Showing papers in "Weather and Forecasting in 1995"

Winter Weather Forecasting throughout the Eastern United States. Part IV: Lake Effect Snow

Abstract: This article is the final installment of a four-part series that examines the challenge of forecasting winter weather throughout the eastern United States. This paper examines the problems and challenges of forecasting lake effect snows. The climatology of lake-induced snowfall is reviewed, and an overview of the characteristics and evolution of these mesoscale precipitation bands is presented. The atmospheric conditions associated with five different types of lake snow bands are discussed. The abilities of remote sensors to resolve, and dynamical models to simulate, these mesoscale events are also explored. Finally, several techniques designed to improve operational forecasts of lake effect snow are described in detail, along with representative case studies.

The Bow Echo: Observations, Numerical Simulations, and Severe Weather Detection Methods

Abstract: Bowing convective line segments (bow echoes) are often associated with swaths of damaging downburst winds and are sometimes accompanied by tornadoes that may reach violent (F4) intensity. Bow echoes range in length from less than 20 km to more than 100 km, the latter forming a broad bowing line segment. In this manuscript, both past and recent observations of bow echo convective systems will be reviewed. The meteorological environments of long-lived derecho-producing convective systems will also be presented. This will be followed by recent conceptual models and numerical simulations of bowing convective systems. Finally, the evolution of two large-scale bow echo convective systems, which produced extensive wind damage and weak tornadoes, will be described. With the ongoing installation of a comprehensive network of Doppler radars, the National Weather Service forecaster will be required to know more about conceptual storm structure models of severe convective storms. It is hoped that this litera...

Evaluation of 33 Years of Quantitative Precipitation Forecasting at the NMC

Abstract: The National Meteorological Center (NMC) initiated Quantitative Precipitation Forecasts (QPF) and an intensive QPF verification program in 1960. These forecast products have evolved from a manual effort, relying on extensive forecast experience to one that placed much greater reliance on the interpretation and modification of numerical models. Verification graphs show steady improvements in forecast accuracy, especially for the longer-range forecasts, which in this context am those in the 24–60-h range. During the 1960s the Threat Score (TS) for day-2 forecasts for 1 in or more of precipitation averaged approximately 0.07. During recent years, that score has nearly doubled, and the 36–60-h period forecast in 1993 had a TS comparable to that for the 12–36-h period during the 1960s. Improvement in accuracy is probably related to a number of diverse factors including improved numerical models, increased forecaster knowledge of the strengths and weaknesses of the operational models, and an increased ...

Advances in the Detection and Analysis of Fog at Night Using GOES Multispectral Infrared Imagery

Abstract: A technique is described for the detection of fog and low clouds at night using multispectral infrared (IR) imagery from Geostationary Operational Environmental Satellites (GOES). The technique requires subtraction and enhancement of digital data from IR window channels at 3.9- and 10.7–11.2-µm wavelengths. Resulting images show stratiform clouds over any type of terrain for a wide range of surface temperature conditions. The bispectral difference images are a considerable improvement over the use of enhanced images for a single window IR channel. An image enhancement technique that displays the approximate depth of fog was developed based on the correlation of brightness differences in the two IR channels to cloud-top heights reported by aircraft. The improved resolution and greater frequency provided by the GOES I-M satellites will result in a substantial improvement in the ability to monitor areas of reduced ceilings and visibilities at night.

A Climatology of the Warm Season Great Plains Low-Level Jet Using Wind Profiler Observations

Abstract: Hourly observations from the Wind Profiler Demonstration Network during the warm season months of 1991 and 1992 were used to develop a climatology of the low-level jet (LLJ) over the Great Plains of the central United States. The maximum overall frequency of LLJ occurrence was in the southern part of the Great Plains, while the maximum frequency of the stronger LLJs extended farther north and east (Kansas through Nebraska). The frequency of occurrence for the weakest LLJs exhibited little diurnal variation. In contrast, the strongest jets were about six times more likely to occur within a few hours of local midnight than during the day. Southerly wind events that did not satisfy the criteria for low-level jets (i.e., those that did not include the low-level local maximum of the wind speed profile) showed comparatively little diurnal variability regardless of speed. Analysis of the synoptic patterns associated with LLJ occurrence showed that the LLJ was promoted by the warm sector of an extratropi...

