Weather Regime-Dependent Predictability: Sequentially Linked High-Impact Weather Events over the United States during March 2016

The large-scale flow pattern across the North Pacific, North America, and the Western North Atlantic during the latter half of March 2016 was characterized by frequent cyclonic wave breaking (CWB).  This large-scale flow pattern enabled three sequentially linked high-impact weather events to occur over the continental (CONUS) United States (U.S.).  The first high-impact weather event was a challenging-to-predict cyclogenesis event on 23–24 March in the central Plains that resulted in both a major snowstorm along the Colorado Front Range and a severe weather outbreak over the central and southern Plains.  The second high-impact weather event was a severe weather outbreak that occurred over the Tennessee and Ohio River Valleys on 27–28 March.  The third high-impact weather event was the development of well below normal temperatures over the eastern U.S. that followed the formation a high-latitude omega block over northwestern North America from 28 March to 1 April.  This omega block established a planetary-scale flow environment favorable for the transport of arctic air in early April after a comparatively winterless winter.

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.