On February 2nd, 2016, alias Groundhog Day, the mascot of this historic and beloved tradition, Punxsutawney Phil, was hoisted high in the air at Gobbler’s Knob, PA, amidst a chorus of cheers.
"Take your jackets off, you're not going to need them!" a representative from the Groundhog Club shouted to the crowd.
Punxsutawney Phil did not see his shadow this year. Since the tradition of Groundhog Day began in 1886, Phil has not seen his shadow a total of 18 times, including this past Groundhog Day. Legend goes that when Punxsutawney Phil doesn’t see his shadow, we will have an early spring.
Whether you’re a steadfast skeptic or you think Punxsutawney Phil is a more reliable predictor of the changing season than all the climate models combined, the end of winter does not mean the end of snow. Spring snowstorms actually make up a large fraction of Vail and Beaver Creek’s annual snowfall. On average, roughly 50 inches of snow falls in the central Rockies during the month of March. So what dictates this sudden flux of fresh pow?
We have the Jetstream to thank for this trend. The Jetstream is an extremely fast-moving, narrow band of air that occurs in the upper atmosphere. Jet streams occur when two air masses of different temperatures or densities collide. This results in an extreme air pressure gradient, which is most pronounced along the transition zone between the two air masses. Typically, the air would flow directly from high to low pressure, but due to the rotation of the Earth, the resulting wind flows along the boundaries of the two air masses, causing the phenomenon of a jet stream. Here in Colorado, when we say the Jetstream, we’re typically talking about the polar jet stream. Both the Northern and Southern Hemispheres have a polar and subtropical Jetstream, but the polar Jetstream is usually stronger than the subtropical Jetstream.
In the wintertime, the polar Jetstream tends to situate itself right over the Rocky Mountains. When wet, heavy air masses make their way into Colorado, the Jetstream helps lift those air masses high up into the mountains. Most of us are probably all too familiar with the extreme temperature difference between Vail Village and the top of Vail mountain; it’s chilly up there! Once all the water vapor hanging out in those wet, heavy air masses is exposed to the cold up in the mountains, the water vapor condenses and falls from the sky as snow.
Now here’s the plot twist. In the winter, the polar jet stream usually sits right over the lower third of the United States, but as we transition into spring, the Jetstream begins to migrate north towards Canada. As the Jetstream moves to higher latitudes, it takes a lot of the cold weather it creates with it, leaving behind warm weather characterized by low-pressure zones.
How could that possibly cause more snow to fall? Low-pressure zones pull in air and moisture, creating stormy weather.
Similarly to the way air behaves within the Jetstream, air masses flow from areas of high to low pressure. When air masses flow into a mountainous low-pressure zone, such as Vail, those masses of air undergo orographic uplift. Orographic uplift is a fancy way of saying that the mountains force the air upwards, causing an increase in relative humidity within the air mass. Under the right conditions, this results in precipitation. Around here, we call that precipitation gnar-pow (or spring snowstorms).
In summation, the Jetstream is a significant piece of the puzzle that contributes to our springtime fun here in Eagle County. While it is never guaranteed that snow will fall, as long as the Jetstream migrates north each spring, then Eagle County is likely to morph into a low-pressure zone and bring us some much-needed snow.
Emily Stuchiner is a winter naturalist at Walking Mountains Science Center. She enjoys long walks in the snow and meteorology.
