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Roman Krzysztofowicz

Abstract

A probabilistic quantitative precipitation forecast (PQPF) is prepared judgmentally by a meteorologist based on a guidance PQPF. The predictand of a judgmental PQPF is the spatially averaged precipitation amount. The predictand of a guidance PQPF produced by a statistical model is the point precipitation amount. Therefore, a procedure is needed for point-to-area rescaling of the PQPF. Theoretically based equations for rescaling are presented. The equations incorporate two predictive parameters, which characterize the precipitation field being forecast: the quotient of the area covered by a precipitation cell to the area of averaging (cell/area quotient), and the degree of certainty about the precipitation pattern (pattern certainty factor). Both parameters can be judgmentally quantified by the meteorologist during PQPF preparation. The same parameters can be entered into an inverse procedure for area-to-point rescaling of the judgmental PQPF.

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Roman Krzysztofowicz

Abstract

From the theory of sufficient comparisons of experiments, a measure of skill is derived for categorical forecasts of continuous predictands. Called Bayesian correlation wore (BCS), the measure is specified in terms of three parameters of a normal-linear statistical model that combines information from two sources: a prior (climatological) record of the predictand and a verification record of forecasts. Three properties characterize the BCS: (i) It is meaningful for comparing alternative forecasts of the same predictand, as well as forecasts of different predictands, though in a limited sense; (ii) it is interpretable as correlation between the forecast and the predictand; and, most significantly, (iii) it orders alternative forecast systems consistently with their ex ante economic values to rational users (those who make decisions by maximizing the expected utility of outcomes under the posterior distribution of the predictand). Thus, by maximizing the BCS, forecasters can assure a utilitarian society of the maximum potential economic benefits of their forecast.

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Roman Krzysztofowicz

Abstract

Consider an event definable in terms of two subevents as, for example, the occurrence of precipitation within a 24-h period is definable in terms of the occurrence of precipitation within each of the 12-h subperiods. A complete forecast must specify three probabilities; these may be marginal probabilities, one for the period and two for subperiods. Theoretical relations between these probabilities are investigated and solutions are presented to three problems encountered in operational forecasting: (i) guaranteeing that the marginal probabilities jointly obey the laws of probability, (ii) structuring admissible procedures for adjusting the initial (guidance) probabilities by forecasters, and (iii) formulating optimal estimators of the probability for period in terms of the probabilities for subperiods.

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Roman Krzysztofowicz

Under the auspices of the Cooperative Program for Operational Meteorology, Education and Training, pilot research has been conducted from 1991 to 1996 on operational methods for producing Probabilistic Quantitative Precipitation Forecasts (PQPFs) and Probabilistic River Stage Forecasts (PRSFs). The first integrated forecasts were produced using operational data and models of the National Weather Service on 19 October 1996. The PQPF was for a 24-h period and the PRSFs were for lead times of 1, 2, and 3 days. This article discusses these pilot forecasts, their interpretation, and their advantages to forecasters and end users with regard to communicating uncertainties and making decisions.

All those whose duty it is to issue regular daily forecasts know that there are times when they feel very confident and other times when they are doubtful as to the coming weather. It seems to me that the condition of confidence or otherwise forms a very important part of the prediction, and ought to find expression.

—W. E. Cooke (1906)

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Roman Krzysztofowicz
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Roman Krzysztofowicz and Thomas A. Pomroy

Abstract

Disaggregative invariance refers to stochastic independence between the total precipitation amount and its temporal disaggregation. This property is investigated herein for areal average and point precipitation amounts accumulated over a 24-h period and disaggregated into four 6-h subperiods. Statistical analyses of precipitation records from 1948 to 1993 offer convincing empirical evidence against the disaggregative invariance and in favor of the conditional disaggregative invariance, which arises when the total amount and its temporal disaggregation are conditioned on the timing of precipitation within the diurnal cycle.

The property of conditional disaggregative invariance allows the modeler or the forecaster to decompose the problem of quantitative precipitation forecasting into three tasks: (i) forecasting the precipitation timing; (ii) forecasting the total amount, conditional on timing; and (iii) forecasting the temporal disaggregation, conditional on timing. Tasks (ii) and (iii) can be performed independently of one another, and this offers a formidable advantage for applications.

