Showing papers in "Weather and Forecasting in 1986"
TL;DR: In this article, an overview of some conceptual models that are useful for nowcasting is given, which represent a variety of systems associated with midlatitude cyclones and also mesoscale convective systems in the tropics and midlatitudes.
Abstract: Imagery from radars and satellites is one of the main ingredients of nowcasting. When used to provide very detailed forecasts of precipitation for a few hours ahead, the imagery needs to be interpreted carefully in terms of synoptic and mesoscale phenomena and their mechanisms. This paper gives an overview of some conceptual models that are useful for this purpose. The models represent a variety of systems associated with midlatitude cyclones and also mesoscale convective systems in the tropics and midlatitudes. Specific phenomena discussed are warm conveyor belts, including those with rearward- and forward-sloping ascent in ana and kata cold frontal situations, respectively; cold conveyor belts ahead of warm fronts; narrow rainbands associated with line convection at the boundary of a pre-cold-frontal low-level jet; wide mesoscale rainbands associated with midtropospheric convection; squall lines in the tropics and midlatitudes; nonsquall mesoscale convective systems in the tropics and midlatitu...
160 citations
TL;DR: The early empirical rules developed by Finley, Showalter and Fulks, Fawbush, Miller and Starrett, and others have been corroborated by more recent theoretical work as discussed by the authors.
Abstract: Techniques that have evolved during the hundred years that scientific severe thunderstorm forecasts have been prepared are reviewed. The early empirical rules developed by Finley, Showalter and Fulks, Fawbush, Miller and Starrett, and others have been corroborated by more recent theoretical work. While significant efforts have been devoted to defining the severe thunderstorm environment, it is now obvious that these storms can occur under a variety of synoptic conditions. Severe thunderstorm forecasting consists in not only identifying the time and place that an environment compatible with such storms will exist but also in identifying suitable triggering mechanisms in that environment.
65 citations
TL;DR: In this article, surface weather data from PAM II stations in the region showed that during daylight hours of 18 December, when the winds were fairly strong, temperatures across Lower Michigan were relatively uniform and strong radiational cooling after sunset resulted in the formation of a very cold pool of air over western Lower Michigan.
Abstract: During the period 16–20 December 1983, the northern United States was enduring a record-breaking cold air outbreak. The cold air brought temperatures as low as −40°C to some areas while bringing considerable lake-effect snows to locations in the Ice of the Great Lakes. In western Lower Michigan much of the snow fell from the wind-parallel snowbands that resulted as the cold, strong northwesterly winds crossed the relatively warm waters of Lake Michigan. Analysis of surface weather data from PAM II stations in the region showed that during daylight hours of 18 December, when the winds were fairly strong, temperatures across Lower Michigan were relatively uniform. By 19 December, the strong winds subsided and strong radiational cooling after sunset resulted in the formation of a very cold pool of air over western Lower Michigan. Separating this cold air from the warm areas near the shore was the land-breeze front. Along the south end of the lake, radiational cooling served to intensify an existing ...
17 citations
16 citations
TL;DR: In this paper, six years of daily temperature and precipitation forecasting were studied for Urbana, Illinois, and the results showed that the consensus forecast was only 17% at the first day and negligible thereafter.
Abstract: Six years of daily temperature and precipitation forecasting are studied for Urbana, Illinois. Minimum temperature forecast skills, measured against a climatological control, are 57%, 48%, 34% and 20% for the respective forecast ranges of one, two, three, and four days. Maximum temperature skills are comparable. Precipitation probability skills of 29%, 19%, 6% and −2% are found for the same respective forecast ranges. However, our skill in predicting precipitation amount, given that a measurable quantity occurs, is only 17% at the first day range and negligible thereafter. An examination of objective National Weather Service (NWS) forecasts shows this guidance to be slightly less skillful than our consensus in forecasting temperature and precipitation. Some temporal improvement is found in both the consensus and guidance temperature forecasts, but none can be found in the more difficult problem of forecasting precipitation. Significant warm and dry biases are frequently found in both our consensu...
15 citations
TL;DR: In this article, the authors report the results of an empirical investigation of some methods for improving the quality of precipitation probability forecasts using past reliability data and combining these two types of forecasts via both averaging and a more sophisticated statistical aggregation procedure.
Abstract: This paper reports the results of an empirical investigation of some methods for improving the quality of precipitation probability forecasts. These methods include 1) techniques for adjusting subjective and objective forecasts using past reliability data and 2) techniques for combining these two types of forecasts via both averaging and a more sophisticated statistical aggregation procedure. The empirical results indicate that forecast performance can be improved through such methods, with the greatest improvements arising from averaging forecasts that have previously been adjusted.
