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Robert P. Harnack and William R. Sammler

Revised and complete verification statistics for mainland United States long-range forecasts made for the period 1976–80 by the 1976 version of the University of Wisconsin model are presented. Corrections to earlier published values are given, as well as skill scores obtained using a much more complete set of stations for which forecasts were made.

The overall skill score for the pentad temperature forecasts made for January, April, July, and October is negative (−0.14), while those for pentad precipitation and individual year July precipitation forecasts are positive (0.12 and 0.04, respectively). The individual year January temperature forecast skill score was unchanged at −0.08 overall.

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Robert P. Harnack and John R. Lanzante

Abstract

North Pacific and North Atlantic SST (sea surface temperature) were used separately and in combination to specify seasonal-mean North American 700 mb heights. One of the goals was to quantify these relationships so that the importance of North Atlantic versus North Pacific SST could be assessed. Sea surface temperature predictors were in the form of EOF (empirical orthogonal function) amplitudes while the predictands consisted of seasonal-mean 700 mb heights at each of 25 locations over North America. Linear regression analysis was used in the data period 1949–77 to build three kinds of models: 1) using the first five North Pacific SST EOFs, 2) using the fist five North Atlantic SST EOFs and 3) using five EOFs from each field, but screening to produce the best five predictor models.

The principal findings can be summarized as:

1) Based on area-averaged skill and percent area of significant skill, North Pacific SST is a better specifier of 700 mb height than North Atlantic SST.

2) Pacific SST models have significant overall skill for all seasons except spring, with area-averaged true skill being greatest in winter (¯S = 0.247) and least in spring (¯S = 0.061).

3) Atlantic SST models do not attain field significance in any season, but perform best overall in winter (¯S = 0.095).

4) A portion of the region studied for winter and summer contained grid point locations where testing indicated that Atlantic SST adds significant information to that of Pacific SST in explaining variations of 700 mb height. This amounted to 13 and 15% of the total area, respectively, which was not enough to declare field significance.

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Robert P. Harnack and John R. Lanzante

Abstract

Seasonal precipitation is specified for the United States by matching various area-averaged precipitation statistics as predictands with three different predictors in turn: 700 mb heights, North Pacific SST and North Atlantic SST. Predictors are in the form of empirical orthogonal function (EOF) amplitude time series. The predictands used in trials include total precipitation and precipitation frequencies derived using three different critical values: 2.5, 12.7 and 25.4 mm. Screening multiple linear regression is used to relate predictands to predictors for samples ranging from 24 to 35 years in length; initially trials are compared in terms of area-averaged true skill and percent area of local significance. In order to assess specification skill on an independent sample, additional tests are made using a jackknife regression approach.

Results suggest that skillful seasonal precipitation prediction will continue to be very difficult using predictors and methods presently in common use based on the use of specification equations on an independent sample. Generally, area-averaged explained variances are less than 10% and the area of significant local skill is less than 50%. Based on the low level of specification skill, predictive skill for precipitation using specification equations with imprecisely known specifier fields (like 700 mb heights) as input would be effectively zero.

Other conclusions are:

  1. 700 mb heights specify seasonal precipitation about equally well in winter, spring and summer, but worse in fall.

  2. Among the three predictor types employed, 700 mb heights are best for all seasons but fall, when Pacific SST does best. Specification using Atlantic SST is poor in all instances and inferior to the use of the other predictor fields.

  3. Overall among the four precipitation statistics used as predictands, the frequency statistics have a slightly better relationship with 700 mb heights or Pacific SST than do precipitation totals.

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Robert P. Harnack and William R. Sammler

The 1976 version of the University of Wisconsin model's ultra long-range forecasts of monthly mean temperature and precipitation were verified for selected United States stations over the period 1976–80. In an overall sense, neither the pentad category forecasts for four months, nor the individual year forecasts for two months, showed significant skill relative to random chance expectation. Slight positive skill was found for the July precipitation forecasts. Considerable variability of skill scores were seen from one month type to another, and from year to year. The lack of demonstrated significant skill overall for the 1976–80 period contrasts with the positive results reported by the modelers for independent sample forecasts made for the period 1961–75.

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R. P. Harnack and H. E. Landsberg

Abstract

The release of isolated summer showers in the Washington D.C. area, as related to the urban heat island, was studied for nine propitious synoptic situations in 1968, 1972 and 1973. Parcel theory, using urban surface temperature and upper air soundings, permitted comparison between predicted and observed cloud behavior. In all cases the urban thermal effect seemed to be the likely trigger force for shower development. Vertical wind data and cloud energetics permitted an estimate of rainfall positioning in the metropolitan area. In eight of the nine cases, this yielded the correct placing of the urban-induced showers. The study further documents this type of inadvertent rainfall augmentation. Three cases are presented here.

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John R. Lanzante and Robert P. Harnack

Abstract

An investigation of the January thaw phenomenon, a period of unseasonable warmth, was conducted using daily maximum temperatures recorded at New Brunswick, New Jersey, from 1858–1981. Student's t-tests, comparing long-term means of daily maximum temperature to values from a fitted seasonal trend curve, indicate temperatures higher on 22–23 January and lower on the 29th than seasonally expected.

It was found that the January thaw does not have a fixed time of occurrence but occurs most frequently from the 19th to the 28th. During this time the interannual variability of daily maximum temperature is significantly higher than during the remainder of the month.

