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Gregory S. Forbes

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Gregory S. Forbes

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A study of radar echoes associated with the tornadoes of the 3 April 1974 outbreak was performed to evaluate the usefulness of echo shape as an indicator of tornadic thunderstorms. The hook shape was usually successful in characterizing an echo as tornadic, with a false alarm rate of 169%. Because hook echoes were relatively rare, however, a less restrictive shape called distinctive was more successful at detecting tornadic thunderstorms, identifying 65% of the tornadic echoes. An echo had a distinctive shape if it possessed a marked appendage on its right rear flank or was in the shape of a spiral, comma or line echo wave pattern (LEWP). Characteristics of the distinctive echo are given.

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Catherine A. Carlson and Gregory S. Forbes

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Horizontal divergence, relative vorticity, kinematic vertical velocity, and geostrophic and ageostrophic winds are computed from Colorado profiler network data to investigate an upslope snowstorm in northeastern Colorado. Horizontal divergence and relative vorticity are computed using the Gauss and Stokes theorems, respectively. Kinematic vertical velocities are obtained from the surface to 9 km by vertically integrating the continuity equation. The geostrophic and ageostrophic winds are computed by applying a finite differencing technique to evaluate the derivatives in the horizontal equations of motion. Comparison of the synoptic-scale data with the profiler network data reveals that the two datasets are generally consistent. Also, the profiler-derived quantities exhibit coherent vertical and temporal patterns consistent with conceptual and theoretical flow fields of various meteorological phenomena. It is suggested that the profiler-derived quantities are of potential use to weather forecasters in that they enable the dynamic and kinematic interpretation of weather system structure to be made and thus have nowcasting and short-term forecasting value.

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Robert E. Hart and Gregory S. Forbes

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This paper presents results from pilot studies of the use of model-generated hourly soundings to forecast nonconvectively produced strong wind gusts. Model soundings from the operational Eta and Meso Eta Models were used for a period of 14 months in 1996 and 1997. Skill does exist in forecasting strong to damaging surface wind gusts, although the forecasts are at the mercy of the model-based boundary layer stability forecast. The wind gust forecasts are more accurate during the daytime, when the boundary layer depth and stability is more accurately forecasted and also more conducive to vertical mixing of boundary layer winds. The results of this preliminary evaluation show that the model sounding–based forecasts provide a reasonable prediction tool for nonconvective strong wind gusts. Additionally, the results warrant more complete evaluations once the dataset has grown to sufficient size.

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Gregory S. Forbes and Jonathan H. Merritt

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The occasional occurrence of wintertime mesoscale lake vortices is documented. The vortices are readily discernible in satellite imagery, in which they take one of three forms: a miniature comma cloud, a swirl of cloud bands (resembling a miniature tropical storm) or a swirl of cloud streets. Despite their impressive appearance in satellite imagery, these vortices are usually relatively mild in comparison with other lake-effect storms and produce only gusty winds and brief snow squalls as they move onshore. The vortices are accompanied by a slightly lowered surface pressure and a weak cyclonic low-level wind circulation.

Fourteen vortices were detected over the Great Lakes in the years 1978–82; they occurred under conditions of relatively weak surface pressure gradient, with a ridge of high pressure usually found over or west of the region. Convergence was generally detected in the surface winds prior to vortex development, apparently related to land breeze circulations. Comparisons are made between the conditions favoring the occurrence of shoreline-parallel cloud bands and lake vortices. Comparisons are also made between lake vortices and polar vortices and i.e., mesoscale vortices occurring in polar airstreams over oceans.

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Gregory S. Forbes and Roger M. Wakimoto

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A remarkable case of severe weather occurred near Springfield, Illinois on 6 August 1977. Aerial and ground surveys revealed that 17 cyclonic vortices an anticyclonic vortex, 10 downbursts and 19 microbursts occurred in a limited (20 km × 40 km) area, associated with a bow-shaped radar echo. About half Of the vortices appeared to have occurred along a gust front. Some of the other appear to have occurred within the circulation of a mesocyclone accompanying the bow echo, but these vortices seem to have developed specifically in response to localized boundary-layer vorticity generation associated with horizontal and vertical wind shears on the periphery of microbursts. Some of these vortices, and other destructive vortices in the literature, do not qualify as tornadoes as defined in the Glossary of Meteorology. A moon pragmatic definition of a tornado is suggested.

