Our methodology includes the use of the North American Regional Reanalysis (NARR) with a horizontal grid resolution of ~32 km for the period from 1979 through 2018. We produce a climatology of measurable precipitation for each NARR grid point that is seasonally adjusted. Based upon the identification of the 80th percentile of measurable three-day precipitation depths, these anomalously large precipitation depths are required to persist for a minimum of five days during successive seven-day periods. These grid points with persistent and extreme precipitation must be sufficiently numerous to produce a minimum 90th percentile areal coverage that persists for a minimum of seven consecutive days.
This definition of an extreme precipitation regime provides the basis for our multi-scale dynamic-thermodynamic analysis. The analysis consists of an examination of planetary-, synoptic-, and mesoscale environments that facilitate such regimes.
We find 15 such regimes with 10 occurring during the most recent 20 years. Such regimes are characterized by spatially elongated regions of extreme persistent precipitation. The principal results of our analysis include an elongated, quasi-persistent, dynamic tropopause jet and planetary-scale lower-tropospheric frontogenetic forcing extending across the region. Extreme precipitation tends to occur on the warm side of the planetary-scale jet in a region of vigorous lower-tropospheric poleward water vapor transports and anomalously high values of equivalent potential temperature. These extreme precipitation regimes are primarily associated with weak, temporally and spatially clustered mesoscale surface cyclones occurring along the zone of frontogenetic forcing. Such small-scale frontal cyclones occur primarily on the warm side of the jet in regions of extreme equivalent potential temperatures with weak stratification.
We offer a dynamical explanation for the preponderance of weak frontal cyclones that contribute to these extreme precipitation regimes. We also provide observational and theoretical explanations for an apparent upward secular trend in these extreme precipitation regimes.