William H. Klein

Forecasting Local Weather by Means of Model Output Statistics

Abstract Experience over the past decade has shown that objective forecasts of local weather elements can best be obtained by using statistical methods to complement the raw output of numerical prediction models. One of the most successful techniques for accomplishing this is called Model Output Statistics (MOS). The MOS method involves matching observations of local weather with output from numerical models. Forecast equations are then derived by statistical techniques such as screening regression, regression estimation of event probabilities, and the logit model. In this way the bias and inaccuracy of the numerical model, as well as the local climatology, can be built into the forecast system. MOS has been applied by the Techniques Development Laboratory to produce automated forecasts of numerous weather elements including precipitation, temperature, wind, clouds, ceiling, visibility, and thunderstorms. In this paper, the derivation and operational application of MOS forecasts for each of these elements...

Specification of Monthly Precipitation over the United States from the Surrounding 700 mb Height Field

Abstract The relations between monthly precipitation in each of 60 climate divisions in the contiguous United States and the simultaneous field of monthly mean 700 mb height over North America and vicinity were investigated for both frequency and amount by applying a stepwise forward regression procedure to 30 years of data from January 1951 to December 1980. For winter, a good grid for specifying precipitation from 700 mb heights was found to consist of 79 points located every 5° of latitude from 20°–60°N and every 10° of longitude from 45°–150°W. Best results were obtained by expressing all data as ordinary anomalies and by pooling all cases for December, January and February. Fitting monthly totals by a gamma distribution and classifying into 21 percentiles gave as good results as those obtained by any other transformation of precipitation amount, with almost 42 percent of the precipitation variance, nationwide, explained by less than 3 heights. However, this was not as good as the 45 percent explained...

Objective Specification of Monthly Mean Surface Temperature from Mean 700 mb Heights in Winter

Abstract The simultaneous relation between temperature and circulation during the winter season is studied by correlating anomalies of monthly mean surface temperature at 109 stations in the United States from 1948 through 1981 with monthly mean 700 mb height anomalies at a network of 133 grid points in North America and surrounding oceans. The data are screened by a stepwise forward selection procedure to yield multiple regression equations for specifying the monthly mean temperature anomaly at each city from the field of simultaneous 700 mb height plus the previous months temperature anomaly. Several sets of multiple regression equations are derived on a dependent sample of 90 winter months by using two different grids of heights and applying various cut-off criteria, such as the F-ratio, additional explained variance and number of variables, to terminate the selection of additional predictors. Optimum termination points are determined by testing 19 alternate criteria on independent data from 1978 to 1...

Skill in Precipitation Forecasting in the National Weather Service

Abstract All known long-term records of forecasting performance for different types of precipitation forecasts in the National Weather Service were examined for relative skill and secular trends in skill. The largest upward trends were achieved by local probability of precipitation (PoP) forecasts for the periods 24–36 h and 36–48 h after 0000 and 1200 GMT. Over the last 13 years, the skill of these forecasts has improved at an average rate of 7.2% per 10-year interval. Over the same period, improvement has been smaller in local PoP skill in the 12–24 h range (2.0% per 10 years) and in the accuracy of “Yea/No” forecasts of measurable precipitation. The overall trend in accuracy of centralized quantitative precipitation forecasts of ⩾0.5 in and ⩾1.0 in has been slightly upward at the 0–24 h range and strongly upward at the 24–48 h range. Most of the improvement in these forecasts has been achieved from the early 1970s to the present. Strong upward accuracy trends in all types of precipitation forecasts wit...

SPECIFICATION OF PRECIPITATION FROM THE 700-MILLIBAR CIRCULATION

Abstract Five-day precipitation amounts observed during 10 recent winters in 40 equal-area circles covering the United States are assigned a numerical index according to the proportion of light, moderate, or heavy precipitation falling within each circle. The synoptic climatology of precipitation is investigated through construction of correlation fields between this index and the simultaneous 5-day mean 700-mb. height departure from normal in North America and adjacent ocean areas. On the basis of the analogy between lines of equal correlation and lines of equal height anomaly, inferences are drawn concerning the association between precipitation and other meteorological factors. Schematic models are then constructed showing preferred portions of the long-wave pattern for heavy and light precipitation in different parts of the nation. By use of a screening program on the IBM 7090, the field of 700-mb. height is found to be more effective than that of either sea level pressure or 700–1000-mb. thickness in...

