Cold, rain & drizzle; then first snow?

6AM, Thursday, September 11, 2014

A very shallow surge of cool air moved through eastern Colorado before sunrise and will likely be strong enough to keep clouds and patchy drizzle in the area today.  Later in the day there should be a colder push of air that deepens the upslope flow into the Front Range.  Moderate intensity snow is falling across northern Wyoming behind that second push.  We should see an increasing coverage of drizzle and some areas of rain by late afternoon.

Snow? The airmass is cold enough to support snow. The question is whether we will have only shallow stratus clouds that tend to produce drizzle and/or freezing drizzle even in cold air. If the clouds develop more vertically then snow is more likely because heavier precipitation helps to cool the atmosphere and ice crystals in tall clouds support snow. Both the current observations and model guidance suggest some east-west bands of deep clouds are likely this evening that will result in rain initially and snow later. In the heavier bands that changeover might occur before midnight. In other areas it could be later in the night, if at all. Although many areas will likely see some snow flakes, accumulations below 6000 feet are likely to range from zero in some areas to about an inch (grassy surfaces) where some of those bands form. Higher elevations may see 1-3 inches.

Temperatures. Friday morning is likely to see temperatures below freezing in the high elevations and around or just above freezing in the lower foothills and plains. After a slow clearing process on Friday, mostly clear skies Friday night should allow more variable low temperatures by Saturday morning in the Denver-Boulder-Fort Collins corridor ranging from the low 30s to the upper 30s. Be prepared for frost.

After the chilly start to Saturday, the weekend is likely to be mainly dry and warmer.

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