Large-Scale Patterns Associated with Severe Summertime Thunderstorms over Central Arizona

Abstract: Severe thunderstorms are relatively rare over Arizona and occur most frequently during the summer monsoon period, that is, July, August, and early September. Forecasting in Arizona during the summertime is quite difficult and skill scores are low for both precipitation and severe thunderstorm watches and warnings. In the past, due to the sparse population of Arizona, severe thunderstorms usually impacted few people and were considered relatively insignificant events. However, over the last 20 years, the population of central Arizona has grown dramatically, and the impact of severe thunderstorm and flash flood occurrences has also increased. Synoptic conditions associated with 27 severe thunderstorm events that occurred in central Arizona during the summer monsoon have been examined systematically and compared to long-term mean July conditions. The period of study covered 1978 to 1990, and cases selected were limited to the high population area of central Arizona. McCollum subjectively identified ...

The Coefficients of Correlation and Determination as Measures of performance in Forecast Verification

Abstract: This paper is concerned with the use of the coefficient of correlation (CoC) and the coefficient of determination (CoD) as performance measures in forecast verification. Aspects of forecasting performance that are measured—and not measured (i.e., ignored)—by these coefficients are identified. Decompositions of familiar quadratic measures of accuracy and skill are used to explore differences between these quadratic measures and the coefficients of correlation and determination. A linear regression model, in which forecasts are regressed on observations, is introduced to provide insight into the interpretations of the CoC and the CoD in this context. Issues related to the use of these coefficients as verification measures are discussed, including the deficiencies inherent in one-dimensional measures of overall performance, the pros and cons of quadratic measures of accuracy and skill vis-a-vis the coefficients of correlation and determination, and the relative merits of the CoC and the CoD. These c...

The Regional Analysis System for the Operational “Early” Eta Model: Original 80-km Configuration and Recent Changes

Abstract: The analysis component of the National Centers for Environmental Prediction (NCEP) operational “early” 80-km eta model, as implemented in July 1993, is described. This optimum interpolation (OI) analysis is fully multivariate for wind and geopotential height (univariate for specific humidity) and is performed directly on the eta model's vertical coordinate. Although the eta OI analysis and model performance has been generally favorable when compared to the Limited-Area Fine Mesh Model (LFM) and the Nested Grid Model (NGM), deficiencies in the eta OI analysis fields have been observed, especially near the surface. A series of improvements to the eta OI analysis is described. A refinement to the eta model orography, which created a more realistic depiction of the model terrain, is also discussed along with the impact of these changes on analysis and model performance. These changes were implemented in the early eta system in September 1994. The operational configuration of the new mesoscale (29 km)...

The LAPS Wind Analysis

Abstract: The Local Analysis and Prediction System (LAPS) wind analysis combines Doppler radar, profiler, aircraft, and surface wind observations into a three-dimensional gridded wind field. Other fields derived primarily from the wind analysis include radar echo steering wind, helicity, and lifted index times vertical velocity (an indicator of convective potential). This report describes LAPS wind analysis procedures. The goal is to combine various data sources to take advantage of the strengths of each source, thereby automating the data synthesis that a human forecaster performs. The utility of these analyses to the operational forecaster is an important part of designing the analysis procedure and output displays. Comparisons of LAPS wind fields with independent aircraft measurements obtained during the Winter Icing and Storms Project experiment indicate agreement generally within 4 m s−1 (rms).

Severe and Convective Weather: A Central Region Forecasting Challenge

Abstract: This paper is one of a series of papers dealing with operational weather forecasting within the Central Region of the National Weather Service. It focuses on the primary Central Region warm season weather producer, the thunderstorm. Some of the unique aspects of Central Region thunderstorms are highlighted. Techniques used to answer four questions routinely posed by forecasters are described. These questions are the following: 1) Will thunderstorms occur that will affect my area of forecast responsibility? 2) If thunderstorms develop, will these thunderstorms reach severe intensity? 3) If these thunderstorms reach severe intensity, what types of severe phenomena are likely with these storms? and 4) If thunderstorms occur, what storm type is most likely to be observed? A case from 14 May 1990 is used to illustrate the application of some of the techniques.

