Snowman weather not in forecast
Prediction for Christmas Day: Sunny with highs in lower 40s.
A white Christmas in Gothenburg?
Not likely, according to a meterologist from the National Weather Service in Hastings.
Unlike last year, when more storm systems brewed and spewed snow on Nebraska and other Plain States, meteorologist Merl Heinlein said colder air has not moved in and settled like it did during the winter of 2010.
“It was pretty harsh, in terms of low temperatures, the past two winters when it couldn’t seem to warm up,” Heinlein said.
Heinlein said the jet stream entrenched itself farther south and the cold didn’t leave.
In fact, on Christmas Day last year, approximately 50 percent of the United States was covered by snow.
What forecasters predict for temperatures and snowfall this winter (December, January and February) in south-central Nebraska is anyone’s guess.
During those months, Heinlein said there are equal chances of above, near or below normal temperatures and precipitation.
“Equal chances means it’s the best chance of having a typical season,” he said.
So far this fall and early winter, he said more typical weather has occurred.
That means high temperatures in the 30s that occasionally climb into the 40s and 50s.
But we all know the days will come when snow, sleet or even rain pummel the plains.
Trying to figure out what type of precipitation will occur in the winter is one of the most difficult tasks for a forecaster.
NWS science and operations officer Rick Ewald said an important piece of the puzzle involves determining the temperature throughout the troposphere.
The troposphere, he said, is basically the lower seven to eight miles of the atmosphere.
Although the temperature usually decreases with height, Ewald said sometimes it actually increases with height in the lower troposphere.
“This can cause problems for the forecaster,” he said.
How does temperature affect precipitation?
In general, Ewald said ice crystals grow in several ways, eventually forming snowflakes. If the entire column of the atmosphere remains below freezing to the ground, snow falls.
If snowflakes encounter a warm layer in the atmosphere that is above freezing, the snowflakes melt and turn to rain.
A warm layer, not quite as warm or as deep (a degree or two above freezing for 500 feet), means snowflakes will partially melt and refreeze as they meet a cold layer closer to the ground.
By the time they hit the ground, they look like tiny, frozen ice balls known as sleet.
The freezing rain process is similar to sleet formation, except that the warm layer melts snowflakes into rain drops.
But before reaching the ground, Ewald said the rain falls through another cold layer. If the air temperature in this layer, and at the ground, is several degrees below freezing then the rain drops will instantaneously freeze wherever they land. This creates a hazardous situation known as freezing rain.
Ewald said radiosondes, or weather balloons, collect temperature profiles of the atmosphere which forecasters use in their work.
But because of cost, he said they are normally launched only twice per day at NWS sites —in Nebraska from the North Platte and Omaha offices.