Likelihood of NWS Watches, Warnings, and Advisories in California on Days with Landfalling Atmospheric Rivers during 2007–2016

California (CA) is frequently influenced by landfalling atmospheric rivers (ARs) that may produce high-impact weather such as flooding, heavy precipitation, and mountain snowfall. The goal of this research is to determine the likelihood of high-impact weather associated with landfalling ARs in California as a function of latitude and intensity during a 10-year study from 2007–2016 in CA. The methodology employs an analysis of the spatial distribution of National Weather Service-issued watches, warnings or advisories (WWAs) over CA and their association with landfalling ARs. The “control” spatial analyses illustrate that liquid WWAs (e.g., floods and flash floods) are issued during both the cold season and warm season, whereas frozen WWAs (e.g., blizzards and winter storms) are primarily issued in the winter over the Sierra Nevada; flash flood WWAs are primarily issued in the warm season in southern CA.

Herein we focus on results for southern California and landfalling ARs at 32N. Composite analysis of the 10 days with the highest number of WWAs over southern California on days that also contained a landfalling AR at 32N illustrated a poleward extension of high integrated water vapor (IWV) >30 mm and integrated vapor transport (IVT) with magnitudes >500 kg m^–1 s^–1. This analysis motivated a further research into the likelihood of different types of WWAs across California on days with landfalling ARs at 32N associated with IVT magnitudes above different thresholds. For example, a flash flood or flood-related WWA occurs 15–30% of the time across the Transverse Ranges during all months when IVT magnitudes are >250 kg m^–1 s^–1, but occurs >50% of the time during December–February when IVT magnitudes are >500 kg m^–1 s^–1. A similar increase in the likelihood of WWAs related to winter weather phenomena is observed across the Sierra Nevada for AR landfalls of different intensities at 32N. Spatial analyses of these results and others will be presented.