Diurnal Corrections of Short-Term Surface Temperature Forecasts Using the Kalman Filter

Abstract: Surface temperature forecasts from numerical weather prediction models are known to have systematic errors, partly due to poor resolution of the topography and deficiencies in the physical formulation. The Kalman filter theory provides an excellent tool for combining observations and forecasts to correct such systematic errors. As detailed in this paper, deviations between temperature forecasts and observations were studied at several locations in Norway. It was found that the sign and magnitude of the deviations varied considerably in both time and space. Moreover, the diurnal variation of the systematic errors increased as the diurnal variation between daytime and nighttime increased. A Kalman filter model has been constructed that allows for diurnally varying corrections. Eight corrections, valid at 0000, 0300, 0600, 0900, 1200, 1500, 1800, and 2100 UTC, are calculated simultaneously. These corrections, which are weighted means of previous differences between observations and forecasts, are ap...

Case Study of a Severe Mesoscale Convective System in Central Arizona

Abstract: A mesoscale convective system (MCS) developed over central Arizona during the late evening and early morning of 23–24 July 1990 and produced widespread heavy rain, strong winds, and damage to buildings, vehicles, power poles, and trees across northern sections of the Phoenix metropolitan area. Although forecasters from both the National Weather Service and National Severe Storms Laboratory, working together in the 1990 SouthWest Area Monsoon Project (SWAMP), did not expect thunderstorms to develop, severe thunderstorm and flash flood warnings were issued for central Arizona between 0300 and 0500 local standard time. This study examines the precursor and supportive environment of the mesoscale convective system, drawing upon routine synoptic data and special observations gathered during SWAMP. During the evening of 23 July and the early morning of 24 July, low-level southwesterly flow developed and advected moisture present over southwest Arizona across south-central Arizona into the foothills and...

Winter Weather Forecasting throughout the Eastern United States. Part I: An Overview

Abstract: The complex combination of synoptic- and mesoscale interactions topographic influences, and large population densities poses a multitude of challenging problems to winter weather forecasters throughout the eastern United States. Over the years, much has been learned about the structure, evolution, and attendant precipitation within winter storms. As a result, numerous operational procedures, forecast applications, and objective techniques have been developed at National Weather Service offices to assess the potential for hazardous winter weather. An overview of the challenge of forecasting winter weather in the eastern United States is presented, including a historical review of several legendary winter storms, from the Blizzard of 1888 to the Halloween Nor'easter of 1991. The synoptic-scale features associated with East Coast winter storms are described. The mesoscale nature of many eastern winter weather events is illustrated through an examination of the Veterans' Day Snowstorm of 11 November ...

A Comparison of Temperature and Wind Measurements from ACARS-Equipped Aircraft and Rawinsondes

Abstract: A comparison was made of temperature and wind observations reported by rawinsonde and Aircraft Communications, Addressing, and Reporting System (ACARS)-equipped commercial aircraft separated by less than 150 km in distance and 90 min in time near Denver, Colorado, in February and March 1992. Only data made on aircraft ascents and descents reported automatically were used. A total of 4440 matched data pairs were obtained for this period. The sample was analyzed in total but also as a function of time and distance separation, height, time of day, and ascent versus descent. A standard deviation temperature difference of 0.97°C and rms vector wind difference of 5.76 m s−1 were found for the entire sample but were reduced, respectively, to 0.59°C and 4.00 m s−1 when only data pairs separated by less than 25 km and 15 min were used. The study provides an upper limit to the combination of rawinsonde and ACARS observation and reporting errors and mesoscale variability, but it is not possible to distingui...

Modernization in the National Weather Service River and Flood Program

Abstract: Hydrologic forecasting is vital not only to the National Weather Service mission of saying lives and protecting property but also to our nation's water management decision makers. Since its inception, the River and Flood Program has continually endeavored to modernize its tools and techniques. Modernization is an innovative process of research and development that strives to make available the best methods, data, and equipment for hydrologic forecasts. This paper addresses the role of modern technology (e.g., automation, computers, and radar) in hydrologic forecasting and also examines some of the problems of river modeling and forecasting and what modernization tools are available to remedy them. In addition, this paper looks at the increasing importance of training personnel to effectively use these modernized methods, procedures, and equipment in their jobs.

