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Friday, May 6, 2016

December 2011 weather returns to normal

Monday, December 19, 2011

This December's mostly above-freezing daytime temperatures and limited snowfall are the norm for this time of year.

The much colder and snowier Decembers of the last few years have been the exception, according to several weather experts.

"Basically for most of the state the past four Decembers have all been unusually snowy and a bit on the cold side," said Harry Hillaker, Iowa state climatologist. "This year has been a walk in the park compared to what those other ones have been like."

Last year, for example, December in Le Mars averaged temperatures of 2.9 degrees colder than normal and a total of 8 1/2 inches of snow fell, Hillaker said.

In 2009 temperatures were 3.6 degrees colder than normal and 24.4 inches of snow fell during December, he said.

Hillaker noted much of that snow fell later in the month in 2009.

That's why the about 5 inches of snow so far this December and temperatures hovering around 30 to 40 degrees may feel a little out-of-the-ordinary.

"It seems a bit more extreme because recent Decembers have been much colder than usual," Hillaker said.

The average normal temperature for today (Dec. 19) is a high of 31 degrees and a low of 12 degrees, which is just below current weather forecasts.

Mike Gillispie, hydrologist, National Weather Service (NWS) Sioux Falls, noted that while temperatures are hovering slightly above normal this month, it's not abnormally warm.

"November ended up being about 3 degrees above normal on average across the area," he said. "It's a little above normal, but we're not sitting here 10, 20 degrees above normal."

He predicted the remainder of December will follow a similar weather pattern with temperatures in the 30s, 40s and potentially some 50s.

"It looks like the end of the month is going to be trending toward some even warmer temperatures than what we've seen in the last several days," Gillispie said last week.

Sunday's high temperature reached 55 degrees, according to the NWS Automated Weather Observation System at the Le Mars Municipal Airport.

Normal temperatures for the last half of December in Le Mars fall between 27 and 30 degrees for highs with lows around 10 degrees, Gillispie said.

Despite temperatures holding steady in the 30s and 40s as is normal, precipitation levels fall in the below normal category from September through November.

For example, in those three months Sioux City received .59 inch of liquid precipitation compared with the normal average 5.7 inches, said Jen Hacker, NWS meteorologist.

The dry autumn was a result of northwest Iowa and surrounding areas being caught between two air currents known as jet streams, Gillispie said.

"The dry fall basically had a real persistent kind of split-flow pattern with the southern jet stream being well south of us and the northern jet stream being north of us and we were kind of sitting there high and dry," Gillispie said.

He said the area is still in that pattern, which is why there haven't been very many weather systems passing through and the ones that have weren't very strong.

Looking at the big picture Gillispie said the area is expected to experience a weak to moderate La Niņa, a tropical pacific weather pattern, throughout the winter months.

Statistically that means wet and cold conditions across the Pacific Northwest, the northern Rocky Mountains and the western Dakotas with warm and dry conditions across the southern plains and southeastern United States, Gillispie said.

"So the area of Iowa, Nebraska, the eastern Dakotas and up into Minnesota is kind of in that little transition area between those areas where there's not a real strong signal," he said. "It can kind of go either way."

For example, last year northwest Iowa experienced a moderate to strong La Niņa and it was cold and wet, Gillispie said.

"This year we're sitting kind of a little on the warmer side and very dry," he said.

The long-range outlook is showing a similar picture of the cold and wet to the northwest of the area and the warm and dry to the southeast, Gillispie said.

"There is a little bit of an indication we might see some cooler temperatures and possibly a little above-normal precipitation getting closer to the area through January and February," he said.

But because the area looks to be in that transition zone, again the outcome remains uncertain at this time, Gillispie explained.

"It's one of those where we could still see quite a bit of snow this winter, but we could also end up getting 20 inches for the whole winter," he said.

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