In the above-mentioned Salt Lake City study, frequency of Big and Super snowstorms was based on the period 1963-1989 (the period for which composite data was available). In order to make a fair comparison, the same period was used for this study.
II. SUMMARY OF RESULTS
A. CLIMATOLOGY RESULTS
A1. Logan, but not by much. During the period 1963-1989, Logan logged a total of 74 major snowstorms, but Salt Lake City was very close on its heels with 69 such storms. -both got the same number of Supers (12) during the period 1963-1989, -Logan had a few more Bigs (62 Logan vs 57 for SLC))
Q2. How frequently do both Logan and Salt Lake City get major snowfalls (6+ inches) from the same storm?
A2. Over one-third (38%) of the time. Thirty`eight percent of the storms where Logan received 6+ inches of snow in 24 hours, Salt Lake City also received 6+ inches in 24 hours (in a few cases the Salt Lake City 24 hour period was offset slightly...shifted forward or back 12 hours). In other words, 28 out of the 74 major storms in Logan were also major storms in Salt Lake City.
Q3. How frequently does Salt Lake City receive significant snowfall (3+ inches) from the same storm that gave Logan 6+ inches?
A3. The majority (69%) of the time. Most of the storms (69%) where Logan received 6+ inches of snow in 24 hours, Salt Lake City received at least 3 inches in the same 24 hour period. In other words, for 51 out of the 74 major storms in Logan, the Salt Lake airport got at least 3 inches of snow during the same period.
Q4. How do seasonal distribution of major snowstorms compare for Logan and Salt Lake City?
A4. Logan gets fewer major snowstorms in spring than Salt Lake, but more during the winter. See Table 1.
B. COMPOSITE RESULTS
A. Yes, a composite study suggests there are two possible scenarios where Logan gets hit by heavy snows, but Salt Lake is left out in the cold. A search was made for all snowstorms during the period 1963-1989 where Logan got 6+ inches, while Salt Lake City received little if any snow. Storms meeting the following criteria were used: 1) Salt Lake received less than 1 inch during the same 24 hour period, and 2) Salt Lake City received 2 inches or less during any adjacent 24 hour period. There were 10 such cases (See Table 2). Half (5) of these occurred with stationary baroclinic bands (SBBs). Three of them occurred with what may be called a weak rex block (RB) pattern. The remaining two had no outstanding characteristics (ODD).
In this scenario, a deep trough parks off the southern coast of British
Columbia with a strong jet and baroclinic band draped across the west from
central California to northern Utah. This stalled baroclinic zone can lead
to heavy precipitation in a number of different ways. Although composites
do not offer enough detail to determine the exact physical mechanism for
snowfall, they do suggest the following possible mechanisms:
1. frontogenesis: strong vertical motions can result from tightening of the baroclinic zone.
2. "overrunning" precipitation can be the primary cause of heavy precipitation in the Cache Valley if a distinct cold front stalls between Logan and Salt Lake City. In such a pattern, areas south of Ogden tend to have a hard time picking up significant precipitation in southerly pre-frontal surface flow while areas from Ogden north can get significant precipitation. Forecasters at the Salt Lake weather office may see "overrunning" as the easiest explanation for the stark difference in snowfall between the Salt Lake and Cache Valleys, but composites do not provide clear evidence of this mechanism. Although composites suggest the presence of a surface trough/cold front extending from Wendover to Ogden at the time of onset of snowfall, they also indicate cold air advection at 700 mb instead of overrunning, warm air advection.
3. jet streak circulations: the presence of jet streaks rippling through the baroclinic zone can produce secondary circulations and vertical motion. It is interesting to note that the composites place Logan under the favored front left exit region of the jet, while Salt Lake City lies under the less favored south (anticyclonic) side of the jet.
See also 4-panels of SBBs at T-12 hours and T+12 hours.
Three of the 10 cases where Logan got hammered, but Salt Lake City remained virtually untouched, were a rather interesting pattern reminiscent in appearance to a weak Rex Block. In this scenario, a large high develops over western Canada and a weak low off the California coast. Northern Utah lies under the confluence of the polar and subtropical jets in the lee of the west coast block. Although there is no surface development evident in the composites, warm air advection has developed as the leading edge of 700 mb cold air has nosed into extreme northern Utah. A very strong northeasterly surface gradient keeps the cold air in place while moist southwest H7 flow overruns the cold dome. This is a clear case of "overrunning" precipitation as Nevada and Utah lie under a ridge at 700 mb and no upper level dynamics are evident.
See also 4-panels of RBs at T-12 hours and T+12 hours.
A good test case for "scenario 1" occurred this past winter and lends credibility to the composite results. On December 21, 1996, the "Stationary Baroclinic Band" scenario set up just as described above and verified remarkably well. Cache Valley received around one and a half feet of snow, neighboring mountains over 4 feet, while little if any snow fell south of Ogden.
III. SUMMARYLogan and Salt Lake City, as expected, have very similar frequencies of major snowstorms. Despite the similar climatology, seasonal distribution varies somewhat with Logan getting more during the winter and fewer during the spring than Salt Lake City. A composite study contrasting the Cache and Salt Lake Valleys suggests two scenarios, the Stationary Baroclinic Band, and the Rex Block, where Logan gets heavy snow but Salt Lake City little if any snow. The common link between these two scenarios is the stationary nature of the synoptic weather pattern with "overrunning" a possible lift mechanism. It is hoped that attention to these possible scenarios by forecasters can help to determine their usefulness as forecasting tools.