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Roman Krzysztofowicz and Ashley A. Sigrest

Abstract

The predictand of a probabilistic quantitative precipitation forecast (PQPF) for a river basin has two parts: (i) the basin average precipitation amount accumulated during a fixed period and (ii) the temporal disaggregation of the total amount into subperiods. To assist field forecasters in the preparation of well-calibrated (reliable) and informative PQPFs, local climatic guidance (LCG) was developed. LCG provides climatic statistics of the predictand for a particular river basin, month, and period (e.g., 24-h period beginning at 1200 UTC and divided into four 6-h subperiods). These statistics can be conditioned on information entered by the forecaster such as the probability of precipitation occurrence and various hypotheses regarding the precipitation amount and timing.

This article describes two probability models of the predictand, details guidance products, and illustrates them for the Lower Monongahela River basin in Pennsylvania. The first model provides marginal climatic statistics of the predictand on an “average” day of the month. The second model conditions the statistics on the timing of precipitation within the diurnal cycle. The resultant characterization of the precipitation process allows the forecaster to decompose the complex assessment of a multivariate PQPF into a sequence of feasible judgmental tasks.

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Roman Krzysztofowicz and Ashley A. Sigrest

Abstract

A comparative verification is reported of 2631 matched pairs of quantitative precipitation forecasts (QPFs) prepared daily from 1 October 1992 to 31 October 1996 by the Hydrometeorological Prediction Center (HPC) and the Weather Service Forecast Office in Pittsburgh (PIT). The predictand is the 24-h spatially averaged precipitation amount. The property of QPF being verified is calibration. Four interpretations of each QPF are hypothesized and verified: an exceedance fractile, a conditional exceedance fractile, the mean, and the conditional mean (with conditioning on precipitation occurrence).

Time series of calibration statistics support the following conclusions. (i) The HPC QPF, which lacks an official interpretation, is calibrated as the 18%–19% exceedance fractile and as the conditional median, on average. (ii) It serves as a useful guidance to local forecasters. (iii) Pittsburgh forecasters adjust the guidance in the correct direction to produce PIT QPF, whose official interpretation is the (unconditional) median. (iv) Relative to this interpretation, HPC QPF has a substantial overestimation bias, which hampers the calibration of PIT QPF. (v) The calibration of each QPF lacks consistency over time. (vi) To improve the potential for good calibration, the guidance QPF and the local QPF should be given the same probabilistic interpretation; the conditional median of the spatially averaged precipitation amount is recommended.

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Roman Krzysztofowicz and Ashley A. Sigrest

Abstract

From 1 August 1990 to 31 July 1995, the Weather Service Forecast Office in Pittsburgh prepared 6159 probabilistic quantitative precipitation forecasts. Forecasts were made twice a day for 24-h periods beginning at 0000 and 1200 UTC for two river basins. This is the first in a series of articles devoted to a comprehensive verification of these forecasts. The property verified herein is calibration: a match between forecast probabilities and empirical frequencies of events.

Monthly time series of calibration statistics are analyzed to infer (i) trends in calibration over time, (ii) the forecasters’ skill in quantifying uncertainty, (iii) the adaptability of forecasters’ judgments to nonstationarities of the predictand, (iv) the possibility of reducing biases through dynamic recalibration, and (v) the potential for improving calibration through individualized training.

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Ashley A. Sigrest and Roman Krzysztofowicz

Abstract

The predictand of a probabilistic quantitative precipitation forecast (PQPF) may be either a point precipitation amount or a spatially averaged precipitation (SAP) amount. At the current state of the art, it is the SAP amount (the volume of water accumulated over an area during a period) that is most predictable. This case study compares the climatic PQPFs of the two predictands within a river basin in the Appalachians, then highlights similarities and distinctions of which the forecasters should be aware. Empirical relations reveal whether or not a given statistic of the point precipitation amount is (i) locally invariant, that is, does not vary appreciably within some area so that a single estimate (e.g., a spatial average) can approximate the statistic at every point within the area, and (ii) amenable to averaging, that is, can be averaged over some area to obtain an approximation to the statistic of the SAP amount. The study also illustrates the effect of elevation on the statistics of point precipitation and highlights seasonal differences. The conclusions point to a need for local climatic guidance to help forecasters in calibrating PQPFs.

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