Using normalized climatological anomalies to rank synoptic-scale events objectively

A method for ranking synoptic-scale events objectively is presented. NCEP 1 2-h reanalysis fields from 1948 to 2000 are compared to a 30-yr (1961-90) r eanalysis climatology. The rarity of an event is the number of standard dev iations 1000-200-hPa height, temperature, wind, and moisture fields depart from this climatology. The top 20 synoptic-scale events from 1948 to 2000 f or the eastern United States, southeast Canada, and adjacent coastal waters are presented. These events include the "The Great Atlantic Low'' of 1956 (ranked 1st), the "superstorm'' of 1993 (ranked 3d), the historic New Engla nd/Quebec ice storm of 1998 (ranked 5th), extratropical storm Hazel of 1954 (ranked 9th), a catastrophic Florida freeze and snow in 1977 (ranked 11th) , and the great Northeast snowmelt and flood of 1996 (ranked 12th). During the 53-yr analysis period, only 33 events had a total normalized ano maly (M-TOTAL) of 4 standard deviations or more. An M-TOTAL of 5 or more st andard deviations has not been observed during the 53-yr period. An M-TOTAL of 3 or more was observed, on average, once or twice a month. October thro ugh January are the months when a rare anomaly (M-TOTAL greater than or equ al to 4 standard deviations) is most likely, with April through September t he least likely period. The 1960s and 1970s observed the fewest number of m onthly top 10 events, with the 1950s, 1980s, and 1990s having the greatest number. A comparison of the evolution of M-TOTAL to various climate indices reveals that only 5% of the observed variance of M-TOTAL can be explained by ENSO, North Atlantic oscillations, or Pacific-North American indices. Th erefore, extreme synoptic-scale departures from climatology occur regardles s of the magnitude of conventional climate indices, a consequence of a nece ssary mismatch of temporal and spatial scale representation between the M-T OTAL and climate index measurements.