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Alain Roberge, John R. Gyakum, and Eyad H. Atallah

1. Introduction Intense precipitation during the cold season on the North American west coast is believed to often be caused by poleward-traveling extratropical cyclones ( Lackmann and Gyakum 1999 ). The amount of water vapor and heat transported is so important that it may cause significant flooding in the mountains ( Colle and Mass 2000 ; Neiman et al. 2002 ; Ralph et al. 2006 ). This is caused by the combination of intense orographic precipitation and fast snowmelt, which may also initiate

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Doug McCollor and Roland Stull

American Ensemble Forecast System (NAEFS; Toth et al. 2006 ). We investigate the quality and value of NAEFS temperature forecasts for 10 cities in western North America. Section 2 describes the forecasts and observations analyzed in the MREF study. Section 3 describes the verification metrics used to evaluate the probabilistic forecasts and discusses the results, and section 4 summarizes the findings and concludes the paper. 2. Methodology a. Ensemble forecasts Raw (unprocessed) temperature

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Renaud Barbero, John T. Abatzoglou, and Katherine C. Hegewisch

the potential utility of such forecasts for water resources and wildfire management (e.g., Hartmann et al. 2002 ). Recent studies have demonstrated the utility of statistical downscaling over raw GCM output for reproducing monthly climatic statistics and extremes for both climate scenarios ( Ning et al. 2012 ; Ahmed et al. 2013 ; Ning et al. 2015 ) and seasonal climate forecasting ( Yoon et al. 2012 ). To address this gap, we assess in this study the skill of North American Multimodel Ensemble

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Brian A. Colle and Michael E. Charles

1. Introduction a. Background This paper is the third in a series verifying extratropical cyclones within the operational models at the National Centers for Environmental Prediction (NCEP). In Parts I and II of this study, Charles and Colle (2009a , b , hereafter referred to as CC09a and CC09b) highlighted the performance of the North American Mesoscale (NAM), Global Forecast System (GFS), and Short-Range Ensemble Forecast (SREF) models around North America and its adjacent oceans for the

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Robert S. Gaza and Lance F. Bosart

314 WEATHER AND FORECASTING VOLUME5Trough-Merger Characteristics over North America ROBERT S. GAZA* AND LANCE F. BOSARTDepartment of Atmospheric Science, State University of New York at Albany, Albany, New York(Manuscript ~:eived 20 October 1989, in final form January 1990)ABSTRACT Split flow, defined by the presence of two separate westerly airstreams in the mid- and upper troposphere, iscommon in middle

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Katherine M. Willingham, Elizabeth J. Thompson, Kenneth W. Howard, and Charles L. Dempsey

1. Introduction During the North American monsoon season, convective storms in the southwestern U.S. Sonoran Desert produce damaging microbursts (e.g., Vasiloff and Howard 2009 ; Fig. 1 ). Severe winds from microbursts can threaten aviation operations, damage property, and interrupt communications, transportation, and electrical power transmission. Because of rapid urban expansion and the rising population density of central Arizona, specifically in the Phoenix metropolitan area, the

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Jeremiah O. Piersante, Russ. S. Schumacher, and Kristen L. Rasmussen

1. Introduction North and South America are home to two of the world’s largest mountain ranges: the Rocky and Andes Mountains. The elevation of these barriers, north–south orientation, and their position on the western side of the continents substantially influence the weather downstream because they modify midlatitude westerly and other associated flows. Interestingly, the regions east of the mountainous terrain in both continents are global hot spots for deep and organized convection, owing

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N. Vigaud, M. K. Tippett, J. Yuan, A. W. Robertson, and N. Acharya

regression (ELR; Wilks 2009 ) have recently provided probabilistic precipitation forecast skill estimates on S2S time scales over different parts of the globe including North America ( Vigaud et al. 2017a , b , 2018 ), but such approaches have yet to be applied to surface temperatures. In the ELR methodology proposed in Vigaud et al. (2017a) , calibration is done at the gridpoint level (i.e., a separate regression model is constructed for every location without using information from neighboring grid

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Shawn M. Milrad, Eyad H. Atallah, and John R. Gyakum

climatological North American storm tracks during the months of October–April are depicted to end in Atlantic Canada ( Fig. 2 ). For this study, St. John’s, Newfoundland, Canada, is chosen as the location of interest. In addition to being situated in a location commonly affected by several different storm tracks ( Fig. 2 ), St. John’s has the most precipitation events during our period of study (1979–2005) of all major stations in Atlantic Canada. This information is shown in Table 1 , where from 1979 to

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Shawn M. Milrad, Eyad H. Atallah, and John R. Gyakum

potential temperature gradient in the SLRV ( Fig. 2b ) and are, thus, conducive to ageostrophic frontogenesis ( Figs. 2a,b ). b. Transitioning tropical cyclones Hart and Evans (2001) found that 50% of landfalling tropical cyclones in the North Atlantic basin have undergone extratropical transition (ET) at some point in their life cycle. Many of these storms bring heavy rain and high winds to areas in eastern North America ( Hart and Evans 2001 ; Jones et al. 2003 ). Most early literature on ET focused

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