Probable Maximum Precipitation: The Smethport Challenge

Abstract

The 18 July 1942 Smethport, Pennsylvania storm produced US and world record rainfall accumulations of 165 mm in 10 min, 240 mm in 25 min and 780 mm in 270 min. Observations from the Smethport storm control sub‐daily Probable Maximum Precipitation (PMP) estimates over a large region of the eastern US. The 18 July 1942 storm is the archetypal example of a Smethport‐Like storm. A prominent feature of Smethport‐Like storms is that they are Northwest Flow events, a class of storm systems linked to development of the summer ridge over the US. The Northwest Flow setting for these storms results in characteristic properties of space‐time variability of rainfall extremes. Polarimetric radar analyses for recent Smethport‐Like storms illustrate the evolution of mesoscale convective systems producing extreme rainfall and provide insights into physical processes associated with peak rainfall rates from the Smethport storm. Smethport‐Like storms are on the extreme high end of the convective intensity spectrum, with storm rotation often contributing to large, intense updrafts. Synoptic scale, mesoscale and storm‐scale properties of Smethport‐Like storms combine to create a near‐optimal environment for extremes of water vapor convergence, a principal ingredient of record, short‐duration rainfall. The central element of the Smethport Challenge is to accurately simulate the climatology of rainfall extremes for Smethport‐Like storms using convection‐permitting climate models.