There have been many headlines about the lack of snow over the conterminous U.S. this winter particularly because the northeastern U.S., a major source region for headlines, has experienced an unusually mild winter. For instance:
The less than normal snow extent also affects many parts of the western U.S. including where I live in Colorado. It was visually evident during the President’s Day weekend when I noticed I could see most of my two-foot tall snow stake at our house at the 8300-ft level in the Colorado mountains:
That contrasted with the conditions of a year ago which were much different. Just how different? This picture from last year’s President’s Day weekend shows the same snow stake at the same location:
Ah yes! What a difference a year makes. Even more striking is the difference in the amount of water contained in the central Colorado snowpack this year as opposed to last. The graph to the right shows the snow water equivalent (SWE), the amount of water in the snowpack measured in inches, for a location not too far from the snow stake comparing Jan 1-Feb 21 of last year with the same period this year (last year=blue, this year=red). Again, what a difference a year makes! Last year the snowpack contained more than 3″ of water (if you were to melt all the snow in a column); this year the pack has about ¾”, just 25% of last year’s amount.
The dearth of snow in the Central Rockies is not confined to the area just around my snow stake. This next map, from the Natural Resources Conservation Service (NRCS—a part of the U.S. Dept. of Agriculture), shows all of Colorado having a below average snow season this year. The NRCS collects data about the snowpack (snow depth, water content, etc.), both remotely and in-situ, and produces a variety of maps depicting the condition of the wintertime snow cover. Their Colorado map set can be accessed from here.
To the right is an even bigger picture view, again from the Natural Resources Conservation Service. Most of the western U.S. except the far northern regions and a few spots in the southern Rockies have had a snowstarved winter. If you are interested in monitoring changes in the snowpack across the western U.S., here’s where you can access daily updates to this map.
Much of this winter’s snowlessness across the U.S. is related to the location of the jet stream. Winter storms generally follow the path of the jet stream (Storms develop along thermal boundaries between cold air to the north and warm air to the south, in the Northern Hemisphere. The jet stream is created by these same thermal differences.) Generally, cold air and snow will be more prevalent along and on the poleward side of the jet stream’s location. This winter, on average, the jet stream has been farther north than normal especially over the western U.S. as this map shows (magenta arrows show average position of the jet stream). Usually, those magenta arrows would be hundreds of miles farther south and the northward bulge over western North America would not be as pronounced. This is a pattern which is often seen during La Niña conditions (cold water over the central Pacific Ocean) like what we have this winter—although this year the jet stream is even farther north than in typical La Niña years.
And, as with all things weather, by the time I post this missive it will be old news. In the past few days the jet stream has shifted south and west (as shown by this satellite picture and wind speed map from today).
It now enters the U.S. over the Pacific Northwest and dives down into the central Rockies. Meanwhile over the eastern U.S., it’s farther south as well (and, not surprisingly, Richmond, VA had a few inches of snow this past weekend). For Colorado, this has meant a return to snowy conditions for our mountains with fresh powder at the ski resorts being measured in feet rather than inches. This is good news for the western watersheds (but bad news for my snow stake). And for the next 10 days, at least, it looks like the jet stream will remain closer to its normal position across the western U.S. leading to more normal wintertime weather.