I. INTRODUCTION:

Following is a composite study of major snowfall events at Cedar City, Utah. Cedar City (CDC) is one of two points for which the Salt Lake City WSFO performs verification of temperature and precipitation forecasts, the other location being Salt Lake City itself. Forecast confidence for Salt Lake City (SLC) is fairly high owing to experience and an abundance of meteorological feedback. In addition, numerous studies for the highly populated Wasatch Front area have been made which have further enhanced forecast confidence for snowstorms. Two such studies include "'Big vs Super' Snowfall events at Salt Lake City, UT: A Composite Comparison" (1994), and "Salt Lake City 'Big vs Super" Snowfall Events Revisited" (1997). These studies have proven helpful in the forecasting of significant snowfalls in the Salt Lake valley. In an attempt to improve forecasting for SLC's other verification point, Cedar City, the author made this parallel composite study of Big and Super snowstorms.

For those not familiar with the Salt Lake City study, a "Super" snowstorm was defined by 10+ inches in a 24 hour period, while a "Big" snowstorm was defined by 6-9 inches in the same period. Major snowstorms occur a little less frequently in Cedar City than Salt Lake City. Although both CDC and SLC get about 2 "Bigs" every year (1.7 CDC vs 2.1 SLC), CDC doesn't get quite as many "Supers" (1 every 2.8 years CDC vs 1 every 2.1 years SLC). This corresponds well with a comparison of annual snowfall for the two stations. CDC airport receives 44 inches of snow annually, while the SLC airport receives 65 inches annually.

In considering the frequency of these storms, it is important to keep in mind that the CDC airport, much like the SLC airport, is somewhat unrepresentative of actual snowfall for the city areas. Both airports are significantly lower than much of the populated areas and usually come in at the bottom end of the snow totals.

II. GENERAL RESULTS

A. Similarities between SLC and CDC snowstorms.

Whether speaking of "Super" or "Big" snowstorms, composites show that major SLC and CDC storms have certain characteristics in common; 1. significant snowfall generally begins with the arrival of the cold front /surface trough, 2. significant snowfall tends to end with the passage of the H5 trough axis(it is estimated that H5 trough passage is about 8 hours slower with the Super storms than the Bigs, hence the longer period of snowfall and greater snowfall amounts) and 3. the center of the surface low tracks roughly overhead (See cold front )

B. Key Characteristics of CDC snowstorms.

The main things to keep in mind with major snowstorms at CDC is that they are developmental. They occur as the flow pattern over the eastern Pacific/western U.S. buckles or splits. The preferred scenario is for ridging and height rises to develop upstream in the eastern Gulf of Alaska and (for the really good storms) into British Columbia (BC). These height rises act like a see- saw; when one end goes up, the other must come down (see Table 1) The bigger the see-saw action, the deeper the west coast trough (and the bigger the storm, assuming the correct storm track). You want the base of the H5 trough to track west and south of Cedar City (roughly from Vancouver, BC to Las Vegas, NV). This is a more northerly track than major snowstorms for northern Utah. This puts Cedar City on the polar/cyclonic side of the jet, generally in a region of pva and diffluence aloft, and close to the H7 circulation center.

Owing to the splitty nature of the weather pattern, Cedar City storms are more likely to develop closed lows at H5 and H7 than Salt Lake City. Northern Utah snowstorms generally occur with deep open troughs (the main exception being spring-time snowstorms in northern Utah which are most often associated with closed/cut-off lows aloft). Of the major snowstorms at Cedar City, about 2/3 of the Supers and 1/3 of the Bigs exhibited closed lows at H5.

III. DETAILED RESULTS

A. How can you distinguish between CDC Supers and CDC Bigs?

When the above described pattern (buckling or splitting of the flow over the E. Pacific/W. U.S.) sets up for Cedar City, the way to tell the difference between the big storms and the really super storms is the magnitude of the sea-saw action. The Supers really stand out from the Bigs in three ways:

1. height rises upstream are double those for the Bigs (see Table 1),
2. a sharp ridge at H5 and H7 is develops in the eastern Gulf of Alaska and BC Canada (Bigs, on average, exhibit a flat ridge), and
3. very strong surface high pressure builds into BC (with Bigs the surface high generally doesn't make it on-shore into BC, but remains well out in the Gulf of Alaska) (See Table 2).

