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Joseph T. Schaefer and Richard L. Livingston

Abstract

Since the probability of precipitation (PoP) appears in the forecasts of the National Weather Service (NWS), it is important that both the forecaster and user fully grasp the subtleties of the meaning of PoP. A brief review of the theory of PoP is presented. It is shown that although the PoP is defined as the average point probability, the guidelines outlined in the National Weather Service Operations Manual (NWSOM) also make the PoP the expected areal coverage of precipitation across the forecast area.

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Charles A. Doswell III and Joseph T. Schaefer

Abstract

No abstract available.

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Richard L. Livingston and Joseph T. Schaefer

Abstract

The medium-range forecast model (MRF) provides the basic guidance for the 3–5-day extended forecast issued by individual Weather Service Forecast Offices. The ability of the MRF to forecast shorter synoptic, subsynoptic, and mesoscale phenomena is examined through the use of the anomaly correlation coefficient and illustrated with several case studies. The operational implications of putting detail in the extended forecast are reviewed. Finally, several suggestions are presented to help operational forecasters determine consistency and accuracy in the extended guidance.

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Joseph T. Schaefer and Charles A. Doswell III

Abstract

By its very nature, interpolation in a vector field is ambiguous, owing to the somewhat arbitrary nature of the vector norm. Since a two-dimensional vector field cm be specified by two scalar quantities. which can be separately interpolated, the ambiguity can be resolved by forcing the interpolated wind field to preserve the vorticity and divergence fields associated with the raw data. A method to calculate divergence and vorticity directly from randomly spaced wind observations is developed and, using analytically generated data, shown to produce more accurate results than conventional computations. Two methods of retrieving the wind field from the analysed scalar fields are presented and also tested on the analytic field. Finally, total analysis, from wind observations to gridded wind fields, is demonstrated on real meteorological data.

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Joseph T. Schaefer, Donald L. Kelly, and Robert F. Abbey

Abstract

One of the principle applications of climatological tornado data is in tornado-hazard assessment. To perform such a hazard-potential determination, historical tornado characteristics in either a regional or tom area are complied. A model is then used to determine a site-specific point probability of a tornado greater than a specified intensity occurring. Various models require different climatological input. However, a knowledge of the mean values of tornado track width, tornado track width, tornado affected area and tornado occurrence rate as both a function of tornado intensity and geographic area, along with a violence frequency distribution, enable Mod of the models to be applied.

The NSSFC-NRC tornado data base is used to supply input for the determination of these parameters over the United States. This climatic data base has undergone extensive updating and quality control since it was last reported. For track parameters, internally redundant data were used to cheek consistency. Further, reports which derivated significantly from the mean wore individually checked. Intensity data have been compared with the University of Chicago DAPPLE tornado base. All tornadoes whose recorded intensifies differed by more than one category were reclassified by an independent scientist so that the two data sets are consistent.

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Jiang Chaowei, Donald L. Kelly, and Joseph T. Schaefer

Abstract

Beijing, People's Republic of China, and Topeka, Kansas, United States of America, are located at approximately the same latitude and are affected by similar synoptic weather patterns. However, their thunderstorm climatology differs significantly. Rowinsonde data from the two stations are compared. It is found that the typical distribution of the oceanic subtropical high pressure areas subtly modifies the synoptic scale environment accounting for the observed differences.

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Donald L. Kelly, Joseph T. Schaefer, and Charles A. Doswell III

Abstract

While the climatology of excessive rain and tornadoes is well-documented, little is known of storms that produce high winds or large hail. The characteristics of the approximately 75 000 severe thunderstorms which occurred in the United States from 1955 through 1983 are analyzed in an attempt to rectify this situation.

The distribution of over 29 000 storms causing hail larger than 19 mm shows marked diurnal, seasonal, and geographic preferences. These storms occur most frequently during the midafternoon hours of May and June in a zone running from central Texas to Nebraska. Spring storms tend to occur south of the Kansas-Nebraska border and summer storms north of it.

Thunderstorm winds which produce either “structural” damage or are reported as faster than 25.8 m s−1 generated about 46 000 reports. These storms typically occur during midafternoon in June and July. While the geographic distribution of violent windstorms is similar to that hailstorms, a zone of weaker severe thunderstorm gusts lies from northern Iowa to central Ohio. During May, windstorms are predominant across the plains area, but by August thew storms are indigenous only to the northern Midwest.

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Mathias Drton, Caren Marzban, Peter Guttorp, and Joseph T. Schaefer

Abstract

Tornadic activity in four U.S. regions is stochastically modeled based on data on tornado counts over the years 1953–98. It is shown that tornadic activity on a given day is mostly affected by the activity on the previous day. Hence, the process can be modeled as a Markov chain. A parametric nonhomogenous Markov chain model is developed based on the well-known increase of tornadic activity in the spring and summer months. This model, with only eight parameters, describes tornadic activity quite well. The interpretability of the estimated parameters allows a diagnosis of the regional differences in tornadic activity. For instance, a comparison of the values of the parameters for the four regions suggests that in the South tornado persistence is specific mostly to the early part of the year. Finally, within the framework of probabilistic forecast verification, it is shown that the Markov chain model outperforms the climatological model, even though the former is far simpler in terms of the number of parameters (8 and 366, respectively). The superior performance of the model is confirmed in terms of several measures of performance in all four regions. The exception is the southern Tornado Alley, where the reliability of the model forecasts is nonsignificantly inferior to that of the climatological ones.

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Edward W. Ferguson, Joseph T. Schaefer, Steven J. Weiss, Larry F. Wilson, and Frederick P. Ostby

Abstract

The tornado events of 1982 are reviewed. Significant and interesting aspects of the 1047 reported storms are noted. The synoptic patterns associated with four major tornado days are examined.

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Ashton Robinson Cook, Lance M. Leslie, David B. Parsons, and Joseph T. Schaefer

Abstract

In recent years, the potential of seasonal outlooks for tornadoes has attracted the attention of researchers. Previous studies on this topic have focused mainly on the influence of global circulation patterns [e.g., El Niño–Southern Oscillation (ENSO), North Atlantic Oscillation, or Pacific decadal oscillation] on spring tornadoes. However, these studies have yielded conflicting results of the roles of these climate drivers on tornado intensity and frequency. The present study seeks to establish linkages between ENSO and tornado outbreaks over the United States during winter and early spring. These linkages are established in two ways: 1) statistically, by relating raw counts of tornadoes in outbreaks (defined as six or more tornadoes in a 24-h period in the United States east of the Rocky Mountains), and their destructive potential, to sea surface temperature anomalies observed in the Niño-3.4 region, and 2) qualitatively, by relating ENSO to shifts in synoptic-scale atmospheric phenomena that contribute to tornado outbreaks. The latter approach is critical for interpreting the statistical relationships, thereby avoiding the deficiencies in a few of the previous studies that did not provide physical explanations relating ENSO to shifts in tornado activity. The results suggest that shifts in tornado occurrence are clearly related to ENSO. In particular, La Niña conditions consistently foster more frequent and intense tornado activity in comparison with El Niño, particularly at higher latitudes. Furthermore, it is found that tornado activity changes are tied not only to the location and intensity of the subtropical jet during individual outbreaks but also to the positions of surface cyclones, low-level jet streams, and instability axes.

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