A few weeks back, the east coast got slammed by a massive winter storm. I wanted to display the North American Mesoscale (NAM) forecast model output for the dynamics and temperatures for this system as it approached the mid-Atlantic. Keep in mind, this is model data, as there is no practical way to observe some of these things with this resolution. Reality did not differ from this model output significantly.
First, we look up high, at the 300 mb level. Notice the deep trough over the Appalachian Mountains, and the weak jetstreak moving northward just off the coast of Georgia and the Carolinas. This was one source of energy for this storm system. I am not sure about the bulb shape at the bottom; this may be significant as well. I will have to look into this.
Second, we descend through the atmosphere, to the 500 mb level. Notice the hook-shaped vorticity pattern in this system. Also, notice how the winds at this level blow across the vorticity gradient from higher vorticity to lower; this was Positive Vorticity Advection (PVA). There was strong PVA over Virginia and eastern North Carolina on this chart.
Diving closer to the Earth’s surface, to the 700 mb level, we see rapidly rising air, near the PVA and the nose of the jetstreak. This rapidly-rising air was causing clouds to form; quite a bit of precipitation fell from these clouds, as we learned on 1/22/16.
Finally, at the 850 mb level, we start to look at thermal advection. Several important features of mid-latitude cyclones are present in this image. First, notice that there were winds blowing across the thermal gradient from warm to cold in eastern North Carolina and south-central Virginia; this was Warm Air Advection (WAA). WAA enhances convection, and thus clouds and precipitation, and in this case, the WAA corresponds to the same area as the PVA. The WAA can be thought of as the warm front. One of these would have been bad enough, but to have them overlap created the rapidly-rising air seen on the 700 mb chart. There was also wind blowing across the thermal gradient from cold to warm; this was Cold Air Advection (CAA), which also closely corresponded to the cold front. This was strongest in southern Georgia.
By looking at the storm through vertical slices, we can understand a little bit more about its internal structure. At some point, I will also post a temporal analysis of a mid-latitude cyclone, so that you can see how a system like this evolves.
Thank you for reading my post.
The upper level NAM charts are from Unisys Weather.