Abstract

Case studies have shown that the Great Lakes can intensify and alter the speed of passing cyclones in winter by contributing latent and sensible heat to the storms. However, the influence of the Great Lakes on cyclones has not been systematically examined using an extensive dataset. In this research, a National Climate Data Center dataset for the period 1965–90 was used to examine the rate of movement and change in mean sea level pressure of 583 cyclones as they passed over the Great Lakes.

The Great Lakes had a strong effect on the passing cyclones during the ice-free/unstable season from September through November. As cyclones approached the lakes during this season, they accelerated. Once in the Great Lakes region, their rate of intensification increased (the change in pressure tendency at the center of the cyclone was negative). The acceleration into the region was less for cyclones during the ice-cover/unstable season, and rates of intensification for these cyclones did not change within the region. Cyclones that traversed the Great Lakes region during the stable season from May through July exhibited essentially the same behavior as those in the ice-free/unstable season.

The authors’ results for the unstable seasons (ice free and ice cover) are consistent with previous modeling case studies of the influence of the Great Lakes on passing cyclones. Because the lakes are generally cooler than the overriding air during spring and summer, a satisfactory explanation for the influence of the Great Lakes on cyclones during the stable season is not apparent.

Abstract

The rainstorm on 17–18 July 1996 in northern Illinois produced three rainfall records. The 43.0-cm total storm rainfall at Aurora was the greatest point rainfall recorded for storm durations of 24 hours or less in this century in Illinois and most surrounding states. The 27.9-cm storm rainfall recorded in the southwestern part of the Chicago metropolitan area was the heaviest 24-h amount ever recorded in that city. The July 1996 storm also produced the heaviest 24-h mean rainfall recorded in Illinois over areas of 5200 and 13 000 km² immediately surrounding the storm center.

An area of approximately 12 000 km² experienced 24-h point rainfall amounts that exceeded those expected to occur, on the average, once in 10 years. Similarly, the 25-, 50-, and 100-yr frequency values were exceeded over areas of 6730, 4920, and 3500 km², respectively.

One concern resulting from a major rainfall event such as this storm is its impact on the rainfall frequency analysis. This new information may result in changes in the estimated rainfall amounts at selected return periods, which are used to design water-handling structures. Based on previous research, the Aurora rainfall appears to exceed the 1000-yr return period. However, fitting a statistical distribution to the annual maximum time series and using regional averages minimized the effect of this storm on rainfall frequency estimates.

Abstract

Cyclones are an important feature of the Great Lakes region that can have important impacts on shipping, lake temperature profiles, ice cover, and shoreline property damages. The objective of this research is to analyze the frequency and intensity of cyclones that traversed the Great Lakes region, the changes of these characteristics since 1900, the interrelationship of cyclone frequency and intensity, and their relationships to circulation patterns and regional temperature and precipitation.

Significant increases in the number of strong (≤992 mb) cyclones over the twentieth century were found for the annual, cold season, November, and December time periods. In contrast, the frequency of all cyclones in the annual and warm season time series and the central pressure of all cyclones in the annual, cold, and warm season time series displayed significant decreases from 1900 to 1939.

Relationships between cyclone frequency and intensity and between cyclone and anticyclone frequency and intensity suggest that there is a partial compensation within the region. As the number of cyclones increases, their intensity decreases. As the number of cyclones increases, so does the number of anticyclones. And, finally, as the cyclones become stronger, so do the anticyclones. Comparisons with the Pacific–North American teleconnection index indicate that lower (higher) cyclone frequency is associated with more zonal (meridional) flow. Comparisons of cyclone characteristics with temperature and precipitation in the Great Lakes region shows that cyclone frequency is inversely related to temperature and directly related to precipitation in most month and season categories. In contrast, the relationships between cyclone intensity and climate variables are inconsistent.

Abstract

The zones of origin for all cyclones that traversed the Great Lakes region from 1899 to 1996 are analyzed using a digital daily record of central pressure and location for individual cyclones. Plots of latitude of formation show that Great Lakes cyclones form (or reform) east of the Rocky Mountains at all latitudes between 25° and 65°N. In winter, about the same number of cyclones originate to the northwest as to the southwest of the Great Lakes region. In spring, the southwest zone is dominant. The number of summertime cyclones is greatly reduced, with the west zone of origin most active, while the fall plot displays a transition between the summer and winter distributions. The proportion of strong Great Lakes cyclones that originate in the southwest zone is greater than for all cyclones; however, the seasonal shifts in the latitudinal distributions of origin in the two datasets are similar.

