EAGLE COUNTY, Colorado - Don't put the summer tires back on just yet. A change in the weather is on its way. But it may take some time.
The National Weather Service's office in Grand Junction predicts that a few minor storms may roll through northwest Colorado starting about the evening of Dec. 5. But the real chance of snow - and, crucially, cooler temperatures - won't come until the week of Dec. 10.
Travis Booth, a forecaster in the National Weather Service's Grand Junction office, said some "minor disturbances" will flow through the region later this week, but those storms will more closely resemble Monday morning's showers, with a bit of snow in the upper valley and rain in Gypsum.
"Farther out there's some promise," Booth said. "The models have been inconsistent, but they do agree there should be a push of cold air."
Meteorologist Joel Gratz of OpenSnow.com said the reasons for the change are complex - weather everywhere is interconnected, he said - but the most simple explanation is that the "jet stream," a flow of air in the upper atmosphere, is shifting to the south. Generally, when the jet stream is tracking to the north, areas to the south stay fairly dry and brown - like the late fall we've had so far. When the jet stream drops to the south, so do the storms. This time of year, that means at least some snow.
Booth said one of the reasons the jet stream has stayed north is that it has been influenced by a persistent low pressure system in the Gulf of Alaska. That low is expected to change to high pressure, which will push the storm track to the south, and much closer to Colorado. When that happens, temperatures drop, so what snow does fall is more likely to stick, at least for a while.
This big question, of course, is whether the storm track will bring feet of white, fluffy powder, a few inches of cement, or something in between? No one knows.
While weather forecasting has become pretty accurate in the short-term, long-term forecasting is still a matter of watching, and history.
"You're looking for trends and consistency between the models," Booth said. "In this case, the models have been consistent, and are trending in the same direction."
In this case, forecasters look for high pressure developing over the Pacific between roughly California and Hawaii. Big, persistent areas of high or low pressure can give forecasters a general idea of what's coming.
In this case, the high pressure should start delivering at least some snow to the Rockies, most likely before Christmas.
"That doesn't guarantee snow - nobody can do that," Gratz said. "But by the later part of the month, people should feel better."
And, of course, snow over the rest of the winter should allow people to feel better about making lemonade, flushing toilets or watering flowers when winter fades into spring and summer next year.
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