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Sean Stelten and William A. Gallus Jr.

(RUC) when the forcing mechanism was a synoptic-scale front rather than a smaller-scale feature like an outflow boundary. The challenge becomes even greater when the initiation takes place some distance away from any synoptic forcing feature. In these cases, smaller-scale features with lower predictability drive initiation, including indirect effects of surface boundaries such as positive thermal advection, as the Great Plains southerly low-level jet (LLJ) rises over those boundaries. The LLJ, a

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Aaron Johnson, Xuguang Wang, and Samuel Degelia

1. Introduction One of the goals of the Plains Elevated Convection At Night (PECAN; Geerts et al. 2017 ) experiment is to improve predictions of nocturnal convection and related features, including mesoscale convective systems (MCSs), convective initiation (CI), bores, and low-level jets (LLJs) “ with a particular focus on the next generation convective-permitting models and advanced assimilation techniques ” ( Parsons et al. 2013 ). Comprehensive observations in the PECAN Integrated Sounding

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Aaron Johnson and Xuguang Wang

layer ( Parker 2008 ; Trier et al.2011 ; Marsham et al. 2011 ; Coleman and Knupp 2011 ; Erlingis and Barros 2014 ) and low-level jets (LLJs; Carbone et al. 1990 ; Trier and Parsons 1993 ; Trier et al. 2006 ; Tuttle and Davis 2006 ; Coniglio et al. 2010 ; Trier et al. 2010 ; French and Parker 2010 ). The notoriously low skill of warm-season quantitative precipitation forecasting is due in large part to the low predictability of nocturnal convective initiation (CI) and subsequent evolution

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Jonathan E. Thielen and William A. Gallus Jr.

remain substantial areas for improvement, particularly in the depiction of finer-scale details of system morphology ( Snively and Gallus 2014 , hereafter SG14 ; Gallo et al. 2017 ; Carlberg et al. 2018 ). The development and life cycle of nocturnal MCSs are closely associated with the low-level jet (LLJ) that frequently develops at night over the Great Plains during the spring and summer months ( Pitchford and London 1962 ; Bonner 1968 ; Augustine and Caracena 1994 ; French and Parker 2010

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