15 citations
TL;DR: In this article, an analysis of more than 11 years of data indicates that the subjective PoP forecasts add information above and beyond that contained in the objective probability of precipitation (PoP) forecasts for all combinations of geographical area, lead time, and season investigated in this study.
Abstract: This paper addresses two specific questions related to the interrelationships between objective and subjective probability of precipitation (PoP) forecasts: Do the subjective forecasts contain information not included in the objective forecasts? Do the subjective forecasts make full use of the objective forecasts? With respect to the first question, an analysis of more than 11 years of data indicates that the subjective PoP forecasts add information above and beyond that contained in the objective PoP forecasts for all combinations of geographical area, lead time, and season investigated in this study. For longer lead times, this conclusion appears to contradict the results of earlier studies in which the two types of PoP forecasts were compared using aggregate skill scores. With regard to the second question, the statistical results demonstrate that the subjective forecasts generally do not make full use of the objective forecasts. However, these latter results are not as strong, in a statistica...
14 citations
TL;DR: In this paper, the authors focused on the development and movement of several mesoscale convective systems over the southern Plains during the night preceding the historic 24 May 1981 flash flood.
Abstract: During the evening of 24 May 1981, rainfall from a slow-moving, multicell thunderstorm exceeded 200 mm (8 in) in just 2 h over western sections of Austin, Texas. This intense precipitation, falling on previously saturated terrain, resulted in record flash flooding. The evolution of the large-scale meteorological setting associated with the flood is briefly examined. The presentation then focuses on the development and movement of several mesoscale convective systems (MCSs) over the southern Plains during the night preceding the flash flood. It is shown that two significant mesoscale convective outflow boundaries stalled and weakened in the Austin vicinity. The flash flood storm appears to have developed along a weak baroclinic zone left behind by these outflows. Additionally, the precursor MCSs primed the hydrologic setting in south central Texas as they produced 75–125 mm (3–5 in) rains. Upper-air soundings are considered and related to the anomalous (relative to other storms over Texas) movemen...
13 citations
TL;DR: In this paper, high resolution images of atmospheric soundings calculated from the VAS radiometer on the GOES satellite are presented for precipitable water and lifted index fields during two midsummer days that contain a wide variety of preconvective and convective conditions.
Abstract: Local forecasts often rely upon the extrapolation of trends seen in images of clouds from the GOES satellite. This work presents correspondingly high resolution images of atmospheric soundings calculated from the VAS radiometer on GOES. These VAS sounding images vividly depict moisture and stability conditions in preconvective regions, as though GOES were observing the United States with “stability detectors” instead of infrared detectors at 1–3 h intervals and 60 km horizontal resolution. False color images are presented for VAS-derived precipitable water and lifted index fields during two midsummer days that contain a wide variety of preconvective and convective conditions. Since each sounding image requires only 5 min to calculate with an automated regression algorithm on a minicomputer, it should be possible to process VAS data operationally for real-time objective analysis of potential convective instabilities.
12 citations
TL;DR: In this paper, the authors examined the thermodynamics of the lifting process, finding that the SLTs are sensitive mainly to surface dewpoint for very warm humid air and to surface temperature for cold air.
Abstract: When the temperature of a parcel lifted adiabatically from the surface to a given pressure level (SLT) is subtracted from the ambient temperature at that level, the result is the surface index (SLI) for that level. This quantity, for the 500-mb pressure level, has been found useful in the very-short-range prediction of deep and often severe convection. We examine the thermodynamics of the lifting process, finding that the SLTs are sensitive mainly to surface dewpoint for very warm humid air and to surface temperature for cold air. Nomograms are presented for determination of SLT's for 850, 700 and 500 mb, given surface temperature and dewpoint and a nominal surface pressure of 950 mb. An approximate correction is given for departures from this pressure. These are intended for use with the routine aviation surface observations. An additional nomogram is provided for use over the sea. An example of application is shown, based on hourly surface observations and on ambient temperatures interpolated b...
10 citations
TL;DR: In this paper, a study of trends in the quality of National Weather Service (NWS) forecasts from 1967 to 1985 is presented, focusing on forecasts of precipitation probabilities, maximum temperatures, and minimum temperatures.
