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Ira S. Brenner

Abstract

The biases of objective forecasts of maximum and minimum temperature for Phoenix are evaluated relative to the observed temperatures. The temperature forecasts were calculated from regression equations that had been derived from model output statistics (MOS). During the analysis period, from October 1982 through September 1984, significant cold biases of ∼1.5–2.5°F (0.8–1.4°C), were determined for the MOS minimum temperature forecast at 24, 36 and 48 h. The maximum temperature forecasts had warm biases <1.0°F (0.6°C) that were significant only at 24 h. The minimum and maximum temperatures from the most recent 30-year normals (1951–80) were, respectively, ∼5°F (3°C) and 1°F (0.6°C) colder than the observed temperatures during the analysis period. The difference between the climatological and observed minimum temperatures is significant at the 1% level and suggests that a local or regional change in weather conditions may be an important factor in explaining the MOS biases.

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Ira S. Brenner

Abstract

Considerable daily variations of summer convective rainfall average areal coverage and rainfall amount were identified in west-central Florida for the period May–September 1997–2000 using a 29-site rainfall network. Pearson correlation coefficients identified the correlations to each from among 16 parameters that can be extracted directly from the 1200 UTC radiosonde data at Ruskin, Florida, and that represent moisture, stability/temperature, and flow. The highest correlations were with all of the moisture parameters—precipitable water, minimum theta- e temperature, wet-bulb zero pressure, and average dewpoints in various layers from the 850- to 500-mb height level.

Multiple linear regression analysis produced a separate prediction equation each for average areal coverage and rainfall amount, which were tested on independent data from May to September 2001. Reliable predictions of the trend direction and magnitude of the change from the observed value of the previous day occurred about 75% of the time with the average prediction error generally within ±10% (areal coverage) and ±0.10 in. (rainfall amount). When the observed trend changed by at least 20% for areal coverage (39 cases), and at least 0.20 in. for average rainfall amount (36 cases), the trend direction was correctly predicted 100% and about 90% of the time, respectively. Of these, the predictions for areal coverage underforecast both the amount of observed increase and decrease by an average of 8% and 6%. For rainfall amount, the predictions underforecast both the magnitude of observed increase and decrease by about 0.18 and 0.06 in., respectively.

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