, and the Norman Forecast Office issued a Winter Storm Warning the morning of the 4th, for the far northwest part of the forecast area. A Snow Advisory took effect at the same time - for much of the rest of the northwest fourth of Oklahoma.

During the evening, rain in the northwest part of Oklahoma changed over to snow. The ground was still relatively warm after a month of above-normal temperatures, so most of the snow melted for the first hour or two. Snowfall became heavy in the evening and began to accumulate as the ground cooled under it. During the evening shift, the predicted snowfall was raised slightly, from "four to six" to "four to eight" inches in the warning area (Harper, Woods, Ellis, and Woodward Counties). The snow subsided overnight and morning totals in the Warning area were mostly six to eight inches. The only exceptions were at Arnett and Laverne, Oklahoma, where the cooperative observers reported totals of 12.1 and 10 inches respectively. Within the Advisory area, amounts were mostly in the two inches or less range.

In addition to the snowfall, the wind also presented some problems. Gusts between 30 mph and 40 mph in some areas caused the snow to blow and drift. Even Interstate 40 was closed for a while in the western part of the Texas Panhandle. Serious travel difficulties also affected northwest Oklahoma during the height of the storm.

As the clouds cleared on the 5th, visible satellite imagery clearly showed the area of snowfall. Rivers and lakes also stood out clearly within the snow field. This is typical in areas where the water bodies have not frozen.

Although the December 4 and 5, 1999, snowstorm was fairly strong, it was otherwise quite typical for an early-season, southern U.S. winter storm. This helped make it more predictable than some of the more subtle winter storms that occasionally result in unpleasant surprises. As the winter progresses, the job of forecasting winter storms usually becomes even more challenging as the ground cools and the risk of ice storms is added to the other winter storm effects.

Reports and Information - 12/05/1999