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James T. Moore, Fred H. Glass, Charles E. Graves, Scott M. Rochette, and Marc J. Singer

near the maximum in θ e advection at 850 hPa, on the southwest edge of the maximum in upper-level (200–300 hPa) divergence, and on the cool side of a west–east-oriented quasi-stationary surface boundary. Both studies emphasized the importance of the southerly low-level jet (LLJ) in organizing and sustaining the elevated convection. Glass et al. (1995) noted that the strongest cases of elevated MCSs with heavy rainfall had an LLJ that was coupled with the 850-hPa θ e advection maximum, and

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Stephen F. Corfidi

component governed by the rate and location of new cell formation relative to existing convection. Building on this idea, and extending the work of Merritt and Fritsch, Corfidi et al. (1996) showed that the propagation component is, in many cases, directly proportional but opposite in direction to the low-level jet. This finding is somewhat surprising given that MCS propagation can be influenced by a myriad of factors such as the distribution of convective available potential energy (CAPE), convective

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Paul J. Croft and Alan E. Gerard

investigation of the event revealed several features that could have made a difference in operational forecasts. These included jet streak features, a midlevel moisture pool, and a deformation zone as identified through satellite imagery and model diagnostics. The event's mesoscale duration (several hours) and extent (central Mississippi), and the inability of both long- and short-range models to capture it, were also of interest in this regard. In fact, only the late run (0000 UTC 14 Dec 1997 Eta) of the

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Keith A. Browning

kata cold frontal situations, respectively;- cold conveyor belts ahead of warm fronts;- narrow rainbands associated with line convection at the boundary of a pre-cold-frontal low-level jet;- wide mesoscale rainbands associated with midtropospheric convection;- squall lines in the tropics and midlatitudes;- nonsquall mesoscale convective systems in the tropics and midlatitudes;- subsynoptic-scale comma clouds associated with cold-air vortices;- polar-trough conveyor belts and instant occlusions.1

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David J. Nicosia and Richard H. Grumm

the average of the most negative values for each case seen in Fig. 16 . Figure 16 indicates that the generation term for EPV was significantly negative with each case, to the warm side of the midlevel frontogenetic regions. All three cases show that the generation term for EPV was highly negative close to the emerging dry tongue jet ( Carlson 1980 ) at midlevels, which is depicted by the relative humidity and wind fields at 500 hPa ( Fig. 16 ). For the February case ( Fig. 16a ) and the January

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Barry N. Hanstrum, Graham A. Mills, Andrew Watson, John P. Monteverdi, and Charles A. Doswell III

1994 ). Some of these have been shown to have supercellular characteristics ( Monteverdi and Quadros 1994 ; Monteverdi and Johnson 1996 ). Lipari and Monteverdi (2000) present a schematic of the synoptic environment associated with tornadic thunderstorms in California's Central Valley. A very favorable shear environment is created there as a surface front and associated short-wave trough–jet streak crosses the California coast. With the front typically south of the Central Valley, topographic

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Joseph J. Gale, William A. Gallus Jr., and Karl A. Jungbluth

overwhelm the ambient low-level line-normal shear circulation, if a rear-inflow jet ( Smull and Houze 1987 ) descended to the surface, it would further strengthen the cold pool circulation and cause faster dissipation. If a rear-inflow jet remained elevated, it would induce a circulation below it opposite the cold pool's circulation, thus aiding the ambient line-normal low-level shear circulation and extending the MCS's lifetime. Other studies briefly suggest possible reasons for MCS dissipation

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Frederick Sanders and Brian J. Hoskins

-level jet entrance. Patternsof Q-vectors and vertical circulations are noted for frontogenetical and frontolytical situations.1. Introduction Quasi-geostrophic theory has provided the conceptual basis for understanding the behavior ofextratropical synoptic systems for more than 30 years. Perhapsits most widely used expression is the omega equation,for diagnosing vertical motion from fields of geopotential height and temperature, as discussed in detailby Durran and Snellman (1987). The traditional

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Alicia C. Wasula, Lance F. Bosart, Russell Schneider, Steven J. Weiss, Robert H. Johns, Geoffrey S. Manikin, and Patrick Welsh

850 hPa points directly toward the region where each tornado episode began ( Fig. 2d ). A very strong (greater than 44 m s −1 ) upper-level jet streak is present at 200 hPa ( Fig. 2e ). The tornado episode begins in the warm sector of the surface low ( Fig. 2f ), and in the region of enhanced ascent in the equatorward-entrance region of the upper-level jet ( Fig. 2e ). A cross section through the jet entrance region confirms the presence of strong ascent over the location of the first tornado

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Mathieu Nuret and Michel Chong

) deployed in the vicinity of the equator. The statistics of the differences between observations and analyses show a systematic underestimation of the analyzed wind speed, close to 1 m s0 1 , reaching 2 m s0 1 at the easterly jet level, and an overestimation of 1 m s0 1 at 1000 hPa. Analyzed humidity is too dry in the midtroposphere and too moist in the low troposphere. Analyzed temperature exhibits a more complicated behavior, but the high troposphere is generally too cold. The statistics

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