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Paul J. Roebber, James M. Frederick, and Thomas P. DeFelice

, the strong composite 500-hPa Pacific jet splits over North America, with the southwesterly (northwesterly) subtropical (polar) jet positioned over the southern United States (northern Canada), leading to confluence downstream of the U.S. midwest. The anomalous nature of this split flow is indicated by the strong positive 500-hPa height anomalies located over Canada ( Figs. 5a and 5b ). Correspondingly, composite surface conditions ( Figs. 6a–d ) were relatively quiescent with broad-scale high

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J. J. Simpson, G. L. Hufford, R. Servranckx, J. Berg, and D. Pieri

event demonstrate that it is an analog of the April 2001 event. See the recent Journal of Geophysical Research special issue (2001, vol. 106, No. D16) for details of the April 1998 event. a. The April 2001 dust event 1) Satellite evidence and time line A time series of the TOMS aerosol index ( Figs. 1a–h ) shows the evolution and dispersion of silicate dust from its east Asia source, across the Pacific basin, and into North American airspace from 7 to 14 April 2001. Aerosol index values as high as

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Richard H. Grumm

results also revealed that the errors in AVN position forecasts of surface cyclones were smaller thanthose found in the NMC Nested Grid Model (NGM). The geographical distribution of the pressure errors weresimilar to those found in the NGM over eastern North America and the adjacent western Atlantic Ocean.Negative pressure errors, indicative of overdeepening of surface cyclones, dominated the mountainous regionsof western North America. Positive pressure errors, indicative of underdeepening of surface

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S. Gubler, K. Sedlmeier, J. Bhend, G. Avalos, C. A. S. Coelho, Y. Escajadillo, M. Jacques-Coper, R. Martinez, C. Schwierz, M. de Skansi, and Ch. Spirig

Hemisphere. For South America, two studies by Coelho et al. (2006a , b) based on both empirical and multimodel approaches showed that the best seasonal precipitation prediction quality during austral summer is found in the tropics and the region around southern Brazil, Uruguay, Paraguay, and northern Argentina. A recent study by Osman and Vera (2017) on both temperature and precipitation confirms the highest performance in the tropics during DJF for temperature, and states that precipitation

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David M. Schultz and Charles A. Doswell III

west coast of North America. As the tropospheric flow crossing the Rocky Mountains increases, lee troughing occurs, fixed to the lee slopes of the mountains by warming due to dry-adiabatic descent. Eventually, a mid- to upper-tropospheric absolute vorticity maximum (i.e., short-wave trough or jet streak) may couple with the lee trough, causing the lee trough to become mobile (departure). A review of previous literature on lee troughing and lee cyclogenesis can be found in Steenburgh and Mass (1994

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Mark R. Sinclair

Whittaker and Horn (1984) and indicates two principal regions of cyclone activity; one extending from near Japan toward the Gulf of Alaska and the other spiraling poleward from east of North America toward Iceland and the Arctic Ocean. Other localized maxima are seen over the Mediterranean Sea, the Great Lakes, east of the the Rockies, east of the Urals (near 80°E), and near the Caspian Sea. In all these details, Fig. 9a is quite similar to Fig. 2a of Whittaker and Horn (1984) , based on manual

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William H. Klein and Hal J. Bloom

1988) ABSTRACT This paper describes an operational system for specifying monthly precipitation amounts in the contiguousUnited States from the concurrent 700-mb monthly mean height field over North America and adjacent oceans.Multiple regression equations are derived for each month of the year at 60 climate divisions by applying a quasiobjective forward selection procedure to 30 yr of data for 1951-80. The resulting specification equations explainan average of 37% of the

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Frederick Sanders and Eric G. Hoffman

large thermal inertia keeps surface air temperatures from varying strongly, and over the adjacent land, where the temperatures can vary widely depending on season, time of day, and the meteorological circumstances. A third region of maximum frequency of strong baroclinic zones, less well defined than the others, runs along the eastern slopes of the North American Cordillera from Montana through Wyoming and Colorado to New Mexico. The frequency in each of these states is larger than in the adjacent

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Michelle L. L’Heureux, Michael K. Tippett, Ken Takahashi, Anthony G. Barnston, Emily J. Becker, Gerald D. Bell, Tom E. Di Liberto, Jon Gottschalck, Michael S. Halpert, Zeng-Zhen Hu, Nathaniel C. Johnson, Yan Xue, and Wanqiu Wang

forecast skill were negative, the distribution would typically be set to climatology. Here, the forecast skill is estimated using the 1982–2010 hindcasts of the Niño-3.4 index from the North American Multimodel Ensemble (NMME; see more details below). Therefore, with only the forecaster input of the mean Niño-3.4 index, probabilities can be obtained for any number of categories or for exceeding any specified threshold. Within this paper, this proposed forecast strategy will be referred to as CPCcalib

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Matthew B. Switanek, Joseph J. Barsugli, Michael Scheuerer, and Thomas M. Hamill

dominant modulator of seasonal-to-interannual precipitation and temperature variability for much of the world ( Ropelewski and Halpert 1987 ) and North America in particular ( Redmond and Koch 1991 ; Cayan et al. 1999 ; Guo et al. 2017 ; Kumar and Chen 2017 ). This has led many in the modeling community to focus on improving dynamical weather models’ representation of ENSO and its influence on global climate ( Vitart 2014 ; Zhu et al. 2015 ). However, recent progress has been slow in improving

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