The challenging nature of the Colorado snowstorm forecast was evidenced by the difficulty that forecasters and models alike had in placing the location of the expected heavy snow far enough westward to along the immediate Front Range until 12–36 h before the event. A science opportunity is to better understand what dynamical and physical processes govern the spatiotemporal distribution of sensible weather element forecast uncertainty in a region of complex terrain. Although the 23–24 March and 27–28 March severe weather outbreaks occurred in conjunction with discontinuous 500-hPa trough retrogression, the downstream responses differed. In the first severe weather outbreak, the leading 500-hPa trough associated with the aforementioned Colorado snowstorm lifted out to the northeast and weakened before reaching the Appalachians while a trailing trough deepened southeastward across the western CONUS. In the second severe weather outbreak, a leading 500-hPa trough was able to maintain its intensity as it moved eastward and crossed the Appalachians as the trailing 500-hPa trough dropped southward along the West Coast and evolved into a cutoff cyclone over the Southwest instead of moving eastward as the previous 500-hPa trough did.
We hypothesize that during discontinuous retrogression over North America when a leading 500-hPa trough weakens and lifts northeastward the associated region of warm-air advection is able to spread northeastward across eastern Canada and the Northeast where it can become mostly removed from the corridor of warm, moist unstable air ahead of the deepening trailing upstream trough. In this case, stratiform precipitation amounts can diminish rapidly from west to east across the central and northern Appalachians in response to loss of a moisture source, weakening forcing for ascent, upper-level ridging, and downslope west-southwesterly flow. We also hypothesize that discontinuous retrogression patterns across the CONUS may be more common during El Nino cool seasons, favor mountain snows over the Rockies, contribute to significant upslope-related precipitation episodes east of the Rockies, support severe weather across the southern and central Plains, produce heavy rain over parts of the Tennessee and Ohio Valleys, and lead to relatively warm and dry conditions east of the Appalachians. Evidence in support of these hypotheses will be presented in conjunction the CWB-related predictability issues for the three high-impact weather events. A subsequent presentation by Winters et al. (2017) will examine the regime-related predictability issues associated with the development of high-impact temperature and precipitation events over the CONUS.