Documentation of a Systematic Bias in the Aviation Model's Forecast of the Atlantic Tropical Upper-Tropospheric Trough: Implications for Tropical Cyclone Forecasting

Abstract: This study uncovers what appears to be a systematic bias in the National Meteorological Center's aviation (AVN) model at 200 mb over the Caribbean Sea. In general, the 48-h forecast in the vicinity of the Tropical Upper Tropospheric Trough (TUTT) underpredicts the magnitude of the westerly 200-mb winds on the order of 5-10 m s−1. This unrealistic weakening of the TUTT and associated cold lows by the AVN results in erroneous values of the vertical (850-200 mb) wind shear. These systematic errors are in the same order of magnitude as the minimum shear threshold for tropical cyclone genesis and development. Thus, 48-h tropical cyclone formation and intensity forecasts based upon the AVN model are often incorrect in the vicinity of the TUTT. Knowing the correct future upper-wind regime is also crucial for track forecasting of more intense tropical cyclones, especially in cases of recurvature. It is shown that simple persistence or climatology of the 200-mb winds south of a TUTT axis is superior to th...

The Challenge of Forecasting Heavy Rain and Flooding throughout the Eastern Region of the National Weather Service. Part I: Characteristics and Events

Abstract: Heavy rain and flooding are of particular concern to forecasters throughout the eastern and southeastern United States. In this paper, the unique combination of synoptic-scale and mesoscale weather conditions and topographic factors that contribute to and enhance convectively produced flash flooding and river flooding are described and categorized. A classification system for synoptic-scale flooding events is developed, which is then used to identify the distribution (by month and synoptic-scale weather system) of major floods in the region of interest. Also, examples of flooding caused by synoptic-scale weather systems are presented. Characteristics of mesoscale heavy-rain events that result in flooding are discussed. Topographical and geographical factors in the region, which play a role in these events, are described, along with examples of mesoscale, flash flood events. Finally, floods produced by warm-top thunderstorms, which pose a unique and challenging forecast problem throughout the east...

Improving Seasonal Hurricane Predictions for the Atlantic Basin

Abstract: This paper demonstrates that improved forecasts of the annual number of hurricanes in the Atlantic tropical basin are possible by separating tropical-only hurricanes from hurricanes influenced by extratropical factors. It is revealed that variables previously shown to have a predictive relationship with the annual number of Atlantic hurricanes have a significantly stronger predictive association with the number of hurricanes formed solely from tropical mechanisms. This stronger relationship exists for extended-range (6-month lead) as well as for short-range (0-month lead) forecast models. Any future study of seasonal hurricane activity over this region should consider tropical-only hurricanes as separate from hurricanes influenced by baroclinic disturbances. The annual number of hurricanes that form or intensify as a result of interactions with baroclinic disturbances appears unrelated to significant tropical or midlatitude atmospheric anomalies and thus should be considered the random component ...

The WSR-88D Severe Weather Potential Algorithm

Abstract: The WSR-88D severe weather potential (SWP) algorithm is an automated procedure for the detection of severe local storms. The algorithm identifies individual thunderstorm cells within radar imagery and, for each cell, yields an index proportional to the probability that the cell will shortly produce damaging surface winds, large hail, or tornadoes. This index is a statistically derived function of the storm's maximum vertically integrated liquid (VIL) and horizontal areal extent. The correlation between these storm characteristics and severe weather occurrence was first documented in the 1970s. Several National Weather Service field offices in the central plains and Northeast regions of the United States have successfully used VIL as a discriminator between severe and nonsevere thunderstorms. This paper describes the observational data and statistical methodology employed in the development of the SWP algorithm, and regional and seasonal variations in the SWP/severe weather relationship. The expec...

Winter Weather Forecasting throughout the Eastern United States. Part III: The Effects of Topography and the Variability of Winter Weather in the Carolinas and Virginia

Abstract: Winter weather in the Carolinas and Virginia is highly variable and influenced by the area's diverse topography and geography. The Gulf Stream, the highest mountains in the Appalachians, the largest coastal lagoonal system in the United States, and the region's southern latitude combine to produce an array of weather events, particularly during the winter season, that pose substantial challenges to forecasters. The influence of the region's topography upon the evolution of winter weather systems, such as cold-air damming and frontogenesis, is discussed. Conceptual models and specific case studies are examined to illustrate the region's vast assortment of winter weather hazards including prolonged heavy sleet, heavy snow, strong convection, and coastal flooding. The weather associated with these topographic and meteorological features is often difficult for operational dynamical models to resolve. Forecasting precipitation type within the region can be especially difficult. An objective technique ...