The Synoptic Climatology of Monthly Mean Surface Temperature in the United States during Winter Relative to the Surrounding 700 mb Height Field

Abstract The synoptic climatology of monthly mean surface temperature in the contiguous United-States is obtained by analysing and summarizing fields of simple linear correlation between the temperature anomaly at 109 stations during 102 winter months and the simultaneous anomaly of monthly mean 700 mb height at 133 grid points over North America and adjacent oceans. In addition, the correlation fields are multiplied by the standard deviation of height anomaly at each grid point to produce composite maps for warm (cold) temperatures at each station. On the basis of these correlation fields and composite maps, inferences are drawn concerning the relations between surface temperature and the surrounding 700 mb circulation, and geographical differences are highlighted. Schematic models are constructed showing preferred locations of above and below normal 700 mb heights for opposite extremes of winter temperature in each of several climatologically similar regions into which the United States can be divided.

A Comparison Of Pointwise Screening and Empirical Orthogonal Functions in Specifying Monthly Surface Temperature from 700 mb Data

Abstract A comparison is made between two types of specification of monthly wintertime surface temperatures over the United States. The specifications are obtained by multiple regression of station temperature anomaly at each of 37 stations onto 700 mb height anomalies represented by 1) grid-point values selected by a forward stepwise screening procedure, and 2) coefficients of the dominant empirical orthogonal functions (EOFs). Various measures of skill show that specifications derived from the pointwise screening are superior in both developmental (dependent) and independent samples. The differences in the skill levels are interpreted as a disadvantage of the spatial generality inherent in the EOF representations.

Space and Time Variations in Specifying Monthly Mean Surface Temperature from the 700 mb Height Field

Abstract Anomalies of monthly mean surface temperature observed at 109 stations in the United States from 1948 through 1981 are related to concurrent monthly mean 700 mb height anomalies at a network of 133 grid points in North America and the surrounding oceans. The data are screened by a stepwise forward selection procedure to yield multiple regression equations for specifying the monthly mean temperature anomaly at each city and month from the field of simultaneous 700 mb height plus the previous months temperature anomaly. A short set of equations selected on a subjective basis and containing an average of five terms per equation tested as well as equations selected by any other cutoff criterion. Various properties of the short specification equations and related atmospheric characteristics are described on a regional, seasonal and month-to-month basis. Six features are mapped for the months of January, April, July and October: the reduction of variance, standard error of estimate, standard deviation...

An operational system for specifying monthly precipitation amounts over the United States from the field of concurrent mean 700-mb heights

Abstract This paper describes an operational system for specifying monthly precipitation amounts in the contiguous United States from the concurrent 700-mb monthly mean height field over North America and adjacent oceans. Multiple regression equations are derived for each month of the year at 60 climate divisions by applying a quasi-objective, forward selection procedure to 30 yr of data for 1951–80. The resulting specification equations explain an average of 37% of the precipitation variance, but values range from 70% along the Pacific Coast in January to 10% in southern New England in July. When applied to prognostic 700-mb charts for 1987 and 1988, the equations have shown more skill than persistence but less skill than official monthly outlooks. Four attempts to improve the specifications are discussed. Best results were obtained by screening the mean precipitation amounts within 10–12 coherent regions, selected by factor analysis, instead of 60 smaller climate divisions. This procedure raised the exp...

RELATION BETWEEN UPPER AIR LOWS AND WINTER PRECIPITATION IN THE WESTERN PLATEAU STATES

Abstract A comparison is made of the relative effectiveness of the cyclonic circulation pattern at four different tropospheric levels in specifying winter precipitation over the intermountain area of the western United States. This is accomplished by developing the synoptic climatology of precipitation resulting from Lows at the 850-, 700-, 500-, and 300-mb. levels. Twelve-hr. precipitation amounts (expressed as a percent of the 7-day normal) at 280 stations in the Plateau States for 13 yr. are related to the positions of nearby low centers through a computer system of moving coordinates. For each level, the upper Lows are classified into three intensity categories according to the departure from normal of their central heights. Average precipitation amount, distribution, and frequency of occurrence are then calculated for the area of the grid system. The dependence of these precipitation characteristics upon the level, intensity, and location of the upper Low is described and illustrated. The geographica...