Well, our once-or-twice yearly real blizzard has hit. There's a noticeable optical depth between my lab window and the building across the sidewalk. I'd guess the visibility is 100 meters or so. But it's supposed to end by tonight, so we'll pick up less than a foot of snow in total.

Here's what the system was doing earlier today, though:

Radar map from for 10:35 CST, several hours after the snow storm was supposed to have reached us.

This sort of thing happens in the summer with rain storms too, though to a lesser extent. That hole developed pretty much right at the edge of the Twin Cities suburbs as the system moved in from the west, and stuck around for five or six hours, meaning that while it was snowing both east and west of us by 7 am, it didn't start here (in the urban core) until almost 1 pm. I don't think it's a heat island thing, exactly, since it was well below freezing here all day. Rather, my theory is that the metropolis has its own associated convection system that may disrupt the convective processes of incident weather systems and thus delay the onset of precipitation. However, this is a pretty obvious phenomenon and plenty of other non-experts have observed the same thing, but local meteorologists have been asked about this on several occasions, and they never seem to know much about it.



It's the angle of the radar. The precip is still there but the particular radar product must be a low-angle only one. I seem to recall they have a product that does low-angles only for a quick refresh cycle.

Good thought, but I doubt that's correct here. I'm pretty sure the precipitation hole shown here is real.

Most significantly, the presence or absence of the hole correlated strongly with whether or not it was snowing in the city. Meanwhile, news reports showed that it was snowing the whole time in outlying regions.

Also, notice the little white cross in the center of the image noting the location of the radar station (out of Chanhassen, MN, in this case). It is nowhere near the center of the hole as you'd expect if it was a radar angle artifact.

Does Chicago exhibit any similar precipitation shadow effect on systems arriving from the west?

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This page contains a single entry by Milligan published on February 26, 2009 1:16 PM.

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