Generalized Additive Models versus Linear Regression in Generating Probabilistic MOS Forecasts of Aviation Weather Parameters

Abstract: The skill of probabilistic Model Output Statistics forecasts generated from Generalized Additive Models (GAM) is compared to that of traditional multiple linear regression techniques. Unlike linear regression, where each predictor term in the additive model is assumed to vary linearly with the predictand (unless specified otherwise by the developer), GAM is a nonparametric tool that makes use of the data to automatically estimate the appropriate functional (curvative) relationship for each predictor term. This relieves the developer from the chore of identifying and computing the correct predictor transformations and helps uncover certain nonlinearities that may have been missed. Forecast equations for each statistical technique are developed for nine regions encompassing a total of 90 stations in the northeastern United States. Three parameters (cloud amount, ceiling height, and visibility) are forecast for eight thresholds and two lead times (12 h and 24 h). The developmental dataset consists o...

Modeling and Forecasting the Daily Maximum Temperature Using Abductive Machine Learning

Abstract: The abductory induction mechanism (AIM®) is a modern machine-learning modeling tool that draws from the fields of neural networks, abductive networks, and multiple regression analysis. This paper introduces AIM as a useful weather modeling and forecasting utility and reports on its use with daily maximum temperatures in Dhahran, Saudi Arabia. Compared with other statistical methods and neural network techniques, this approach has the advantages of faster and highly automated model synthesis as well as improved prediction and forecasting accuracies. AIM models developed using daily data for 18 weather parameters over 1 yr that were used to predict the maximum temperature on a given day from other parameters on the same day. Evaluated on data for another full year, these models give 97% yield in the ±3°C error category. Various models for 3-day forecasting have been developed and evaluated. First-day forecasts give 77% yield in the same error category, and they compare favorably with official forec...

Lightning from Two National Detection Networks Related to Vertically Integrated Liquid and Echo-Top Information from WSR-88D Radar

Abstract: Two national cloud-to-ground (CG) lightning-detection systems were operating across the contiguous United States during 1993. These two networks are compared to each other and to WSR-88D radar information collected by the Twin Lakes, Oklahoma, radar on 9 June 1993 as a squall line moved through central Oklahoma. Next Generation Weather Radar Information Dissemination Service data are used to investigate the relationship of CG lightning to WSR-88D echo tops and vertically integrated liquid (VIL). Such relationships appear not to have been investigated in such fine time and space resolution to date. When lightning is normalized by the frequency of occurrence of 4 km × 4 km resolution echo-top areas, the greatest percentage of echoes with lightning occurs when echo-top heights exceed 50 000 ft (15.2 km). The percentage of echoes with lightning drops significantly as echo tops decrease. The relationship of VIL with lightning is not as clearly defined. The frequency of echoes with lightning increases ...

A Probabilistic Forecast Contest and the Difficulty in Assessing Short-Range Forecast Uncertainty

Abstract: Results are presented from a probability-based weather forecast contest. Rather than evaluating the absolute errors of nonprobabilistic temperature and precipitation forecasts, as is common in other contests, this contest evaluated the skill of specifying probabilities for precipitation amounts and temperature intervals. To forecast optimally for the contest, both accurate forecasts and accurate determination of one's uncertainty about the outcome were necessary. The contest results indicated that forecasters over a range of education levels produced skillful forecasts of temperature and precipitation relative to persistence and climatology. However, in this contest forecasters were not successful in assessing the uncertainty of their maximum or minimum temperatures from day to day, as measured by the correlation of interval width and absolute error. Though previous experiments have shown more optimistic results, the seasonal variation of forecast uncertainty can account for much of the observed ...

Three-Dimensional Computations of Equivalent Potential Vorticity

Abstract: Moore and Lambert showed how a quantity called equivalent potential vorticity (EPV) can provide quantitative values to assess conditional symmetric instability (CSI), also known as slantwise instability. Expanding the EPV equation into three dimensions, the equation becomes a function of the geostrophic wind shell, the horizontal equivalent potential temperature gradient, the absolute geostrophic vorticity, and the vertical equivalent potential temperature gradient, all of which are easily computed from gridded data. The equation reduces further by recognizing that the geographic wind shear is a function of the horizontal equivalent potential temperature gradient in a saturated environment. This reduced equation is difficult to evaluate quantitatively because of its dependence on the local value of the moist adiabatic lapse rate. Nevertheless, it shows that horizontal temperature gradients will always act to promote slantwise convection. EPV is a mixture of upright potential instability and slant...