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Gregory S. Forbes and Howard B. Bluestein

This article is one in a series of papers from the Symposium on the Mystery of Severe Storms: A Tribute to the Work of T. Theodore Fujita. It reviews Fujita's vast contributions to the understanding of tornadoes and tornadic thunderstorms and briefly summarizes a few complementary studies on these topics by other authors. This article also reviews Dr. Fujita's applications of photogrammetry to the study of thunderstorms and tornadoes. Comments about Ted Fujita's influence on the authors' careers are interspersed throughout the paper and in the acknowledgments.

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Robert E. Hart, Gregory S. Forbes, and Richard H. Grumm

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Since late 1995, NCEP has made available to forecasters hourly model guidance at selected sites in the form of vertical profiles of various forecast fields. These profiles provide forecasters with increased temporal resolution and greater vertical resolution than had been previously available. The hourly forecast profiles are provided for all of NCEP’s short-range models: the Nested Grid Model, Eta Model, and Mesoscale Eta Model. The high-resolution forecasts aid in the timing of frontal passages, low-level jets, and convective initiation. In addition, through time–height cross sections of Richardson numbers, forecasters can alert pilots to the potential for clear air turbulence several hours to a day in advance. Further, the profiles are useful in prediction of cloudiness and the dissipation of low-level stratus and fog. Time–height cross sections of wind velocity have proven extraordinarily useful in visualizing and forecasting inversion heights, frontal passage timing, boundary layer depth, and available environmental and storm-relative helicity during convective events.

The hourly model forecasts were found to be exceptionally helpful when combined with hourly surface observations to produce enhanced real-time analyses of convective parameters for use in very short term forecasting. High-resolution analyses of lifted index, CAPE, convective inhibition, moisture flux convergence, and 2-h changes in these fields aid the forecaster in anticipating convective trends. The introduction of model forecast error into these real-time analyses was minimized by using the latest available Eta or Mesoscale Eta Model runs. Therefore, the model data used to enhance the analyses are typically no more than 6–12 h old.

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Paul J. Roebber, Lance F. Bosart, and Gregory S. Forbes

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An outstanding issue in the assessment of forecast skill (and value) is whether any advantage that can be obtained through regional knowledge not readily available to distant forecasters supersedes the leveling effect of information obtained by all forecasters (through the interpretation of numerical weather predictions). An analysis of 1 yr of data from the National Collegiate Weather Forecasting Contest was conducted in order to evaluate whether physical separation from the forecast site (defined by distances outside of and within 1000 km, with a minimum separation of 100 km) has a measurable effect on skill. The results indicate that regional effects (on the meso-α scale) are manifested in forecasts of both temperature (maximum and minimum) and precipitation amount (by category). Furthermore, these effects are a function of the experience level of the forecaster. Specifically, experienced forecasters are able to use regional knowledge to their advantage in forecasting temperature and precipitation amount, while their less-experienced counterparts cannot advantageously use such information for either type of forecast. The implication of these results with respect to the allocation of National Weather Service resources is also addressed.

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Gregory S. Forbes, Dennis W. Thomson, and Richard A. Anthes

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An interesting ice storm of moderate severity occurred along the east slopes of the Appalachians on 13–14 January 1980. Though surface temperatures were initially below freezing in most of this region, objective guidance indicated that large-scale warm would render the atmosphere conducive to rain. Warm advection did occur above about 900 mb, but below this level warm advection was prevented by a cold ~ shaped ridge of high pressure which became entrenched along the east slopes. Temperature in the lowest 0.5–1 km remained below freezing and an ice storm resulted.

This case study documents the evolution of the wedge ridge and the temperature and wind fields associated with it. Comparisons are made between the evolution of these fields within the quasi-stationary wedge ridge (a weather regime known as cold-air damming and their evolution during the preceding period, when the pressure ridge was progressing eastward across the Midwest The processes controlling the charges of temperature in these regimes are analyzed; cold advection and upslope flow maintain the cold dome. Cross sections are used to present detailed analyses of the vertical structure and evolution of the temperatures and winds within the damming region. Interesting features include the development of an “extended coastal front”—the sloping inversion separating the trapped cold dome from the warm onshore flow above, a jet parallel to the mountain at low levels, and an enhanced flow over the mountain near its crest. Apparently due to the lack of vertical resolution sufficient to capture such features operational numerical models exhibited substantial errors in this case.

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