Patrick S. Market

Stability and Charging Characteristics of the Comma Head Region of Continental Winter Cyclones

AbstractThis paper presents analyses of the finescale structure of convection in the comma head of two continental winter cyclones and a 16-storm climatology analyzing the distribution of lightning within the comma head. A case study of a deep cyclone is presented illustrating how upper-tropospheric dry air associated with the dry slot can intrude over moist Gulf air, creating two zones of precipitation within the comma head: a northern zone characterized by deep stratiform clouds topped by generating cells and a southern zone marked by elevated convection. Lightning, when it occurred, originated from the elevated convection. A second case study of a cutoff low is presented to examine the relationship between lightning flashes and wintertime convection. Updrafts within convective cells in both storms approached 6–8 m s−1, and convective available potential energy in the cell environment reached approximately 50–250 J kg−1. Radar measurements obtained in convective updraft regions showed enhanced spectral ...

A Climatology of Thundersnow Events over the Contiguous United States

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 FUTURE OF HUMANS IN AN INCREASINGLY AUTOMATED FORECAST PROCESS

ur role as humans in the forecast process has been a very sensitive and highly debated issue within the meteorological profession since the advent of numerical weather prediction (NWP) models in the 1960s. NWP model guidance contin-ues to improve to such a degree that forecasters are discovering their ability to add value to NWP model forecasts is outpaced (Brooks et al. 1996). This has resulted in an increas-ing reliance on NWP model guidance, an issue first described by Snellman (1977). Since that time, new roles for forecasters have been contemplated in an effort to determine the optimum role for humans in the fore-cast process in order to produce the best fore-cast products possible for all users of weather forecast information. This article represents results from a collab-orative effort of the forecast community to iden-tify the ways in which these roles might continue to change in the future.Reliance on NWP model guidance to initialize a gridded forecast database has become particularly evident in the National Weather Service (NWS) since the late 1990s. Since then, forecasting has shifted from the manual production of text-based forecasts to the

Formation of a Sharp Snow Gradient in a Midwestern Heavy Snow Event

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 acros...

A Case Study of Severe Winter Convection in the Midwest

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 fai...

Calculated Height Tendencies in Two Southern Hemisphere Blocking and Cyclone Events: The Contribution of Diabatic Heating to Block Intensification

Abstract The Zwack–Okossi vorticity tendency equation was used to calculate 500-hPa height tendencies in two intensifying Southern Hemisphere blocking events. The National Centers for Environmental Prediction–National Center for Atmospheric Research gridded reanalyses were used to make each of these calculations. The block intensification period for each event was associated with a deepening surface cyclone during a 48-h period beginning at 1200 UTC 28 July and 1200 UTC 8 August 1986, respectively. These results demonstrate that the diabatic heating forces height rises through the sensible and latent heating terms in these two Southern Hemisphere blocking events. The sensible heating was the larger contributor, second only to (about the same as) the vorticity advection term in the first (second) event. The vorticity advection term has been shown by several studies to be associated with block intensification.

The Application of a Simple Method for the Verification of Weather Forecasts and Seasonal Variations in Forecast Accuracy

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.

Forcing, instability and equivalent potential vorticity in a Midwest USA convective snowstorm

This paper investigates a case of convective snow that was observed with an occluded-type cyclone. Cross-section and plan-view analyses show that convection resulted from the release of elevated, potential instability by isentropic uplift associated with an easterly trowal airstream (a western extension of the warm conveyor belt) located between 700 and 850 hPa. Forcing for ascent was supplied by low-level frontogenesis, as well as an intensifying 500hPa tropopause fold and its associated potential vorticity anomaly. The latter not only provided a source of very cold, dense air, but also was responsible for lower tropospheric cyclogenesis and subsequent trowal generation. Copyright

The Role of Sublimational Cooling in a Late-Season Midwestern Snow Event

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.

The Impact of Writing Area Forecast Discussions on Student Forecaster Performance

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—Columbias 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.