An Interesting Mesoscale Storm-Environment Interaction Observed Just Prior to Changes in Severe Storm Behavior

Abstract: Satellite images are presented to illustrate an interesting interaction that occurred between a severe thunderstorm and a mesoscale feature that originated in its nearby environment. Immediately following that interaction, a series of tornadoes began, starting with a long-lived F5 tornado that produced major damage in Hesston, Kansas. Some speculation is presented regarding the physical processes that may have contributed to the observed changes in thunderstorm behavior.

Lightning Casualties and Property Damage in Colorado from 1950 to 1991 Based on Storm Data

Abstract: Forty-two years of lightning casualty and damage reports in Colorado are summarized. The data are from NOAA's Storm Data, which is compiled monthly by the National Weather Service. The dataset contains 103 deaths, 299 injuries, and 191 damage reports from 1950 through 1991. Time fluctuations of these lightning events are shown by classifying the data by year, month, and hour of the day. Additional information is provided on age and gender of lightning victims. Houses were the most common objects reported as damaged by lightning. Fire was the most common source of lightning damage to structures or objects. Comparisons are made of these and other results, when possible, with previous studies of lightning in other locations around the world. Geographical patterns of lightning incidents for the state are shown in maps of total number of casualties, and number of casualties normalized by population density for each county. Much higher casualty rates per population and area occur over the mountains and...

Fog Forecasting Objectively in the California Coastal Area Using LIBS

Abstract: Weather forecasts and warnings are the most important services provided by the meteorological profession. As one aspect of these services, a year-round fog forecasting objective is selected for California coastal sites and their surrounding areas, both alongshore and offshore. The effort was to provide a simple, straightforward forecasting method using primarily data from the coastal site, which might be supplemented and improved using the forecaster's experience, any new technology, and new results of research. The meteorological objective is to forecast light, moderate, and dense fog, the probability of each occurring, and the expected times of beginning and ending of each period of reduced visibility, both for terminals and for limited nearby areas. Forecasts of five days' duration are sought using a frequently observed sequence of synoptic events. Methods in use were reviewed. The synoptic approach, emphasizing sea–air relationships, has produced the most promising results at this time. This ...

An Object-Oriented Approach to Nowcasting Showers

Abstract: The use of a simple conceptual model depicting the life cycle of a convective cell, including the production of daughter cells, has been examined as a way of improving nowcasts in certain showery situations. The evolution of the cell from birth through to dissipation is represented as a sequence of six stages, with the cell being described throughout by a fixed set of attributes whose values change as the cell evolves. Cells are identified and their attribute values set automatically from radar and satellite observations, whereupon the conceptual model is used to predict their subsequent evolution. Application of the technique in real cases suggests that it may improve nowcasts in those cases when daughter-cell production has a significant impact on storm development and movement. This “proof of concept” study has highlighted a number of areas for further work; these include allowing the life cycle to adapt to the convective environment, the use of higher-resolution observations, and investigatin...

An Unusual Summertime Downslope Wind Event in Fort Collins, Colorado, on 3 July 1993

Abstract: An unseasonal, severe downslope windstorm along the eastern foothills of the Colorado Rocky Mountains is described. The storm, which occurred on 3 July 1993, produced wind guts in Fort Collins, Colorado, over 40 m s−1 and resulted in extensive tree and roof damage. The synoptic pattern preceding the wind event resembled a pattern typical of that for a Front Range late fall or wintertime wind storm, including a strong south–southwest-oriented height gradient at 700 mb and a strong west to east sea level pressure gradient across the Front Range. A particularly interesting facet of the event was that one small geographical area in and near Fort Collins experienced wind gusts nearly 40% stronger than any other location involved in the event. The mesoscale forecast version of the Regional Atmospheric Modeling System (RAMS) with 16-km grid spacing over Colorado was run for the storm. Consistent severe winds were not predicted by the model in this configuration. Increasing resolution in postanalysis to ...