11:55 AM, Monday, December 19, 2016
Across Colorado and Wyoming the week should be mainly dry, not as cold, but windy at times in and near the eastern foothills. The next chance for a storm will come on Christmas weekend, at least for the mountains and West Slope areas, with big uncertainty regarding impact along the Front Range.
Breezy, cool front Wednesday
Milder temperature may be accompanied by occasional strong gusty winds in and near the foothills Monday afternoon and Tuesday. A cold front on Wednesday will not bring cold arctic air like the last two. Its main impact should be to disrupt the warming trend with slightly colder temperatures and temporarily spread gusty winds out onto the plains.
Recap of snow
I did not anticipate the persistence of a west-east band of heavy snow Friday evening. Given that it occurred over the Denver area, its impacts were certainly well observed. 7-11 inches of snow fell mainly in 6 hours on parts of Jefferson, Denver, and western Adams counties. DIA also set a record low when the temperature bottomed out at -15 just before midnight Saturday. Boulder bottomed out at -10 about the same time but did not set a record.
As with any weather phrase, “chinook” is sometimes attached to any wind in the winter. Although the Front Range is the national hot spot for chinook winds, a chinook describes a certain pattern. Strong west-to-east wind into and above the high peaks can be deflected by the mountains. Under certain conditions the airflow can get deflected upward at the mountain barrier and then downward over the east slope (similar to the flow of water in a creek when it encounters a row of boulders). If the downward deflection is strong enough the strong upper level winds can reach down to the surface in and near the foothills. So chinook winds come from above. That’s why you don’t necessarily see them observed at nearby stations before they hit. They can be very sporadic and variable in both space and time.