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Julia H. Keller, Christian M. Grams, Michael Riemer, Heather M. Archambault, Lance Bosart, James D. Doyle, Jenni L. Evans, Thomas J. Galarneau Jr., Kyle Griffin, Patrick A. Harr, Naoko Kitabatake, Ron McTaggart-Cowan, Florian Pantillon, Julian F. Quinting, Carolyn A. Reynolds, Elizabeth A. Ritchie, Ryan D. Torn, and Fuqing Zhang

downstream, and the next downstream ridge builds, which signifies the downstream propagation that arises from the initial local changes in the jet near the site of ET ( Fig. 1c ). Meanwhile, Nuri reintensifies into a strong extratropical cyclone and initiates cyclonic wave breaking over the western North Pacific ( Fig. 1c ). Subsequently, the upper-level wave pattern amplifies farther downstream, establishing a high-amplitude ridge–trough couplet over North America. A heat wave develops in the high

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Jacopo Riboldi, Christian M. Grams, Michael Riemer, and Heather M. Archambault

) documented how a series of three TCs (Florence, Helene, and Leslie), recurving in the North Atlantic between September and October 2000, led to the continuous invigoration of atmospheric blocks over the North Atlantic and to repeated Rossby wave breaking over western Europe associated with extreme flooding over southern Switzerland and northern Italy. It has been hypothesized that a link exists between ET in the western North Pacific and enhanced blocking activity over western North America during the

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Volkmar Wirth, Michael Riemer, Edmund K. M. Chang, and Olivia Martius

gradual than that of the carrier wave (dotted) or the RWP signal (blue). Fig . 1. Schematic of a Rossby wave packet (RWP) at a specific time. The blue line represents , the black dotted line is the underlying carrier wave , and the two red lines depict plus (upper line) and minus (lower line) the amplitude . A real world example is presented in Fig. 2 . Figure 2a shows the midlatitude jet with large meridional undulations over North America. Over the rest of hemisphere, the jet is more zonally

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Paolo Ghinassi, Georgios Fragkoulidis, and Volkmar Wirth

bust” for the majority of the operational forecast models, showing a huge drop in the medium-range forecast skill over Europe ( Rodwell et al. 2013 ). The authors associated this poor performance to the misrepresentation of moist convective processes over North America a few days earlier, and this error was subsequently communicated downstream embedded in a RWP. Data are retrieved from the ERA-Interim reanalyses ( Dee et al. 2011 ) with a horizontal resolution of 2° × 2° on 20 pressure levels

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Michael Maier-Gerber, Michael Riemer, Andreas H. Fink, Peter Knippertz, Enrico Di Muzio, and Ron McTaggart-Cowan

events, because they primarily occur in proximity to the eastern seaboard of North America ( McTaggart-Cowan et al. 2008 ). In a recent study, Wang et al. (2018) examine reforecasts in terms of their skill to predict tropical cyclogenesis in the North Atlantic, using the pathway classification of McTaggart-Cowan et al. (2013) . The authors conclude that the two TT categories are less predictable than the others, a finding that they attribute to forecast errors of the deep-layer shear and the

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Gabriel Wolf and Volkmar Wirth

propagation. On the other hand, the diagnostic in Fig. 4a completely misses the generation of the wave packet over the Beaufort Sea. Instead, it connects the North American RWP with an anomaly over the mid-Pacific. This is consistent with the path of the RWP (see the trajectory on the latitude–longitude map atop the Hovmöller diagram in Fig. 4a ), which starts at about 45°N and 170°W. The poor representation of meridionally propagating RWPs is a known deficiency of the method of Zimin et al. (2003

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Stephan Rasp, Tobias Selz, and George C. Craig

of precipitation is related to WCBs ( Pfahl et al. 2014 ), indicating that convective processes can also represent an important source of upper-level PV modification. Rodwell et al. (2013) suggest that MCSs can significantly degrade medium-range forecasts. In their study, the presence of high values of convective available potential energy (CAPE) over the North American continent was correlated with “forecast busts,” a pronounced decrease in forecast skill downstream over Europe about 6 days

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Marlene Baumgart, Paolo Ghinassi, Volkmar Wirth, Tobias Selz, George C. Craig, and Michael Riemer

), large error amplification occurs in the Western Hemisphere: an extended region of large error is found around 150°–110°W and secondary maxima are found around 180° and over an “error belt” extending from North America toward Europe. Between day 15 and 20 ( Figs. 9c and 9d ), error growth continues further both in magnitude and in its spatial extension. At day 20 ( Fig. 9d ), the error affects almost the whole mid- to high latitudes of the Northern Hemisphere. At this stage, the predictability time

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Mirjam Hirt, Stephan Rasp, Ulrich Blahak, and George C. Craig

-D labeled arrays and datasets in Python . J. Open Res. Software , 5 ( 1 ), . 10.5334/jors.148 Jankov , I. , and Coauthors , 2017 : A performance comparison between multiphysics and stochastic approaches within a North American RAP ensemble . Mon. Wea. Rev. , 145 , 1161 – 1179 , . 10.1175/MWR-D-16-0160.1 Jankov , I. , J. Beck , J. Wolff , M. Harrold , J. B. Olson , T. Smirnova , C. Alexander , and J

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