|Fig 1 -- Objective analysis of surface temperature (colored shading), sea-level pressure (contours) and winds for 18Z, Dec. 6 2010. From the HOOT website.|
|Fig 2 -- Base reflectivity composite image for 1818Z, Dec. 6, 2010. From http://www.weather.gov/radar.|
|Fig 3 -- Base reflectivity from Exeter, Ontario (WSO) radar at 1830Z, Dec 6, 2010. From the Environment Canada website.|
|Fig 4 -- 300mb analysis of winds and geopotential height at 12Z, Dec. 6, 2010. From the HOOT website.|
But let's turn our attention to the center of the country. Notice in figure 4 how the analysis has (somehow) drawn a jet streak coming down through Manitoba and into Minnesota. There are not a whole lot of observations supporting this jet streak on this map (and I KNOW Canada has more upper air observations than are being plotted here), so I will assume that this analysis used other supporting data to conclude that there is a jet streak here. Notice the curvature of this jet streak--as air flows through it, it's curving from northwesterly winds to more northerly winds. This is a clockwise curvature of the winds, or anticyclonic curvature (winds around a cyclone go counterclockwise, so clockwise winds are anticyclonic).
Remember out jet streak adage about us seeing strong divergence in the exit region of a cyclonically-curved jet? It turns out that the opposite is true for anticyclonically curved jets. In the exit region of anticyclonically curved jets we expect to see convergence aloft. Since we generally assume air does not rise above the tropopause, convergence aloft generally leads to subsidence and rising pressure at the surface (and often divergence at the surface to compensate). To illustrate, I drew up a quick diagram:
|Fig 5 -- Luke's somewhat crudely drawn diagram to attempt to illustrate how convergence aloft leads to subsidence and increasing pressure at the surface.|
So, whereas in the exit region of cyclonically curved jets we'd expect to see surface lows form, we might expect to see high pressure building at the surface under the exit region of an anticyclonically curved jet--under an area of convergence aloft. We can also see that the central plains is also somewhat under the left entrance region of the straight or slightly cyclonically curved jet streak across the southeast--another favored area for convergence aloft. All of this would theoretically support rising surface pressures. Do we see this?
|Fig 6 -- 3-hour surface pressure changes and wind vectors at 12Z, Dec 6, 2010. From the College of DuPage website.|
We usually don't focus on the formation of high pressure centers--they're somewhat tricky and they don't usually bring "interesting" weather. There are many things that could be going on here to explain why we aren't seeing pressure rises. Here are my thoughts on a few of them:
- High pressure "centers" are usually spread out over a much larger area than low pressure "centers" which are very point-focused. You can even see in figure 1 above how sprawling the high pressure center contour is compared to the very small low-pressure center contour. So while we do have subsidence and general sinking motion, it's spread out over such a large area here that the overall pressure rise due to this sinking motion is minimized or masked by other local effects.
- On a somewhat technical note, in the solution to the geostrophic wind balance equations, a quadratic comes up with different roots depending on if the flow is cyclonic (around a low) or anticyclonic (around a high). When you attempt to calculate these different terms, there actually ends up being a limit for "strong" a high pressure center can get but there is no limit for how "strong" a low pressure center can get (in the theory). So there reaches a point of diminishing returns where increasing the subsidence won't do anything to raise the pressure--it will just increase the winds.
- I really like in figure 6 above how the winds are calm everywhere in the central US except for where they just start to blow eastward across southern Minnesota and northern Iowa--right underneath where we'd guess there was maximum convergence aloft. I strongly suspect that all the subsiding air underneath that convergence is being evacuated out by the winds toward the low pressure center to the east. In this way, there is no net pressure gain because the subsidence is being exactly balanced by those divergent winds evacuating air at the surface. I think this is the biggest reason we don't see much of a pressure rise.