Monday, December 6, 2010

Anti-cyclogenesis?--and more lake effect snow

Cold air has surged south all the way to the Gulf of Mexico behind a rather strong cold front this weekend.  Freeze warnings are out as far south as Florida with this event.  It's a chilly day for most of the northeastern 2/3 of the country:
Fig 1 -- Objective analysis of surface temperature (colored shading), sea-level pressure (contours) and winds for 18Z, Dec. 6 2010.  From the HOOT website.
Note that rather strong cold air advection is still going on behind this front--and those closely packed pressure contours point to some pretty strong winds over the Great Lakes.  Several areas of the upper midwest are reporting 20 knot winds out of the west to northwest.  Of course, what does lots of cold air moving over the still relatively warm Great Lakes mean?  Another lake-effect snow outbreak, of course.  Here's a radar composite from early this afternoon.
Fig 2 -- Base reflectivity composite image for 1818Z, Dec. 6, 2010.  From
Lots of lake-effect snow bands oriented parallel to the wind flow.  However, the long axis of most of the Great Lakes is not parallel to the wind flow, so instead of one big band of lake effect snow we're seeing several small bands.  The only lake that comes close to being oriented in that direction is Lake Huron, and if we check the Canadian radar in Exeter, Ontario, we do see one big dominant band of lake effect snow coming off of Lake Huron.  There are several smaller bands as well, though--partially because Lake Huron is such a wide lake compared to the much narrower Lakes Erie and Ontario.
Fig 3 -- Base reflectivity from Exeter, Ontario (WSO) radar at 1830Z, Dec 6, 2010.  From the Environment Canada website.
But I already have talked a bit about lake effect snow recently.  Right now I want to briefly look at the current upper air pattern to see what's going on over the central US.  So--here's a 300mb analysis from 12Z this morning.
Fig 4 -- 300mb analysis of winds and geopotential height at 12Z, Dec. 6, 2010.  From the HOOT website.
A few features stand out on this map, most notably the huge trough sitting over the northeastern US.  Note how the surface low center in figure 1 was analyzed up northeast of Maine--away from the main jet streak as shown in this 300-mb analysis.  This would tend to support the conclusion that that low is already well-occluded and starting to fill in (the surface temperatures surrounding the surface low are also very similar with no strong gradients--more evidence of an occluded cyclone).

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.
In looking at a 3-hour pressure change map from this morning, there's really no clear signal across the central US.  Lots of zero-pressure-change contours are floating around on the plains.  We don't see strong pressure rises.  However, we do see strong winds picking up across the upper midwest, conveniently starting somwehwere in southern Minnesota and northern Iowa and blowing eastward toward the surface low in the Canadian maritimes.

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:
  1. 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.
  2.  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.
  3. 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.
So there's a quick look at the structure and nature of the high pressure center in the central US and why it may or may not be strengthening.  What we can say conclusively is that the upper air conditions are not favorable for cyclogenesis at all, so we can expect a precipitation-free day for the entire central part of the country.  That is, unless you live downwind of a large lake...

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