rdale
EF5
pressure falling rapidly. A decrease in station pressure at a rate of 0.06 inch of mercury or more per hour which totals 0.02 inch or more.
http://www.nws.noaa.gov/oso/oso1/oso12/fmh1/fmh1appa.htm#appalink
http://www.nws.noaa.gov/oso/oso1/oso12/fmh1/fmh1appa.htm#appalink