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Patrick S. Market

Abstract

A brief study is provided on the forecast performance of students who write a mock area forecast discussion (AFD) on a weekly basis. Student performance was tracked for one semester (11 weeks) during the University of Missouri—Columbia's local weather forecast game. The hypothesis posed is that student performance is no better on days when they compose an AFD. A nonparametric Mann–Whitney test cannot reject that hypothesis. However, the same test employed on precipitation forecasts (for days when precipitation actually fell) shows that there is a statistically significant difference (p = 0.02) between the scores of those students writing an AFD and those who do not. Similar results are found with a chi-square test. Thus, AFD writers improve their precipitation score on days when significant weather occurred. Forecaster confidence is also enhanced by AFD composition.

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Patrick S. Market and David Cissell

Abstract

A case study of the 13–14 March 1999 heavy snow event across southern Missouri and neighboring states is presented. Of the many features that made this storm notable, the very sharp gradient on the northern periphery of the snowfall field was most intriguing. Moreover, that the snowfall field was confined to the southern half of the state resulted in snow-free regions across central Missouri where significant accumulations had been predicted. The focus of this study was thus to reveal the cause of such large snowfall gradients. Little evidence exists of convective snowfall over Missouri through 1200 UTC 14 March 1999, when this study ends. Analyses confirm that the release of neither convective instability nor conditional symmetric instability was responsible for the large snowfall gradient on the northern boundary. Instead, the juxtaposition of dry and moist airstreams from the north and south, respectively, as components of a deformation zone ultimately defined the large snowfall gradient across southern Missouri.

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Anthony R. Lupo and Patrick S. Market

Abstract

The evaluation of weather forecast accuracy has always been a difficult subject to address for many reasons. In this study, a simple semiobjective method is used to examine the accuracy of zone forecasts issued by the Weldon Spring (Saint Louis) National Weather Service (NWS) Office for mid-Missouri over a period of 416 days with the goal of demonstrating the utility of this method. Zone forecasts were chosen because these forecasts are typically what the public will receive either directly or indirectly from various media outlets. Not surprising, the evaluation method used here demonstrates that forecasts issued by the NWS and the Nested Grid Model (NGM) model output statistics (MOS) represent a considerable improvement over persistence or climatological baseline forecasts. NWS forecasts were slightly better than NGM MOS forecasts, especially when considering temperature and precipitation only. All forecasts showed distinct seasonal variability. The NWS winter-season forecasts were superior to those issued in the summer season, and this superiority was found to be a function of the precipitation forecast parameter. This technique might represent an easily understandable and concise method for providing weather forecast performance information to the general public in such a way that it would instill or reinforce public confidence in the accuracy of weather forecasts.

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Patrick S. Market, Chris E. Halcomb, and Rebecca L. Ebert

Abstract

A “climatology” (climatological description of spatial, temporal, and synoptic characteristics) of snow events with thunder is presented for the contiguous United States. Based upon 30 yr of 3-hourly reports from 204 stations in the 48 contiguous United States, 229 reports are extracted from the 3-hourly observations (consistently bearing the present-weather group in each surface observation) that featured thunder with snow only. When these reports are plotted spatially, the central United States, the intermountain west, and the Great Lakes region emerge as the preferred regions for thundersnow occurrence. A thundersnow event is then defined. Isolated thundersnow reports clearly constitute a thundersnow event. Also, multiple thundersnow reports that are not separated spatially by over 1100 km or temporally by 6 h are considered part of one event. The location reporting the first occurrence of thunder with snow in such a collection of stations then carries the representative time and location for the event. The 229 individual reports make up 191 thundersnow events. Temporal analysis of thundersnow events reveals a nationwide seasonal preference for occurrence in March but no clear diurnal preference. Most thundersnow events are typically reported at only one station and only rarely in consecutive 3-hourly observations. These results thus reinforce the notion of thundersnow as a fairly localized phenomenon of limited duration. In terms of intensity, the thundersnow events investigated in this study feature light snow about one-half of the time, with the remaining events split nearly evenly between moderate (25%) and heavy (23%) snowfall. Further analysis classifies each event according to the meso- to synoptic-scale environment in which it forms. Most events (52%) form in association with a transient midlatitude cyclone; other event types include an orographic class, events that occur with a coastal cyclone, events associated with an arctic front, lake-effect events, and those resulting from upslope flow.

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Patrick S. Market, Ronald W. Przybylinski, and Scott M. Rochette

Abstract

Analysis is provided of a surprise late-season snow event over eastern Missouri and western Illinois. While snow totals failed to exceed 15 cm (6 in.) at any single location, the system was noteworthy because of the poor performance of public, private, and media forecasts in anticipating the event. Using observed data and a successful simulation with a mesoscale numerical model, the event is scrutinized to determine the forcing mechanisms for the precipitation over a small area. A region of enhanced frontogenesis is diagnosed over the region both in the observed data as well as the model output. That the precipitation fell as snow is shown to be the result of a dry layer of air between the surface and the cloud base that saturated and cooled due largely to snow sublimation–evaporation in just a few hours to permit the fall of snow uninhibited from the cloud base to the ground.

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Brian P. Pettegrew, Patrick S. Market, Raymond A. Wolf, Ronald L. Holle, and Nicholas W. S. Demetriades

Abstract

Between 2100 UTC 11 February 2003 and 0200 UTC 12 February 2003, a line of thunderstorms passed swiftly through parts of eastern Iowa and into north-central Illinois. Although this storm somewhat resembled a warm season, line-type mesoscale convective system, it was unique in that the thunderstorm winds exceeded the severe criterion (50 kt; 25.7 m s−1) during a snowburst. While the parent snowband deposited only 4 cm of snow, it did so in a short period and created a treacherous driving situation because of the ensuing near-whiteout conditions caused by strong winds that reached the National Weather Service severe criteria, as the line moved across central Illinois. Such storms in the cold season rarely occur and are largely undocumented; the present work seeks to fill this void in the existing literature.

While this system superficially resembled a more traditional warm season squall line, deeper inspection revealed a precipitation band that failed to conform to that paradigm. Radar analysis failed to resolve any of the necessary warm season signatures, as maximum reflectivities of only 40–45 dBZ reached no higher than 3.7 km above ground level. The result was low-topped convection in a highly sheared environment. Moreover, winds in excess of 50 kt (25.7 m s−1) occurred earlier in the day without thunderstorm activity, upstream of the eventual severe thundersnow location. Perhaps of greatest importance is the fact that the winds in excess of the severe criterion were more the result of boundary layer mixing, and largely coincident with the parent convective line. This event was a case of forced convection, dynamically linked to its parent cold front via persistent frontogenesis and the convective instability associated with it; winds sufficient for a severe thunderstorm warning, while influenced by convection, resulted from high momentum mixing downward through a dry-adiabatic layer.

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