Evidence of a tendency for a secondary thaw maximum to occur, centered on the 26th, is evident in several different analyses. Examination of daily temperature curves for 10-, 20- and 40-year periods reveals a shift in the mean thaw date from 22–23 January to the 26th. This change has evolved over the last 30–40 years. It was concluded that the January thaw is more pronounced when the mean circulation is characterized by a contracted polar vortex over North America and abnormally strong midlatitude westerlies.

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John R. Lanzante and Robert P. Harnack

Abstract

The specification of summer season precipitation in the contiguous United States from summer season fields of 700 mb height, sea level pressure (SLP) and Pacific sea surface temperature (SST) was carried out using stepwise multiple linear regression. The specifier fields were characterized by their first five Empirical Orthogonal Functions (EOF's). The objectives were to assess the overall skill in specifying summer season precipitation, examine the differences among predictands with regard to both spatial averaging and type of statistic, compare the usefulness of the specifier fields, and to look at spatial variations in specification skill.

Overall, the strongest relationships between actual summer season precipitation and the predictors were found for 700 mb heights (R 2 ∼ 0.24) followed by Pacific SST’s (R 2 ∼ 0.21) and SLP (R 2 &sim 0.12). The use of large area averages (∼ 105 km2) for the predictand produced slightly greater R 2 values than for individual climatic division averages (∼ 1O4 km2).

The use of transformed summer season precipitation statistics to account for precipitation skewness, did not improve upon the use of actual summer season precipitation as the predictand. However, frequency of precipitation greater than 0.1 inch resulted in an almost doubling of explained variances over actual precipitation (0.47 versus 0.24) when 700 mb heights were used as the specifier field.

The areas of weakest relationship (west of the Rockies and southern states) between predictor and summer precipitation statistic generally had R 2 values less than 0.3, even for the best models. Elsewhere, the R 2 values generally ranged from 0.5 to 0.7 for the best model (700 mb heights and precipitation frequency). After accounting for artificial predictability which results from imperfect estimates of the statistics, skill values (explained variances) cast of the Rockies ranged from 0.01 to 0.44.

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Robert P. Harnack, Donald T. Jensen, and Joseph R. Cermak III

Abstract

Analyses of proximity soundings and upper-air fields for 37–51 Utah severe wind cases (WIND), reported in the months of May–September and occurring within 3 h after upper-air observation time, are presented. In addition, a comparison of sample mean values between the WIND cases and a climatological sample (CLIM) is made using a standard t test to determine which variables are significantly different between the two samples. This study seeks to determine if the synoptic-scale-derived fields play a significant role in producing severe wind for a region in which subsynoptic effects, attributed to uneven terrain, are important. The WIND sample environment had the following important differences when compared to CLIM:

  1. more convergent wind in the lower troposphere (700-mb moisture and wind convergence),

  2. greater moisture at 500 mb (dewpoint, mixing ratio),

  3. greater positive vorticity advection (500 mb) and differential vorticity advection (700–500 mb),

  4. a larger lapse rate based on various stability indices,

  5. more southerly component flow at levels from 500 to 200 mb,

  6. higher absolute vorticity at levels from 300 to 200 mb,

  7. greater 500-mb wind speeds, and

  8. larger thermal advection (warm) at 200 mb.

Taken together, the statistical results combined with examination of individual cases and composite maps, suggest that severe wind events in Utah are commonly associated with an approaching upper-level trough system that provides enhanced lift, increased thermal instability, and increased midlevel moisture. These changes to the environment, when added to the normally dry, well-mixed, neutrally stratified boundary layer of the afternoon–evening hours, likely promotes high-based convection with severe downbursts at times. Discriminating effects on the subsynoptic scale cannot be determined in this study since only the standard upper-air station network of observations is employed and no surface data is used. Sample mean differences are small and intrasample variability is large, so results must be used with considerable caution in forecasting applications.

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Robert P. Harnack, Kirk Apffel, and Joseph R. Cermak III

Abstract

The first of an anticipated multipart study of atmospheric conditions occurring before and during heavy precipitation events in New Jersey, representative of the mid-Atlantic region, is presented. Upper-air data parameters were analyzed for 81 cases of heavy precipitation observed in the period 1958–93. These variables consisted mainly of standard level (850–100 mb) temperature, moisture, wind, equivalent potential temperature, vorticity, and height, plus selected advections, divergences, and stability indices. Means and standard deviations of variables, stratified by season, were calculated over the event location near the start time of the event. In addition, sample mean values were compared between the event sample and a climatological sample. A standard t test was used to determine which variables are significantly different between the two samples.

Composite maps are presented for selected variables of interest that confirm that the large-scale environment near the beginning of events is altered significantly from the background climatology, as expected. Histograms are used to show frequency distributions for variables that had associated high levels of significance, which further illustrate the upper-air changes that occur and form a basis for selecting key values of important variables for possible operational use. A key values table is presented to facilitate the operational use of these results.

The most common findings among the four seasons are higher moisture from 850 to 400 mb, moisture convergence in the lower troposphere, wind divergence in the upper troposphere and convergence in the lower troposphere, warm advection in the low to midtroposphere, positive moisture advection (except for summer), and higher temperatures at most levels (except for summer). While these results are not unexpected, the magnitude of midtropospheric moisture and wind divergence difference (obtained using radiosonde wind observations) between samples is somewhat surprising.

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