We will get one more dry and mild day on Thursday before a cold low drops out of the northwest and brings some winter back into the region. After partly cloudy skies and a high in the upper 50s Thursday more clouds will bring us a 50-50 chance of rain on Friday, with a better chance of snow up in the mountains. Snow levels will drop to the valley floor Friday night and high temperatures will drop to the mid-40s by Saturday with a pretty good chance of snow showers. Some minor valley accumulations are possible, but the mountains could get a half a foot or more out of the deal by Sunday.

As many of you probably know, winter storm systems are often composed of cold fronts that come in from the west. But a reader wrote in with the following question: “I’ve been on this earth 67 years now and I would like to ask: has there ever been a cold front move from east to west instead of what we see now of moving west to east?”

Sure… the front that moved in last week isn’t a bad example. It dropped down from the Northeast, so at least had some east to west movement. The reason that cold fronts (at least at our latitude) generally move in from the west is the upper-level winds and jet streams tend to flow that way. The reason for that is kind of complex and outside the scope of the question, but it is related to the way the sun hits and heats the earth, along with the direction the earth is spinning. This west to east progression of global “waves” generally pushes stuff like fronts “downstream”, as it were. But cold fronts can move the other way, just like an eddy in a creek can move upstream for a bit. You need to have a cold low pressure center retrograde back toward the west. While this does occur occasionally, usually this kind of pattern only lasts a short period of time. The low may migrate back toward the west for a bit, but almost always gets caught back up in the flow and starts to move back towards the east.