I was reading the paper The Washington DC tornado of 24 september 2001: pre-storm environment and radar perspectives which had this passage:

At 1200 UTC 24 September 2001, at 500 hPa a deep (560 [gpdm]) cut-off extratropical low (midlatitude cyclone that is cut off from the jet stream) was located over Wisconsin with a central core temperature of -20 C." ().

Is 560 dm deep for that part of the country and that time of year?

I have revised the question based on suggestions fomr the comments.

  • $\begingroup$ I'm not sure any set criteria for deep low, even at the surface, as it's a rather subjective criteria, and also varies by season and nearby pressures. As to why they use it as the plot variable rather than pressure on isoheight surfaces... I don't know, a good question. It does allow direct thickness calculation... but if you use the hydrostatic approximation, you can switch to p coordinates. Perhaps it's because soundings historically reported the special pressure levels, and so you'd have needed to record/transmit more data for isoheight? $\endgroup$ Commented Nov 16, 2016 at 12:09
  • $\begingroup$ As to what you ask about hydrostatic pressure changing with gravity, I'm not sure I understand your question fully? If you're asking do variations of gravity come into it, I suppose they would... but for the troposphere, gravity is for all-intents-and-purposes constant. And hydrostatic is already an approximation indeed... such gravity variations are likely of a smaller scale than other neglected terms. But perhaps this is better as it's own question and with more explanation\details? $\endgroup$ Commented Nov 16, 2016 at 12:12
  • $\begingroup$ @spilthrill - so maybe you need to reword your question. Climatologically what geopotential heights are associated with low pressure systems for the USA ? $\endgroup$
    – user1066
    Commented Nov 22, 2016 at 7:56

1 Answer 1


Great question!
First, let's look at typical data: here are the 1981-2010 Reanalysis mean and standard deviation:

enter image description hereenter image description here

So looks like the mean 500 height in September for Wisconsin ranges from about 571-580 Dm, with a standard deviation of 36-42 Dm. These are mean pressures, not lows or highs, so already, not that far off your low. If (a big if) meteorological statistics are normally distributed, using this calculator, this would suggest a 560 Dm height would be in about the 29th-40th percentile, depending upon location. A little lower than usual, but apparently fairly typical.

And just for good measure, looking back at the University of Wyoming, I was able to fairly quickly come upon:

enter image description here

A 554 or 555 Dm low. If you want to look for more images (perhaps even the event you are reading about), you could also look in the Pix directory for the dates of interest at Iowa State's MTArchive.

But it appears that 560 Dm would be a fairly typical height for a low/trough during early fall. Some people might still call it deep (30 Dm of contours on that plot from September would be fair), but it's nothing too abnormal.

  • $\begingroup$ I think I remember that event, even living out in Oklahoma at the time. Coming after September 11th, it stood out I guess. Think I might've chased the system when it came through the Plains, though it wasn't as noteworthy as October 9th. But the strength of the low isn't as vital as the surface conditions and phasing. $\endgroup$ Commented Nov 22, 2016 at 16:17
  • $\begingroup$ No problem, keep up the good questions! And, a better place to look for info on the day than the link I did would be spc.noaa.gov/exper/archive/events, or particularly spc.noaa.gov/exper/archive/event.php?date=20010924. It doesn't have day-before maps, but has morning of, and perhaps would help you look into it more. $\endgroup$ Commented Nov 23, 2016 at 6:09

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