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Sean Stelten and William A. Gallus Jr.

-ARW model with 4-km horizontal grid spacing and 50 vertical levels. The domain spanned 1200 km × 1200 km and was centered on the initiation location of the observed PNCI. Initial and lateral boundary conditions were provided every 6 h from 12-km North American Mesoscale Forecast System (NAM) analyses. Runs began at 1200 UTC the day of the nocturnal initiation event, to capture the diurnal evolution of the boundary layer prior to the nocturnal events. The four PBL schemes used included two local mixing

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Aaron Johnson, Xuguang Wang, and Samuel Degelia

b ), and part of night 1 at the 12.7 mm h −1 threshold ( Fig. 6c ). The 1900 UTC ensemble forecast skill is generally less than or equal to the 1300 UTC skill, despite the later initialization time ( Fig. 6 ). The skill of the operational (using 12-km grid spacing and cumulus parameterization) North American Mesoscale (NAM 1 ; Janjić 2003 ) model 3-hourly forecast precipitation is plotted in Figs. 6d–f , along with the 3-hourly precipitation forecasts from the PECAN forecasts for comparison

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Jonathan E. Thielen and William A. Gallus Jr.

.0.CO;2 . 10.1175/1520-0434(1994)009<0116:LTPTNM>2.0.CO;2 Benjamin , S. G. , and Coauthors , 2016 : A North American hourly assimilation and model forecast cycle: The Rapid Refresh . Mon. Wea. Rev. , 144 , 1669 – 1694 , . 10.1175/MWR-D-15-0242.1 Bluestein , H. B. , and M. H. Jain , 1985 : Formation of mesoscale lines of precipitation: Severe squall lines in Oklahoma during the spring . J. Atmos. Sci. , 42 , 1711 – 1732 ,

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Aaron Johnson and Xuguang Wang

used in this study. The GSI-based DA system uses a 40-member ensemble based on the Advanced Research version of WRF (ARW) version 3.6.1 with the EnKF configuration described in Johnson et al. (2015) . Conventional surface and upper-air observations from the data stream of the operational North American Mesoscale Forecast System at the National Centers for Environmental Prediction (NCEP), including surface and mesonet stations, Aircraft Communication, Addressing, and Reporting System (ACARS), NOAA

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