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Richard H. Grumm and Anthony L. Siebers

operationalnested grid model (NGM) run at the National Meteorological Center (NMC). Preliminary results covering aperiod from 1 December 1988 to 31 August 1989 reveal that the NGM predicts the central pressure of surfaceanticyclones to be too low over much of central and eastern North America during the winter and spring,especially along the track of transient anticyclones. The NGM tends to predict surface anticyclone pressure tobe too high over the eastern Pacific and portions of the western Atlantic during

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Michael E. Charles and Brian A. Colle

number of extratropical cyclones have been verified around North America within the National Centers for Environmental Prediction (NCEP) operational models. For example, Silberberg and Bosart (1982) investigated forecast errors within the Limited-Area Fine Mesh model (LFM) in 1972, which was operational at 190.5-km horizontal grid spacing. They found that the LFM underdeepened Atlantic and Pacific cyclones by 6–10 mb at forecast hour 48, and overdeepened cyclones to the east of the Rockies through

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Blaine C. Thomas and Jonathan E. Martin

1. Introduction One of the most significant synoptic-scale winter weather phenomena affecting central North America is the Alberta clipper, defined in the Glossary of Meteorology ( Glickman 2000 ) as “a low pressure system that is often fast moving, has low moisture content, and originates in western Canada (in or near Alberta province).” Clippers 1 develop in the lee of the Canadian Rockies, a region characterized by a relative maximum in cyclogenesis events ( Petterssen 1956 ; Reitan 1974

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Jason M. Cordeira, Nicholas D. Metz, Macy E. Howarth, and Thomas J. Galarneau Jr.

influence high-impact weather events over North America (e.g., Archambault et al. 2013 ). The cause of these large-scale flow reconfigurations is often attributed to synoptic-scale transient disturbances that contribute to the onset of blocking (e.g., Sanders and Gyakum 1980 ; Shutts 1983 , 1986 ; Colucci 1985 , 1987 ; Mullen 1986 ; Pelly and Hoskins 2003 ; Berrisford et al. 2007 ), the development of persistent large-scale positive and negative geopotential height anomalies (e.g., Namias 1955

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Steven L. Mullen and Bruce B. Smith

form 15 December 1992) ABSTRACT Sea level cyclone errors for two contrasting planetary-scale flow regimes, a long-wave trough verses a longwave ridge over western North America, are computed for the National Meteorological Center's Nested GridModel (NGM) and "Aviation Run" of the Global Spectral Model (AVN). The study is performed for the1987/88 and 1989/90 cool seasons (1 December-31 March). All available 24- and 48-h forecast cycles areanalyzed for North America and

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Andrew C. Winters, Daniel Keyser, and Lance F. Bosart

downstream large-scale flow pattern over North America. To this end, Griffin and Martin (2017) employed time-extended EOF analyses (e.g., Weare and Nasstrom 1982 ; Wilks 2011 , chapter 12) of 250-hPa zonal wind data from the NCEP–NCAR reanalysis dataset ( Kalnay et al. 1996 ) to construct composite analyses of the large-scale flow evolution over the North Pacific and North America during the 10-day period preceding and following the development of each NPJ regime. The Griffin and Martin (2017

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Trevor I. Alcott and W. James Steenburgh

, personal communication), but does not explicitly account for riming or processes occurring below cloud base. The method also relies upon vertical velocities from the North American Mesoscale (NAM) or Global Forecast System (GFS) models, which fail to predict the distribution and intensity of terrain-induced vertical motions in regions where the topography is poorly resolved. The aforementioned studies use data from numerous sites to construct their SLR algorithms. While studying more than one site

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Frederick Sanders

cyclngenesis was evaluated for the period I September 1986-30 April1987. Manual analyses covering the eastern North Pacific, North America and the North Atlantic eastward to20-W were used as ground truth. The criterion for a bomb event in the analyses of the forecasts was a deepeningof the center of at least 24 mb at '60-N, normalized geostrophically at other latitudes, in a period of 24 h,beginning at 0000 or 1200 UTC. Both models displayed skill out to 48 h for the NGM and 60 h for the GLBL. The NGM

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Rebecca M. Westby and Robert X. Black

1. Introduction Anomalous temperature regimes (ATRs) occurring during the boreal cool season [December–January( DJF)], including cold-air outbreaks (CAOs) and warm waves (WWs), are common phenomena over eastern North America ( Konrad and Colucci 1989 ), as well as other areas of the world ( Ashcroft et al. 2009 ; Schultz et al. 1997 , 1998 ). ATRs are generally defined as events where temperatures are significantly above (WWs) or below (CAOs) average on a synoptic time scale (2–7 days). These

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David A. Lavers, Ervin Zsoter, David S. Richardson, and Florian Pappenberger

), 3) tropics (between 20°N and 20°S), 4) Europe (35°–75°N, 12.5°W–20°E), 5) North America (25°–70°N, 170°–50°W), and 6) western North America (25°–70°N, 170°–117°W). The statistical significance of the differences in ROC areas between IVT and precipitation was investigated using a bootstrap procedure. The bootstrap process involved resampling the 253 forecasts (for a particular forecast day) with replacement and calculating the resultant ROC area and then repeating the process 1000 times. 3

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