Areal Probability of Precipitation in Moist Tropical Air Masses for the United States

Cade Reesman,Rebecca D’Antonio,Kevin Gilmore,Paul Miller,Ben Schott,Chris Bannan

doi:10.3390/atmos12020255

Cade Reesman, Rebecca D’Antonio + Show 4 more

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https://doi.org/10.3390/atmos12020255

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Abstract

Moist tropical (MT) air masses routinely host convective precipitation, including weakly forced thunderstorms (WFTs). These short-lived, isolated events present recurring forecasting challenges due to their spatially small footprints and seemingly erratic behavior in quiescent warm-season environments worldwide. In particular, their activity is difficult to accurately characterize via probability of precipitation (POP), a common forecast product for the general public. This study builds an empirical climatological POP distribution for MT days over the continental United States using Stage IV precipitation estimates. Stage IV estimates within MT air masses between May–September (i.e., the boreal warm season) 2002–2019 are masked into precipitation (≥0.25 mm) and nonprecipitation (<0.25 mm) areas and standardized by the number of MT days. POPs are higher when MT air masses are present. For the Southeast U.S., POP generally increases ~15% compared to the overall warm-season value. At 1800 UTC (1–2 PM LT) daily, POPs are confined to coastal areas and east-facing ridges, and advance inland by 2100 UTC (4–5 PM LT). Climatologically, ~50% of the warm-season precipitation in the Southern U.S. occurred in MT environments. This study emphasizes the need for better forecasting tools and climatological analyses of weakly forced environments.

Highlights

Despite advances in high-resolution, convection-allowing numerical weather prediction systems, operational meteorologists still struggle to reliably forecast afternoon precipitation in moist tropical air masses
Moist tropical air masses are often conducive to convection, during the warm season
When moist tropical (MT) air masses are displaced from midtropospheric dynamical forcing, as is often the case for the summertime Southeast U.S, weakly forced thunderstorms (WFTs) are the prevailing convective mode due to the unstable, weakly sheared convective environment

Summary

Introduction

Despite advances in high-resolution, convection-allowing numerical weather prediction systems, operational meteorologists still struggle to reliably forecast afternoon precipitation in moist tropical air masses. As the jet stream retreats poleward during the midlatitude warm season, synoptic dynamical forcing for convection becomes less frequent and thunderstorm activity predominantly develops due to diurnal heating and associated parcel buoyancy [1]. In these weakly forced settings, convective cells are typically conceptualized as small, short-lived, and isolated [2], earning them the designation weakly forced thunderstorms (WFT) [1]. Operational forecasters in the United States openly acknowledge the challenges of forecasting WFT-related phenomena [3] with forecast verification metrics confirming this attitude via depressed success statistics [4,5]

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