During the weekend of March 12-13, 2021, significant weather was forecast to occur across much of the western and central United States, including a severe weather outbreak in the Plains states and a major snowstorm in the Rocky Mountains. These events were associated with a low pressure system over eastern Colorado, the development of which was caused by several synoptic factors.
A good place to analyze synoptic-scale weather systems is the jet stream level (250 or 300 mb). This is because convergence and divergence of the ageostrophic wind at this level is directly related to vertical motion in the mid-troposphere: if divergence is occurring in the upper levels of the atmosphere, air from below will rise to replace it, causing upward vertical motion in the mid-troposphere. This causes an area of low pressure to form or strengthen at the surface and is generally associated with clouds and precipitation. Figure 1 shows the forecast 300 mb wind and height for 0000 UTC 14 March 2021.
Figure 1: GFS 300 mb height and wind (12Z 11 March 2021, valid 00z 14 March 2021)
The map shows an upper-level trough centered over the Four Corners region and an upper-level ridge located over the eastern half of the country. Ageostrophic divergence occurs in regions with an upstream trough and downstream ridge, such as much of the central United States. In addition, a jet streak is shown on the eastern side of the trough base, centered over west Texas and New Mexico. Eastern Colorado is in the left exit region of this jet streak, a region associated with ageostrophic divergence. Since both the jet streak and trough/ridge positions are contributing to upper-level divergence over eastern Colorado, this is an especially favorable area for upward vertical motion.
Another way to forecast the development of synoptic-scale storms is to use the QG omega equation. This equation has a lot of complicated math, but in general it relates upward or downward vertical motion in the mid-troposphere to two variables: the change in vorticity advection with height, and temperature advection. According to the equation, positive vorticity advection increasing with height and warm advection in the low-mid troposphere are associated with upward vertical motion. Both of these were forecast to occur over northern and eastern Colorado, and the GFS model indicated significant upward vertical velocities at 700 mb in this region as a result (Fig. 2).
Figure 2: GFS 700 mb height, vertical velocity (18Z 11 March 2021, valid 00z 14 March 2021)
The end result of all this is the formation of a low pressure cyclone over eastern Colorado. While severe weather may be possible over the Texas and Oklahoma panhandle region, perhaps more interesting is the potential for a major snowstorm in parts of Wyoming and Colorado. The forecast maps below (figure 3a) produced by the Cheyenne, WY National Weather Service office illustrates this well: forecast totals in some cities are up to three feet. The Low End Amount graphic (figure 3b), which shows the snow amount with a 90% chance of being exceeded, is equally interesting. As the map shows, forecast confidence is quite high in very large snow accumulations, with a 90% chance of at least 16 inches of snow in the city of Cheyenne. Similar amounts were forecast for the mountains of Colorado and the Denver metropolitan area: both the NWS offices in Cheyenne and Boulder used rather strong wording in their forecast bulletins, reflecting the potential for a historic event.
Figure 3: National Weather Service Cheyenne, WY forecast maps for a) total snow accumulation and b) low end amount (90% confidence that this amount of snow falls).