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Comparative Verification of Guidance and Local Quantitative Precipitation Forecasts: Calibration Analyses

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  • 1 Department of Systems Engineering and Division of Statistics, University of Virginia, Charlottesville, Virginia
  • | 2 Department of Systems Engineering, University of Virginia, Charlottesville, Virginia
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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.

Corresponding author address: Professor Roman Krzysztofowicz, University of Virginia, Thornton Hall, SE, Charlottesville, VA 22903.

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.

Corresponding author address: Professor Roman Krzysztofowicz, University of Virginia, Thornton Hall, SE, Charlottesville, VA 22903.

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