Six years ago yesterday/today: an incredible mid-latitude cyclone developed across the central portion of the United States and deepened as it moved northward into the northern Plains. This low pressure system is honestly one of the more incredible meteorological anomalies that I have ever seen, as it was an absolutely beautiful example of the foundations of mid-latitude meteorology.
The surface low was a classic Colorado low, with a fairly intense surface low developing on the lee side of the Rockies in southeastern Colorado. This occurs because on the leeward side of Mountains, the descending of air creates what is called vortex stretching, which acts to enhance one of the primary lifting mechanisms in the atmosphere, vorticity. The intensification of this often leads to the development and quick intensification of the surface low, as this process is much like that of an ice-skater pulling in their arms and spinning faster.
As this was occurring, the exit region of an anomalously intense upper-level jet approached the central plains, and this is key in the development and sustaining of any intense surface low. The atmosphere is examined in a sort of “Top-down” approach, as what occurs in the mid and upper-levels of the atmosphere forces surface conditions.
Additionally, placement in jet streaks such as this one is key. The left exit region is often considered to be the most favorable location for upward motion, and thus the sustainment and fostering of surface lows/surface convection. But often times, the exit region in general is a favorable location for both of these, and this was the case with this system. While the divergence across the southern Colorado low wasn’t as intense as its fellow low to the north in the upper Midwest, the divergence/large scale upward motion due to other factors was intense enough.
This allowed for this low to be fostered until large scale forcing merged the two surface lows, and allowed for them to intensify together. At every level, from the upper levels of the atmosphere down to the surface, there was a closed low pressure system, and this is key. For the majority of your life, you have likely heard that low pressure systems move along the surface. That isn’t entirely true, as low pressure systems aren’t like a tire or a wheel that moves along the ground. Low pressure systems propagate along the surface with favorable upper level divergence. That is why it is so important for the atmosphere to have these closed off mid and upper level lows, but also for them to be offset somewhere to the left of the low pressure system. This allows for the region ahead of the low pressure system to have air that rises ahead of it, which forces there to be a deficit of air at the surface, forming a low pressure system. Basically, a low propagates along the surface with favorable upper level dynamics, and dies in its old place as those dynamics move along. This low pressure system was an excellent example of this process.
The surface low is fostered by upper level divergence, and that is seen until the point where the lows at the surface and in the low, mid and upper-levels of the atmosphere are vertically stacked over top of each other. This cuts off the ventilation of the surface low, and allows for it to begin weakening. The problem was that the surface low just sat underneath very strong mid and upper level upward motion for quite sometime, and just kept intensifying. The surface low got all the way down to 955.2 mb, which is absolutely absurd. This was 0.3 mb away from the record for a non-tropical low pressure system in the continental United States.
Other than being an incredible meteorological anomaly, this produced fairly extreme sensible weather conditions. First of all, the pressure gradient around this low pressure system was absolutely incredible, with record pressures being set far away from the low pressure center itself. Some records this broke:
- Lowest sea level pressure ever measured in Wisconsin at 961.3 mb
- Duluth, MN set a record pressure of 960.2 mb
- International Falls, MN set a record of 956.0 mb, which is 15.9 mb lower than the previous record
The system produced a large, and fairly intense surface wind field (that comes into play with the low, mid and upper level wind fields later, but I’ll mention that in a second). With a low pressure system as intense as that was, surface winds are inherently going to be high within that. Winds consistently were recorded gusting into the 50-70 mph range across a large portion of the north-central US.
Across the Ohio and Tennessee River Valleys, a severe weather outbreak occurred! This outbreak was a fairly typical late fall/winter severe weather outbreak, as a very intense mid-latitude cyclone, and associated intense wind field, combined with a strong surface cold front to force a strong line of convection that raced eastward quickly. There was very little in the way of instability ahead of the line, but the wind field in the vertical atmosphere was absolutely unbelievable.
Wind shear from the surface up to 6 kilometer in the atmosphere was in the 75 to 100 knot range across much of the Ohio Valley. This level of shear, along with strong frontal forcing, helps to force and sustain strong convection as it moves eastward, even without significant instability.
The system produced multiple reports of tornadoes, and over 300 reports of wind damage across the Ohio and Tennessee Valleys. The SPC had even issued a high risk across the region because of the set up.
Overall, the event in 2010 was an even that I personally won’t forget because of how incredibly dynamic it was. It also broke surface pressure levels and affected much of the central and eastern United States. Hopefully we won’t see anything similar this season!