This two-part study examines the damaging potential and genesis of low-level, meso-g-scale mesovortices formed within bow echoes. This was accomplished by analyzing quasi-idealized simulations of the 10 June 2003 Saint Louis bow echo event observed during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). This bow echo produced both damaging and nondamaging mesovortices. A series of sensitivity simulations were performed to assess the impact of low-and midlevel shear, cold-pool strength, and Coriolis forcing on mesovortex strength. By analyzing the amount of circulation, maximum vertical vorticity, and number of mesovortices produced at the lowest grid level, it was observed that more numerous and stronger mesovortices were formed when the low-level environmental shear nearly balanced the horizontal shear produced by the cold pool. As the magnitude of deeper layer shear increased, the number and strength of mesovortices increased. Larger Coriolis forcing and stronger cold pools also produced stronger mesovortices. Variability of ground-relative wind speeds produced by mesovortices was noted in many of the experiments. It was observed that the strongest ground-relative wind speeds were produced by mesovortices that formed near the descending rear-inflow jet (RIJ). The strongest surface winds were located on the southern periphery of the mesovortex and were created by the superposition of the RIJ and mesovortex flows. Mesovortices formed prior to RIJ genesis or north and south of the RIJ core produced weaker groundrelative wind speeds. The forecast implications of these results are discussed. The genesis of the mesovortices is discussed in Part II.

This two-part study examines the damaging potential and genesis of low-level, meso-g-scale mesovortices formed within bow echoes. This was accomplished by analyzing quasi-idealized simulations of the 10 June 2003 Saint Louis bow echo event observed during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX). In Part II of this study, mesovortex genesis was investigated for vortices formed at different stages of convective system evolution. During the early ''cellular'' stage, cyclonic mesovortices were observed. The cyclonic mesovortices formed from the tilting of baroclinic horizontal vorticity acquired by downdraft parcels entering the mesovortex. As the convective system evolved into a bow echo, cyclonicanticyclonic mesovortex pairs were also observed. The vortex couplet was produced by a local updraft maximum that tilted baroclinically generated vortex lines upward into arches. The local updraft maximum was created by a convective-scale downdraft that produced an outward bulge in the gust front position. Cyclonic-only mesovortices were predominantly observed as the convective system evolved into the mature bow echo stage. Similar to the early cellular stage, these mesovortices formed from the tilting of baroclinic horizontal vorticity acquired by downdraft parcels entering the mesovortex. The downdraft parcels descended within the rear-inflow jet. The generality of the mesovortex genesis mechanisms was assessed by examining the structure of observed mesovortices in Doppler radar data. The mesovortex genesis mechanisms were also compared to others reported in the literature and the genesis of low-level mesocyclones in supercell thunderstorms.

No abstract

The 22 March 2014 Oso landslide was one of the deadliest in U.S. history, resulting in 43 fatalities and the destruction of more than 40 structures. We examine synoptic conditions, precipitation records, and soil moisture reconstructions in the days, months, and years preceding the landslide. Atmospheric reanalysis shows a period of enhanced moisture transport to the Pacific Northwest beginning on 11 February 2014. The 21–42-day periods prior to the landslide had anomalously high precipitation; we estimate that 300–400 mm of precipitation fell at Oso in the 21 days prior to the landslide. Relative only to historical periods ending on 22 March, the return periods of these precipitation accumulations are large (25–88 yr). However, relative to the largest accumulations from any time of the year (annual maxima), return periods are more modest (2–6 yr). In addition to the 21–42 days prior to the landslide, there is a secondary maximum in the precipitation return periods for the 4 yr preceding the landslide. Reconstructed soil moisture was also anomalously high prior to the landslide, with return periods relative to the particular day that exceeded 40 yr about a week before the event.

Hydrologic models operated by the National Weather Service call for an accurate, consistent, high‐resolution, multi‐decade, continental‐scale record of hydrometeorological fields to serve as forcing data for model calibration. To serve this purpose, the Analysis of Record for Calibration was developed, and version 1.1 of the dataset is described in this study. Geospatial and scientific requirements, methods used in dataset generation, and input data sources are described. Given the prominent role of precipitation in model calibration, accurate and consistent precipitation is a particularly high priority for the analysis. To evaluate the analysis from this perspective, its daily precipitation is compared with surface observing stations over 43 years. The analysis exhibits low bias compared with other similar products. It also displays nonstationary bias behavior after 2015 due to the lack of a climatological constraint, as well as frequent occurrences of heavy‐to‐extreme precipitation that are often difficult to verify. These findings should be taken into account when the product is used for model calibration.