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David R. Novak, Lance F. Bosart, Daniel Keyser, and Jeff S. Waldstreicher

= −12 h are summarized in Fig. 5 . A surface cyclone is found in the Carolinas ( Fig. 5a ) associated with a 500-hPa trough ( Fig. 5b ). There is evidence that the cyclone is deepening since the disturbance tilts toward the west with height, and progressively exhibits a more open structure with height ( Figs. 5a–c ). A strong 300-hPa jet is found at the base of the trough, and another 300-hPa jet is found in an area of confluence off the northeast U.S. coast ( Fig. 5b ). This pattern resembles the

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John M. Lanicci and Thomas T. Warner

particular event occurred on 8May 1983 and was associated with a strong intrusionof low-level moisture through the Rio Grande valley;lid soundings over Del Rio and Brownsville, Texas;and a short-wave trough moving through the subtropical jet across northern Mexico. The return-flow stage is represented by weak returnflow with anticyclonic flow aloft (Fig. 8c). In this composite analysis, there is evidence of two separatelyemerging EML areas over Nebraska and Texas. Theseareas may have separate origins

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Yuh-Lang Lin, Sen Chiao, Ting-An Wang, Michael L. Kaplan, and Ronald P. Weglarz

. Chu and Lin (2000) also found that heavy upslope rain might occur in the first two regimes. Recently, Chen and Lin (2001) have extended the work of Chu and Lin (2000) to include a three-dimensional mountain and found that heavy upslope rainfall may be produced in the presence of a low-level jet (LLJ). This is consistent with what is often observed in heavy orographic rainfall events, which occur in the United States (e.g., Maddox et al. 1978 ; Pontrelli et al. 1999 ), the European Alps (e

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Andrew Hazelton, Zhan Zhang, Bin Liu, Jili Dong, Ghassan Alaka, Weiguo Wang, Tim Marchok, Avichal Mehra, Sundararaman Gopalakrishnan, Xuejin Zhang, Morris Bender, Vijay Tallapragada, and Frank Marks

-globalnest ( Figs. 10a–e ), especially at 072 h and shorter lead times, but the operational GFS generally missed the magnitude of the asymmetric wind maximum ( Figs. 10f–h ). The structure of the wind maximum, as well as the position of Humberto in the right-entrance region of a jet streak, suggest the presence intrusion described as a “sting jet” (e.g., Browning and Field 2004 ; Stewart 2020 ). Taking a closer look at some of the relevant variables, including relative humidity and reflectivity ( Fig. 11

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Todd D. Sikora, Karen S. Friedman, William G. Pichel, and Pablo Clemente-Colón

1. Introduction Intense mesoscale cyclones that form in cold, marine air masses poleward of major jet streams and frontal zones have inspired numerous modeling and observational studies over the past several decades. Businger and Reed (1989) provide an early review of the literature while Heinemann and Claud (1997) discuss more recent research efforts. Names that have been given to these disturbances include polar lows, comma clouds, arctic instability lows, arctic hurricanes, and cold air

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Luca Mathias, Volker Ermert, Fanni D. Kelemen, Patrick Ludwig, and Joaquim G. Pinto

-sustaining ( Houze 2004 ). The convective line of an MCS can be connected to leading, trailing, or parallel stratiform precipitation ( Parker and Johnson 2000 ). Linear MCSs also exhibit a variety of distinct radar echo patterns, such as a squall line, bow echo structures ( Fujita 1978 ), or line echo wave patterns ( Nolen 1959 ). Fujita (1978) first described the typical life cycle of a bow echo and its dynamical characteristics, such as the rear-inflow jet (RIJ) and the downbursts at the bow’s apex. Bow

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Michael Layer and Brian A. Colle

; Mass and Dotson 2010 ), isallobaric wind ( Crupi 2004 ; Niziol and Paone 2000 ), tropopause folds ( Browning and Reynolds 1994 ; Schultz and Meisner 2009 ), and sting jets ( Browning 2004 ). Some of these mechanisms are likely not common over the northeastern United States, such as sting jets, which typically result from the bent-back front of landfalling oceanic cyclones making landfall along the western coast of Europe ( Browning 2004 ) and the U.S. Pacific Northwest ( Steenburgh and Mass 1996

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Peter Knippertz and Jonathan E. Martin

isentropic or moist-isentropic) trajectories relative to the movement of the center of the cyclone ( Carlson 1980 ; Browning 1990 ). The WCB is located ahead of the surface cold front, where strong horizontal temperature gradients cause the formation of a front-parallel low-level jet through the thermal wind relationship. Typically the warm and moist WCB air originates in the planetary boundary layer (PBL), often over a warm ocean surface, and then ascends and contributes to the formation of frontal

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Steven E. Koch and Christopher O’Handley

properties. Another class of gravity waves known as lee waves, which are forced by orography, is fundamentally important to the maintenance of the global momentum balance and to the energetics of the upper atmosphere. Gravity waves are excited by a wide range of phenomena, including convection, density impulses, geostrophic adjustment associated with unbalanced jet streaks and strong frontal systems, topography, and vertical shear instability. Some forecasters are aware that a particular class of gravity

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Kathleen L. McInnes, John L. McBride, and Lance M. Leslie

situations where the manual analyses gave the front a double structure including a prefrontal trough,the numerical analysis-prognosis system combined these into one sharp trough. Cross sections normal to thefrontal surface reveal much deeper cold air and a stronger and deeper warm-air jet than the equivalent castwest sections. Isentropic relative flow diagnostics reveal close agreement with the equivalent diagnostics in theAustralian Cold Fronts Research Programme.1. Introduction One of the most

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