Salt Lake City, Utah "Big vs Super" Snowfall Events Revisited:

a Composite Comparison

David C. Hogan and Lawrence B. Dunn - WSFO Salt Lake City


Introduction:

In December of 1994 a study entitled "'Big vs Super' Snowfall events at Salt Lake City, UT: A Composite Comparison" was completed by several members of the Salt Lake City WSFO. It has proven to be a very useful reference in the forecasting of major snowfall events for the Wasatch Front. In the process of doing parallel studies for other major cities around the state it was found that some refinements in the initial study could be made which give a clearer picture of the frequency and the weather patterns surrounding these heavy snowfalls in Salt Lake City.

Methodology:

The first step was to search through climate records for all "Super" (10 inches or greater in 24 hours) and "Big" (6-9 inches in 24 hours) snowfall cases during the period 1963-1989 (the period for which a complete set of CD Rom Gridpoint Data is available for compositing). This search resulted in the discovery of additional "Super" and "Big" snowstorms which were not included in the previous study. Thus it became necessary to 1. revamp the Salt Lake City climatology for these large storms, and 2. check to see if the addition of numerous events made any difference in the results.

Results:

Updated Snowstorm Climatology: Results of revamping the large snowstorms climatology for Salt Lake City showed a doubling of the frequency of "Big" snowstorms, from one per year up to two every winter. The frequency of "Super" snowstorms took a less significant jump; one every other year instead of one every third year.

Updated Composite Results: Composites of "Big" versus "Super" snowstorms (with the inclusion of 30 more storms) showed that the results of the previous study were still good. Despite this, it was observed that compositing tended to wash out some of the characteristics of the individual storms. In other words, it became apparent that sub-groupings of "big" and "super" snowstorms could be made which better illustrated their unique storm characteristics.

Super Snowstorm Types: It was found that the "super" snowstorms could be conveniently sub- divided into "winter" (December-February) and "spring" (March-May) events, each having distinctly different signatures. The "winter" supers occurred with a broad, fairly stationary long-wave trough (at H5 and H7) over the Pacific Northwest and short-waves ejecting out over Utah. These will be referred to as "broad trough supers" (BTSs). On the other hand, the "spring" super storms occurred during initial stages of closed H5 low development over the Great Basin and exhibited a tell-tale H7 closed low over Idaho/Wyoming. These will be referred to as "closed low supers" (CLSs). A subdivision for "fall" events was unnecessary since there was only one such case.

Big Snowstorm Types: The "big" snowstorms on the other hand showed more variety of weather patterns and didn't fit cleanly into scenarios by season like the "supers". The "bigs" were sub-divided by storm type with the following four categories (in order of frequency of occurrence):

Type I. "broad trough bigs" (BTBs; hard to distinguish from the "broad trough supers"; 15 cases)
Type II. "closed-low bigs" (CLBs;little brother to the closed-low "supers"; 15 cases)
Type III. "sharp trough bigs" (STBs; progressive long-wave trough at H5 and H7; 11 cases)
Type IV. "rex block bigs" (RBBs;H5/H7 high pressure over the eastern Gulf of Alaska parked on top of H5/H7 low pressure over northern California; 4 cases)

Note: 12 of the 57 "Bigs" did not fit cleanly into one of these four groups and were left out of the composites.

The following pages show snap-shots of each of these types of snowstorms at three stages of development: T-12, approximately 12 hours before onset of snowfall; T00, near onset of snowfall; and T+12 hours, about 12 hours after snowfall began. Each page contains three charts: 500 mb heights, 700 mb heights and temperatures, and mean sea level pressure. These charts are accompanied by a brief discussion of the defining characteristics and how these compare/contrast to the other storm types.

SUMMARY OF "BIG" vs "SUPER" FREQUENCIES:
PERIOD OF RECORD: 1963-1994 

# OF SUPERS = 15    15 CASES/32 YEARS = 1 EVERY 2.1 YEARS

# OF BIGS   = 67    67 CASES/32 YEARS = 2.1 PER YEAR


  • Send COMMENTS, COMPLAINTS OR COMPLIMENTS to: David Hogan   (david.hogan@noaa.gov)