After last Saturday’s busted waterspout chase, I’ve become curious about what goes into predicting waterspouts. It’s an area I haven’t paid much attention to, but after reading a paper on waterspouts sent to me by Mike Kovalchick, I’m interested in learning their forecasting parameters.

I had always thought there were just two categories of waterspout: non-mesocyclone and mesocyclone. But the paper presents four categories: tornadic, upper low, land breeze, and winter. All of them fall within a range of variables depicted on a “waterspout nomogram” that correlates convective cloud depth and the difference between water temperature and 850 mb temperature.

Tornadic waterspouts cover a broad swath of the nomogram. The remaining three kinds fall within more specific territory:
* Land breeze waterspouts require a minimum convective cloud depth of 5,000 feet, stretching all the way up to 32,500 feet, and water/H85 temp differences between 11 and 19 degrees C.
* Upper low waterspouts require a minimum convective cloud depth of 6,500 feet, stretching up to 36,500 feet, and water/H85 temp differences between 9 and 19 degrees C.
* Winter waterspouts, as one would expect, are a different animal. Convective cloud depths range from 2,250 feet to 9,750 feet, with water/H85 temp differences starting at 24 C and apparently extending beyond that indefinitely.
* All of the above presume 850 mb wind speeds of less than 40 knots.

This is obviously an extremely simplified summary which I’ve extrapolated from the waterspout nomogram. The nomogram brings out variables that I haven’t addressed here, and it’s well worth checking out in the aforementioned paper (see above for link).

Developed by Wade Szilagyi of the Meteorological Service of Canada, the nomogram is in use for predicting Great Lakes waterspouts, and evidently is under consideration for use in the Mediterranean Sea as well. It looks to be an easy-to-understand tool, and one I’ll surely be using as the Lake Michigan waterspout season ramps up.