Banded Snowfall

This winter season has featured little in the way of widespread snowfall resulting in the snowfall deficit we face currently.  Instead of a more widespread and more predictable type of snow pattern we have had numerous instances of snow bands producing moderate and heavy snow along the Front Range resulting in widely variable snowfall amounts in short distances.  This can be attributed to the disorganized nature and relative weakness of the storm systems that have moved through.  Banding of heavier precipitation is common and occurs with almost every mid-latitude storm system, but can be caused in a number of different ways.  To better understand the difficulty in predicting the intensity and location of snow bands I thought it would be good to look at a few different scenarios that cause banded snowfall here along the Front Range.

The first case is the northwest flow pattern..  In this case the upper level jet stream is oriented in a NW to SE direction over the state.  Typically northwest flow results in generally dry conditions or light snowfall with majority of the snow remaining in the northwest mountains.  Here’s the general weather pattern:

The jet stream sags over the state with the continental divide separating cold Canadian air to the east from relatively warmer Pacific air to the west.  This is marked by a cold front that backs up against the Front Range foothills, typically with an east/northeast wind.  This front supplies low level moisture that can result in low level clouds, fog, freezing mist/drizzle or light snow.  Unfortunately the “upslope flow” is shallow due to the northwest winds aloft drying the air as it moves “downslope” from the high mountains.  This results in a dry layer of air above the saturated layer or cloudy laden air mass near the surface which limits the vertical development of the clouds frequently resulting in just light snowfall or wintry mix type conditions versus heavy snow.  However, with the jet stream aloft the additional energy it provides at upper levels can support snowfall east of the continental divide due evacuation of air aloft forcing the air upwards at the surface like a vacuum stronger than the “downslope” effect from the mountains.  These areas where the upper level dynamics overrule can generate more intense areas of precipitation which falls in banded form due to the winds aloft.  Below is a picture trying to describe what we have just talked about here:

 

In the northwest flow pattern case the orientation (NW->SE) can be easily predicted by the upper level winds, but the intensity and location of these snow bands cannot!  Once a band of heavier precipitation sets up it may move very little geographically due to little changes in upper level conditions, but radar echoes or individual storm cells within the band itself may move very quickly!

The southwest flow pattern can result in very similar in conditions at the surface compared to the northwest flow pattern, but in the upper levels there are significant differences.  First of all the cold air is usually supplied by a cold front that has moved well to the south with cold air hanging on to eastern areas of the state.  The jet stream is oriented from southwest to northeast in this case which results in the southwest to northeast oriented snow bands.  The southwest flow ahead of an approaching trough of low pressure acts much like a warm front bringing moisture from the desert southwest overriding the colder at the surface on the eastern plains.  This can result in light snow, but some of our largest freezing rain/mist/drizzle events can occur in this scenario.  The heavier bands develop as the jet stream energy rides overhead supplying additional energy and lift which gets carried onto the plains in the form of heavier precipitation bands.

The third snow banding case is more theoretical than anything as I have yet to see this really happen for any duration of time or very frequently.  For this newsletter we will call this the “Cold Air Wedge” case which occurs when colder air is banked up against the Front Range foothills and acts like a front forcing the incoming air to rise before being forced by the terrain to the west.  In theory the cold air wedge essentially extends the foothills eastward in the eyes of the incoming upslope flow placing the normally heavier amounts of snow on the plains instead of along the base of the foothills.  Here are a couple of pictures to illustrate what I am trying to say here:

 

There are more than just a few ways that snowfall can become banded or enhanced resulting in large differences in snowfall amounts within a small geographical area.  The southwest and northwest flow scenarios are the most common occurrences and we have seen the effects of both this winter season even with the lack of precipitation.  There are many other ideas and theories on how banded snow develops such as Conditional Symmetric Instability(CSI) and others, but would take a book and not a newsletter to cover all details on this subject.