Contribution of large-scale midlatitude disturbances to hourly precipitation extremes in the United States

Renaud Barbero,John T Abatzoglou,Hayley J Fowler

doi:10.1007/s00382-018-4123-5

Abstract

Midlatitude synoptic weather regimes account for a substantial portion of annual precipitation accumulation as well as multi-day precipitation extremes across parts of the United States (US). However, little attention has been devoted to understanding how synoptic-scale patterns contribute to hourly precipitation extremes. A majority of 1-h annual maximum precipitation (AMP) across the western US were found to be linked to two coherent midlatitude synoptic patterns: disturbances propagating along the jet stream, and cutoff upper-level lows. The influence of these two patterns on 1-h AMP varies geographically. Over 95% of 1-h AMP along the western coastal US were coincident with progressive midlatitude waves embedded within the jet stream, while over 30% of 1-h AMP across the interior western US were coincident with cutoff lows. Between 30–60% of 1-h AMP were coincident with the jet stream across the Ohio River Valley and southeastern US, whereas a a majority of 1-h AMP over the rest of central and eastern US were not found to be associated with either midlatitude synoptic features. Composite analyses for 1-h AMP days coincident to cutoff lows and jet stream show that an anomalous moisture flux and upper-level dynamics are responsible for initiating instability and setting up an environment conducive to 1-h AMP events. While hourly precipitation extremes are generally thought to be purely convective in nature, this study shows that large-scale dynamics and baroclinic disturbances may also contribute to precipitation extremes on sub-daily timescales.

Highlights

Precipitation extremes have received much attention due to the hazards they pose to human lives and infrastructure (Ashley and Ashley 2008; Terti et al 2017)
While cutoff lows (COL) contribute to approximately 5–25% of annual precipitation at Hourly precipitation data (HPD) stations across the interior western United States (US), approximately 25–50% of 1-h annual maximum precipitation (AMP) were coincident with COL (Fig. 4, left panel)
Most 1-h AMP are associated with jet stream events along the western coastal states windward of the primarily north-south topographic barrier extending from the coast ranges in southern California, northward through the Sierra Nevada mountains and Cascades, while this contribution drops off in the interior western US (Fig. 4, right panel)

Summary

Introduction

Precipitation extremes have received much attention due to the hazards they pose to human lives and infrastructure (Ashley and Ashley 2008; Terti et al 2017). We can look to some well-known events where COL were partly responsible for extreme precipitation events on daily or even shorter timescales These include Fayetteville, North Carolina on 29 September 2016, where a COL advected moisture from a tropical storm and was responsible for over 250 mm of precipitation in less than 24 hours, triggering severe flash flooding in some areas. Another example is Boulder, Colorado on 12 September 2013, where the absolute record of 1-day precipitation accumulation coincided with a COL In view of these examples, it is of interest to quantify the contribution of these organized midlatitude systems to hourly precipitation extremes, given the importance of this timescale to storm impacts (Terti et al 2017). This research question was addressed by pairing hourly precipitation observations across the conterminous US (CONUS) along with chronologies of locations of the jet stream and COL events from 1979–2011

Data and methods

Results

Concluding remarks