B. How do SLC Supers and CDC Supers compare?

In general, Cedar City Supers occur with the development of a deep closed low over the western U.S. in combination with strong ridging over BC. This deep closed low drops out of the north-northwest and tracks west and south of CDC. Salt Lake City Supers, on the other hand, generally occur with a broad, open, fairly stationary trough over the western U.S.. The two key things to look for in forecasting the formation of a closed low over the western U.S. are development of:

1. a sharp H5 ridge in E. Gulf of Alaska, and consequently
2. a strong surface high in BC.

Whereas the individual SLC Supers could be separated into 2 distinct groups (broad trough/ winter Supers and closed low/spring Supers), the individual CDC Supers exhibited a wider range of patterns and could not be broken down by season. Of the 10 CDC Super snowstorms, the following three divisions were made with 3 storms in each division: 1. Negative Tilt Troughs, 2. Progressive Troughs, and 3. Blocking Patterns.

C. What are the key differences between the 3 types of CDC Supers?

The negative tilt and progressive storms share a number of common characteristics. Both are developmental, were associated with strong trough amplification over the western U.S., and exhibited north-northwesterly storm tracks. The negative tilt Supers showed a strong short-wave dropping down the lee side of a sharp eastern Gulf of Alaska ridge into a pre-existing area of troughiness. The progressive trough Supers occurred as a split in the flow developed across the western U.S.. Perhaps the most tell-tale difference between the two patterns is that the H7 trough and H7 thermal trough are out of phase with the negative tilt storms, but in phase with the progressive storms.

The blocking pattern was the odd-man-out exhibiting a pattern distinctly different from the other two categories. The number of CDC superstorms due to a blocking pattern is disproportionately large, considering the infrequency of strong blocking patterns over the West Coast. In other words, blocking patterns occur much less frequently than the developmental patterns, but when they DO occur they are more likely to produce warning category snowfalls (assuming a favorable position of the blocking low).

C. How do SLC Bigs and CDC Bigs compare?

A snapshot of SLC and CDC bigs at T00 (near onset of snowfall) shows them to be very similar. The H5 patterns at T00 are nearly identical (relative to the station of interest). That is, if you take the SLC pattern and shift it south 250 miles, you've got the CDC pattern; a deep, open trough at H5 and H7, and identical 5490 m h5 heights over the respective stations. The surface patterns are also similar. Keep in mind that the favored storm track (whether for CDC or SLC) is for the H5 low or base of the H5 trough to track just west and south of the station, which in the case of CDC requires a deeper split in the flow.

D. How many Big and Super snowstorms did SLC and CDC have in common?

Answer: Very Few! (which attests to the differences between the two locations). During the period 1963-1989, Cedar City and Salt Lake City had 58 and 69 major snowstorms (total of supers and bigs) respectively. Of these 127 storms only seven were common to the 2 cities (i.e. occurred within 24 hours of each other). From this we can conclude that it is rare to have a heavy snow warning verify for both SLC and CDC, although it is not too uncommon for the SLC Weather Service office to issue a heavy snow warning for all of western Utah.

What types of storms were these that hit both ends of the states with heavy snows? Three of the seven occurred with blocking patterns, three occurred with deep slow-moving long-wave trough (similar to the SLC broad trough/winter Supers pattern) , and one with a progressive long-wave trough patterns. Considering the infrequency of deep blocking lows over the west coast, we can conclude that the chances of statewide heavy snow are highest with blocks. In these patterns a nearly stationary east-west cold front/baroclinic zone sets up over Utah which frequently sags south across the state. Snowfall under the band is fairly widespread and continuous as strong moist southwesterly flow is pumped over the dome of cold air.

E. How many of the CDC Bigs didn't fit the general pattern (north- northwest track, developmental)?

Out of the 48 CDC Bigs, all but seven of them did not occur with developmental troughs digging out of the north-northwest. Two of these odd-balls occurred when stationary closed lows off the California coast ejected out over the Great Basin. Two others occurred with a slow-moving broad trough over the Great Basin, similar to the broad trough/winter Supers pattern for Salt Lake City, but at a lower latitude. The remaining three exceptions all occurred during January of 1982 when a very deep, stationary, open trough parked over the west coast (in many regards similar to the blocking pattern described above, but without the closed low features).

F. Comprehensive review of all of the storm types discussed in this study.

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 key characteristics listed in bullet format for easy reference and comparison. They are listed in the following sequence: 1. CDC Supers, 2. CDC Bigs, 3. CDC Negative Tilt Supers, 4. Progressive Supers, 5. Blocking Supers, 6. SLC Supers, and 7. SLC Bigs. Begin by clicking on Comprehensive Review.