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THE PREDICTION OF SURGES IN THE SOUTHERN BASIN OF LAKE MICHIGAN

Part III. The Operational Basis for Prediction

LAWRENCE A. HUGHES

Abstract

The development of operational surge prediction in southern Lake Michigan is reviewed through the 10-year span starting with the disastrous surge of June 26, 1954 which took several lives in the Chicago area. Particular emphasis is given to the application of the work of others, especially Platzman, to the surge-prediction problem. Considerable detail is given on the surge of August 3, 1960, for which a successful prediction was made. This example, with its messages to the public, could serve as a model for future surge predictions. Finally a set of steps is given by which a prediction is made, followed by comments on those items still needing research before we can evaluate all parameters for an operational surge prediction.

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Lawrence A. Hughes

The methods for computing “instantaneous” upper-level pressure or height tendencies are revised to allow computation of the 500 mb height tendency using the observed surface (or sea-level) pressure tendency and an appropriate portion of the 1000–500 mb thickness advection. An evaluation is made as to what constitutes an appropriate portion of the thickness advection. Use of the method is discussed and an example is given.

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Lawrence A. Hughes

Abstract

Data from a number of reconnaissance flights into large Pacific tropical cyclones are combined to obtain a generalized pattern of winds at low level (about 1000 feet) under both stationary and non-stationary conditions. With these winds, a number of dependent computations are made including relative trajectories, divergence, vertical motion, rainfall, energy, and relative vorticity.

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Lawrence A. Hughes

Abstract

A number of problems that arise in making probability of precipitation (PoP) forecasts are noted and discussed, and solutions are given where known. This effort is based on a long-standing extensive verification of PoP's issued by 66 offices of the National Weather Service's Central Region. Verification results were sent monthly to each office and each forecaster for the past 13 years. Some additional problems and solutions dealing in general with verification and evaluation of PoP'S are discussed. These include modifications of scores generally used, plus factors to consider when trying to compare scores of offices having different climatological conditions.

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Lawrence A. Hughes and Wayne E. Sangster

Abstract

Using screening regression procedures, an attempt has been made to standardize probability of precipitation Brier scores for difficulty. Climatological factors affecting the difficulty of forecasting used are: precipitation frequency, time persistence and small amount frequency. Standardizing equations were derived for three-month seasons from seven years of data. Four-term regression equations were developed for each season and lead time. Local forecaster improvement over guidance scores varied inversely as the Model Output Statistics (MOS) scores, indicating that poor machine forecasts are easier to improve upon than good machine forecasts.

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Lawrence A. Hughes and David W. Saxton

An incipient severe polar outbreak that eventually affected practically all of the United States is considerably delayed by retrogression of a major trough in the westerlies. An investigation of this retrogression shows how long-wave principles aid in the prediction of such outbreaks.

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Lawrence A. Hughes and Wayne E. Sangster

Abstract

Two methods are discussed for combining the routine forecasts of the 12 h probability of precipitation made by the National Weather Service, for use when longer period probabilities are desired but cannot be created independently. Both apply a year's forecasts from 28 forecast offices to basic equations of probability to adjust for the obvious dependence of the precipitation events among the forecast periods. Both methods suggest that warm season precipitation events are more independent than cold season ones, as would be expected. One method gave unrealistic results for probability combinations outside the range of those actually used. The other method applied realistic constraints to eliminate this undesirable feature. The largest deviations from probabilities for independent events occurred when combining probabilities of 60%, but the deviations wore only about 5% in the warm season and 10% in the cold season. Tables and an equation for combining probabilities are given.

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Sverre Petterssen, M. A. Estoque, and Lawrence A. Hughes

Abstract

An experiment was conducted during the period 11 January to 21 February 1956, for the purpose of determining the manner in which graphical integrations can best be used in providing prognostic charts for sea level and for the 500-millibar level. While the graphical integrations were used as a first approximation, supplementary techniques were applied to obtain final forecasts of the pressure distribution at sea level. The results of the verification are discussed and compared with those pertaining to other forecasting procedures. The geographical distribution of errors is discussed and interpreted.

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Lawrence A. Hughes, Ferdinand Baer, Gene E. Birchfield, and Robert E. Kaylor

It is believed that the severity of the storm hitting Canada on October 15, 1954 was due to the addition of an independent development to the dying hurricane Hazel. The problem of forecasting this event is discussed in the light of forecasts made at the time. The presence of a secondary development is verified.

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Paul J. Neiman, Lawrence J. Schick, F. Martin Ralph, Mimi Hughes, and Gary A. Wick

Abstract

This study utilizes multiple decades of daily streamflow data gathered in four major watersheds in western Washington to determine the meteorological conditions most likely to cause flooding in those watersheds. Two are located in the Olympic Mountains and the other two in the western Cascades; and each has uniquely different topographic characteristics. The flood analysis is based on the maximum daily flow observed during each water year (WY) at each site [i.e., the annual peak daily flow (APDF)], with an initial emphasis on the 12 most recent water years between WY1998 and 2009, and then focusing on a 30-year interval between WY1980 and 2009. The shorter time period coincides with relatively complete passive microwave satellite coverage of integrated water vapor (IWV) over the Pacific basin. The combination of IWV imagery and streamflow data highlights a close link between landfalling atmospheric rivers (ARs) and APDFs (i.e., 46 of the 48 APDFs occurred with landfalling ARs). To complement this approach, the three-decade time series of APDFs, which correspond to the availability of the North American Regional Reanalysis (NARR) dataset, are examined. The APDFs occur most often, and are typically largest in magnitude, from November to January. The NARR is used to assess the composite meteorological conditions associated with the 10 largest APDFs at each site during this 30-year period. Heavy precipitation fell during the top 10 APDFs, and anomalously high composite NARR melting levels averaged ~1.9 km MSL, which is primarily above the four basins of interest. Hence, on average, mostly rain rather than snow fell within these basins, leading to enhanced runoff. The flooding on the four watersheds shared common meteorological attributes, including the presence of landfalling ARs with anomalous warmth, strong low-level water vapor fluxes, and weak static stability. There were also key differences that modulated the orographic control of precipitation. Notably, two watersheds experienced their top 10 APDFs when the low-level flow was southwesterly, while the other two basins had their largest APDFs with west–southwesterly flow. These differences arose because of the region’s complex topography, basin orientations, and related rain shadowing.

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