Abstract: This paper describes the results of a study of trends in the quality of National Weather Service (NWS) forecasts from 1967 to 1985. Primary attention is focused on forecasts of precipitation probabilities, maximum temperatures, and minimum temperatures A skill score based on the Brier score is used to verify the precipitation probability forecasts, whereas the temperature forecasts are evaluated using the mean absolute error and percentage of errors greater than 10°F. For each element, trends are examined for objective forecasts produced by numerical-statistical models and for subjective forecasts formulated by NWS forecasters. In addition to weather element, type of forecast, and verification measure, results are stratified by season (cool and warm), lead time (three or four periods), and NWS region (four regions and all regions combined). At the national level, the forecasts for these three weather elements exhibit positive and highly significant trends in quality for almost all of the various ...
TL;DR: In this article, a case in which profiler winds, taken between the standard upper air observing times of 1200 and 0000 GMT, contributed to real-time experimental forecasts of severe convective weather is discussed.
Abstract: Wind profilers are capable of obtaining horizontal wind data from approximately 1–17 km at hourly intervals. A network of four profilers is being tested in Colorado. This note discusses a case in which profiler winds, taken between the standard upper air observing times of 1200 and 0000 GMT, contributed to real-time experimental forecasts of severe convective weather.
TL;DR: In this paper, the authors describe new operational and experimental forecasting programs at the Swedish Meteorological and Hydrological Institute (SMHI) designed to provide users with more detailed and more useful weather forecasts.
Abstract: This paper describes new operational and experimental forecasting programs at the Swedish Meteorological and Hydrological Institute (SMHI) designed to provide users with more detailed and more useful weather forecasts. User groups currently served by these programs include construction contractors, farmers, electric power companies, street and highway departments, and ski resorts. The programs represent a major component of a SMHI-wide effort to develop products to meet the needs of the public and private sectors in Sweden for meteorological and hydrological information. An important feature of these programs is that many of the forecasts are expressed in probabilistic terms, and some results of the probability forecasting components of four programs are presented here. These subjective forecasts specify the likelihood of occurrence of various precipitation, wind speed, temperature, and cloud amount events, and they generally involve relatively short lead times and/or valid periods. The probabili...
TL;DR: In this article, the authors evaluated the biases of objective forecasts of maximum and minimum temperature for Phoenix relative to the observed temperatures and found that a local or regional change in weather conditions may be an important factor in explaining the MOS biases.
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.
TL;DR: In this article, the authors investigated the reasons for the failure of the forecast models of supertyphoon Abby (1983) and found that for a large part of Abby's lifetime, nearly all forecast tracks were almost 90° to the left of the actual track of the cyclone.
Abstract: Supertyphoon Abby (1983), although not one of the most destructive on record, received a great deal of attention from the typhoon forecasters in Guam. For a large part of Abby's lifetime, nearly all objectively predicted tracks were almost 90° to the left of the actual track of the cyclone. This study is an attempt to understand the reasons for the failure of the forecast models. The intensity and size (horizontal extent) of the supertyphoon are hypothesized to be the main factors contributing to such a forecast failure. After intensifying to a maximum wind speed of 75 m s−1 (145 kt), Abby continued to grow, with the radius of 15 m s−1 (30 kt) winds extending beyond 600 km. Abby's circulation, which can be readily identified on synoptic charts, apparently affected the performance of the dynamical models. The “steering flow” vector as estimated from the operational analyses is found to be almost normal to the motion vector of Abby, which might provide a partial explanation of the forecasts by the ...
TL;DR: In this paper, an investigation was undertaken to determine how different the weather forecasts of telecasters are from those of the National Weather Service for the same areas and times, and the sources of information telecasters use when they modify the NWS forecast or develop their own.
Abstract: This investigation was undertaken to determine how different the weather forecasts of telecasters are from those of the National Weather Service for the same areas and times, and the sources of information telecasters use when they modify the NWS forecast or develop their own. Questionnaires were mailed to 453 television weatherpersons, most of whom were seal holders of the AMS; 67 percent were returned. It was determined that 1) only 6% of all respondents do not consider the NWS forecast at all, whereas for almost 60% this consideration is moderate or heavy; 2) the percentages of those who transmit a.forecast different from that of the NWS 0–10%, 40–60% and 90–100% of the time are 16, 30 and 8, respectively; 3) when the forecast issued was different from that of the NWS, the elements most likely to be different were precipitation (84%), temperature (82), state of sky (66), and wind speed and direction (34); 4) the factors that influence the telecaster to issue a different forecast are, in order,...