The United States is experiencing contrasting severe weather events. The Front Range of Denver is experiencing light snow falls, periodic high wind gusts, along with some sunny days and warmer temperatures. The recent winter snow blizzard ‘Joanas,’ managed to dump between 2 to 3 feet of new snow, and shut down parts of New Jersey, New York, Phillidelphia, Pennsylvania, Maryland, and Washington, D.C. While the east coast digs out from under record snow falls, Colorado has experienced mixed weather with warmer temperatures, light snowfalls, and high wind events.
Colorado’s mountains and plains provide for unique weather patterns, winter is no exception. Colorado front range areas can develop a regional cyclone flow, referred to as the Denver Convergence Vorticity Zone (DCVZ). The Denver cyclone often sets-up in spring, when Colorado experiences convective storms. However, the Denver cyclone basin can set-up a strong front range pattern in fall, and winter, producing amazingly strong upslope winter storm environments. Colorado’s front range weather patterns can also contribute to a thermal inversion cap, trapping cold air and pollution into the Denver metropolitan area. Often, when Denver experiences a strong inversion cap in winter, air pollution levels can increase drastically. Inversion caps can be released again by changing thermal parcels, and movement of frontal boundaries.
Recently, on January 16th, 2016, Colorado experienced another unique weather phenomena, gale force ‘down slope’ or Chinook winds.
Chinook winds, at surface levels, can create EF1-2 scale straight line wind gusts, and can result in heavy straight line wind damage to forests and property. On the 16th, of January, 2016, located a bit east of South Table Mountain, down slope wind gusts, exceeded 86 mph. Chinook down slope winds were responsible for high wind events in mid-December, and also the previous month, in mid-November, gusts exceeded more than 100 mph in numerous areas from Ft. Collins, to Loveland, Berthoud, Longmont, Boulder, Broomfield, Golden, and Morrison, Colorado. There was considerable tree damage along the foothills, first reported in Golden, Colorado, during that particular Chinook wind event, with hundreds of splintered and up-rooted trees scattered across roadways. Split phone poles were also reported just north of Eldorado Springs Canyon off HWY 93, and again between Berthoud and Ft. Collins, where public service utility managers reported there were more than 200 split or broken phone poles, and a number of staggering power outages.
Down slope winds are not uncommon to Colorado. Colorado will likely experience another 100+ mph wind event before spring. Colorado most often gets strong down slope winds in fall, and in spring. Transitioning weather patterns, moving primarily in a northeastward track through the State, are responsible for many storms from convective thunderstorms to winter blizzards. Several days ago, Colorado had both, with several convective snow storms that produced thunder.
Generally, meteorologists tend to agree that advancing low pressure systems, coupled with strong west to eastward flowing mid level jet streams that move air up and over the Continental Divide, and aid formation of down slope or Chinook wind phenomena on the lee side of mountain ranges..
The way meteorologists often explain Chinook wind events, is the result of air traveling at a certain altitude has a specific gravity, and commensurate air density for that elevation. Air is more dense at sea level than it is at 14,000 feet (asl) above sea level. However, air columns tend to prefer their ideal elevation, based on air parcel density. Therefore, if a jet stream flow forces denser air up and over a mountain range, then that denser air will later descend the leeward side of a mountain, creating drag as the air parcel descends to a preferred equilibrium elevation. During descent of a forced air parcel, this drag creates friction, and often a Chinook down slope air parcel will ‘heat up,’ as air mass descends into a higher pressure region. It all sounds sort of strange, that warm air can go down hill, but that often is the case during a Chinook wind events. In addition to very strong surface wind gusts, Chinook winds may be quite dry and may be considerably warmer then the surrounding ambient air mass present on leeward slopes. This is a reason that native Americans refer to Chinook winds as ‘snow eaters,’ because they literally melt and evaporate snow quickly. Along with warm strong wind gusts, on the leeward side of mountain ranges, the plunging air can rise again, once heated. Above areas of strong surface winds, there may be UFO-shaped lenticular clouds. Lenticular clouds can stack up above each other like plates, at the peak of a wind field amplitude, making for beautiful and surreal sunsets. On the leeward side of a Chinook wind environment there are often large oscillating ‘gravity waves’ that form just east of leeward slopes. Oscillating gravity waves decrease in amplitude, as the mid-level and lower jets encounter flatter terrain over the eastern plains. Predictably, Colorado will face a number of additional Chinook wind events this year, until the springtime adiabatic heating stabilizes foothill mountain zones and high plains areas.