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Louis J. Wicker, Michael P. Kay, and Michael P. Foster

Abstract

During the spring of 1995, an operational forecast experiment using a three-dimensional cloud model was carried out for the north Texas region. Gridpoint soundings were obtained from the daily operational numerical weather prediction models run at the National Centers for Environmental Prediction, and these soundings were then used to initialize a limited-domain cloud-resolving model in an attempt to predict convective storm type and morphology in a timely manner. The results indicate that this type of convective forecast may be useful in the operational environment, despite several limitations associated with this methodology. One interesting result from the experiment is that while the gridpoint soundings obtained from the NCEP models generally overforecast instability and vertical wind shear, the resulting convective storm evolution and morphology in the cloud model was often similar to that of the observed storms. Therefore the “overforecast” of mesoscale environment’s instability and vertical wind shear still resulted in a thunderstorm-scale forecast that provided useful information to operational forecasters.

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Michael E. Baldwin, John S. Kain, and Michael P. Kay

Abstract

The impact of parameterized convection on Eta Model forecast soundings is examined. The Betts–Miller–Janjić parameterization used in the National Centers for Environmental Prediction Eta Model introduces characteristic profiles of temperature and moisture in model soundings. These specified profiles can provide misleading representations of various vertical structures and can strongly affect model predictions of parameters that are used to forecast deep convection, such as convective available potential energy and convective inhibition. The specific procedures and tendencies of this parameterization are discussed, and guidelines for interpreting Eta Model soundings are presented.

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Nathan M. Hitchens, Harold E. Brooks, and Michael P. Kay

Abstract

A method for determining baselines of skill for the purpose of the verification of rare-event forecasts is described and examples are presented to illustrate the sensitivity to parameter choices. These “practically perfect” forecasts are designed to resemble a forecast that is consistent with that which a forecaster would make given perfect knowledge of the events beforehand. The Storm Prediction Center’s convective outlook slight risk areas are evaluated over the period from 1973 to 2011 using practically perfect forecasts to define the maximum values of the critical success index that a forecaster could reasonably achieve given the constraints of the forecast, as well as the minimum values of the critical success index that are considered the baseline for skillful forecasts. Based on these upper and lower bounds, the relative skill of convective outlook areas shows little to no skill until the mid-1990s, after which this value increases steadily. The annual frequency of skillful daily forecasts continues to increase from the beginning of the period of study, and the annual cycle shows maxima of the frequency of skillful daily forecasts occurring in May and June.

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Harold E. Brooks, Charles A. Doswell III, and Michael P. Kay

Abstract

An estimate is made of the probability of an occurrence of a tornado day near any location in the contiguous 48 states for any time during the year. Gaussian smoothers in space and time have been applied to the observed record of tornado days from 1980 to 1999 to produce daily maps and annual cycles at any point on an 80 km × 80 km grid. Many aspects of this climatological estimate have been identified in previous work, but the method allows one to consider the record in several new ways. The two regions of maximum tornado days in the United States are northeastern Colorado and peninsular Florida, but there is a large region between the Appalachian and Rocky Mountains that has at least 1 day on which a tornado touches down on the grid. The annual cycle of tornado days is of particular interest. The southeastern United States, outside of Florida, faces its maximum threat in April. Farther west and north, the threat is later in the year, with the northern United States and New England facing its maximum threat in July. In addition, the repeatability of the annual cycle is much greater in the plains than farther east. By combining the region of greatest threat with the region of highest repeatability of the season, an objective definition of Tornado Alley as a region that extends from the southern Texas Panhandle through Nebraska and northeastward into eastern North Dakota and Minnesota can be provided.

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Charles A. Doswell III, Harold E. Brooks, and Michael P. Kay

Abstract

The probability of nontornadic severe weather event reports near any location in the United States for any day of the year has been estimated. Gaussian smoothers in space and time have been applied to the observed record of severe thunderstorm occurrence from 1980 to 1994 to produce daily maps and annual cycles at any point. Many aspects of this climatology have been identified in previous work, but the method allows for the consideration of the record in several new ways. A review of the raw data, broken down in various ways, reveals that numerous nonmeteorological artifacts are present in the raw data. These are predominantly associated with the marginal nontornadic severe thunderstorm events, including an enormous growth in the number of severe weather reports since the mid-1950s. Much of this growth may be associated with a drive to improve warning verification scores. The smoothed spatial and temporal distributions of the probability of nontornadic severe thunderstorm events are presented in several ways. The distribution of significant nontornadic severe thunderstorm reports (wind speeds ≥ 65 kt and/or hailstone diameters ≥ 2 in.) is consistent with the hypothesis that supercells are responsible for the majority of such reports.

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John S. Kain, Michael E. Baldwin, Paul R. Janish, Steven J. Weiss, Michael P. Kay, and Gregory W. Carbin

Abstract

Systematic subjective verification of precipitation forecasts from two numerical models is presented and discussed. The subjective verification effort was carried out as part of the 2001 Spring Program, a seven-week collaborative experiment conducted at the NOAA/National Severe Storms Laboratory (NSSL) and the NWS/Storm Prediction Center, with participation from the NCEP/Environmental Modeling Center, the NOAA/Forecast Systems Laboratory, the Norman, Oklahoma, National Weather Service Forecast Office, and Iowa State University. This paper focuses on a comparison of the operational Eta Model and an experimental version of this model run at NSSL; results are limited to precipitation forecasts, although other models and model output fields were verified and evaluated during the program.

By comparing forecaster confidence in model solutions to next-day assessments of model performance, this study yields unique information about the utility of models for human forecasters. It is shown that, when averaged over many forecasts, subjective verification ratings of model performance were consistent with preevent confidence levels. In particular, models that earned higher average confidence ratings were also assigned higher average subjective verification scores. However, confidence and verification scores for individual forecasts were very poorly correlated, that is, forecast teams showed little skill in assessing how “good” individual model forecasts would be. Furthermore, the teams were unable to choose reliably which model, or which initialization of the same model, would produce the “best” forecast for a given period.

The subjective verification methodology used in the 2001 Spring Program is presented as a prototype for more refined and focused subjective verification efforts in the future. The results demonstrate that this approach can provide valuable insight into how forecasters use numerical models. It has great potential as a complement to objective verification scores and can have a significant positive impact on model development strategies.

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