Meteorology ‘101’ from potentially historic snowstorm in the Rockies this weekend

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This weekend, parts of the Rockies, including Colorado and Wyoming, are likely to experience a potentially historic snowstorm. In at least one National Weather Service map, I have seen a high-end 50-inch forecast for snowfall in parts of the area. While this is probably an outlier, the most likely range of solutions is still impressive and life changing. The National Meteorological Service issued the following statement for the event, “Winter storm warning issued for the mountains, Front Range foothills and urban corridor from midnight Friday through Monday morning. Travel will be difficult, if not impossible, throughout the weekend.“While reading their discussion of forecasting, several key meteorological terms were used. Here’s a weather breakdown of what it all means.

Snow totals are forecast in the 2 to 3 foot range (graph above) in the mountains and major towns in the Front Range area. From a historical perspective, this storm has a significant chance of overtaking the Heavy Snow Sword of March 23, 2016 according to the national weather service. Peak wind gusts on Saturday and Sunday (graph below) could exceed 30mph, so locals should be prepared for associated risks such as power outages. Obviously, there are important things expected in the atmosphere as spring approaches.

The National Weather Service (NWS) -Boulder highlights several things that I want to explain. A key component of the storm is leeward cyclogenesis. The Friday morning discussion on the NWS said, “…. cyclogenesis will develop over northeastern New Mexico and southeastern Colorado from today until tomorrow ….Cold air dam east of the Rockies is expected to keep the center of the surface cyclone toward the southern edge of the model solutions possibly ending in northeastern New Mexico. Ok Dr Shepherd, what does all this mean?

The American Meteorological Society glossary defines lee cyclogenesis as “The synoptic scale development of atmospheric cyclonic circulation on the leeward side of a mountain range.” The term “lee” relates to the background air flow, according to the glossary. The mountain barrier is essential for the formation of a depression (“cyclogenesis”) because it can redistribute vorticity (“fluid rotation”) as the wind blows over the mountains. The intensity of cyclogenesis may be greater if the mountains interact with a baroclinic wave, a wave associated with differences in temperature, wind shear and frontal surface characteristics. The National Weather Service website has an excellent analysis of a case from 2019 to this connect since my word count is limited here.

The discussion of the NWS forecast for this storm also mentions, “There are many factors that this system has going for it that will come together to create a significant lift in our region. The elevator is the key to these types of events. Here are some of the major players in meteorology:

  • A left exit region of a sequence of rolls higher level above. The left exit region of the jet sequence, an area of ​​stronger winds in the jet stream, is often associated with due upward movement. The map above shows some of the characteristics of the upper level jet stream for this storm.
  • Positive vorticity advection (PVA) upstream of the trough. AVP is generally a good indicator of the potential for air elevation.
  • ALoft hot air cavity (TROWAL) which will bring the advection of warm air and humidity to the area. According to the National Weather Service glossary, a TROWAL is “a tongue of relatively warm / humid high altitude air that winds north and west of a mature cyclone … During a winter storm, the strongest Snowfall occurs frequently along and north of the TROWAL axis.
  • Strong updraft from the east up to 45 knots.
  • Instability on Saturday during the day and until Saturday evening. In fact, I wouldn’t be surprised to see thunderstorms in parts of the storm as well.

If there’s one good thing about the storm, it’s that it happens over the weekend. However, significant weather events like this are always high impact events and in this time of year our fight against COVID-19 is making them worse as well.


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