An analysis of differences in frequencies by zone of origin for Great Lakes cyclones during months characterized by positive and negative Pacific–North American (PNA) index patterns reveals a statistically significant relationship between the midtropospheric flow pattern and cyclogenesis. The results indicate that the number of cyclones per month for the positive (PNA index > 0.5) and negative (PNA index < −0.5) categories are approximately equal and that the combined frequencies for positive and negative PNA pattern categories for the northwest, west, and southwest zones of origin are similar. The study supports the intuitive assertion that more Great Lakes cyclones originate from the northwest during months characterized by positive PNA index values than the negative pattern while more cyclones from the west and southwest are associated with the negative PNA index pattern than the positive one.

Approximately 20% of the cyclones that traversed the Great Lakes from 1899 to 1996 originated in the region. The most noteworthy and puzzling finding of the study is that cyclogenesis over the lakes as a proportion of cyclone presence in the region is highest in the summer months. This result corresponds with the finding that cyclones traversing the Great Lakes region in May–July accelerate as they approach the region and increase their rates of deepening over the lakes.

Abstract

The severe 2013/14 winter (December–March) in the Midwest was dominated by a persistent atmospheric circulation pattern anchored to a North Pacific Ocean that was much warmer than average. Strong teleconnection magnitudes of the eastern Pacific oscillation (−EPO), tropical Northern Hemisphere pattern (+TNH), and second-lowest Hudson Bay 500-hPa geopotential height field are indicators that led to severe winter weather across the eastern United States. Unlike in previous cold and snowy midwestern winters, Atlantic Ocean blocking played little to no role in the winter of 2013/14. The primary atmospheric feature of the 2013/14 winter was the 500-hPa high pressure anchored over the North Pacific in response to the extremely warm sea surface temperature anomalies observed of +3.7 standard deviations. Only one other severe midwestern winter (1983/84) since 1950 featured a similar Pacific blocking. The accumulated winter season severity index, which is a metric that combines daily snowfall, snow depth, and temperature data for the winter season, was used to compare the 2013/14 winter with past winters since 1950. Detroit, Michigan, and Duluth, Minnesota, experienced their worst winter of the 55-yr period. Seven climate divisions in northern Illinois, eastern Iowa, and parts of Wisconsin experienced record-cold mean temperatures. These climate conditions were associated with a number of impacts, including a disruption to the U.S. economy, the second-highest ice coverage of the Great Lakes since 1973, and a flight-cancellation rate that had not been seen in 25 years.

Abstract

Soil erosion is a major global challenge. An increased understanding of the mechanisms driving soil erosion, especially the storms that produce it, is vital to reducing the impact on agriculture and the environment. The objective of this work was to study the spatial distribution and time trends of the soil erosion characteristics of storms, including the maximum 30-min precipitation intensity (I ₃₀), storm kinetic energy of the falling precipitation (KE), and the storm erosivity index (EI) using a long-term 15-min precipitation database. This is the first time that such an extensive climatology of soil erosion characteristics of storms has been produced. The highest mean I ₃₀, KE, and EI values occurred in all seasons in the southeastern United States, while the lowest occurred predominantly in the interior west. The lowest mean I ₃₀, KE, and EI values typically occurred in winter, and the highest occurred in summer. The exception to this was along the West Coast where winter storms exhibited the largest mean KE and EI values. Linear regression was used to identify trends in mean storm erosion characteristics for nine U.S. zones over the 31-yr study period. The south-central United States showed increases for all three storm characteristics for all four seasons. On the other hand, higher elevations along the West Coast showed strong decreases in all three storm characteristics across all seasons. The primary agricultural region in the central United States showed significant increases in fall and winter mean EI when there is less vegetative cover. These results underscore the need to update the storm climatology that is related to soil erosion on a regular basis to reflect changes over time.

Abstract

Climate studies of precipitation have generally focused on daily or longer time scales of precipitation accumulation. The main objective of this work was to identify the precipitation characteristics of storms based on 15-min precipitation data, including storm total precipitation, storm duration, mean storm intensity, and maximum 15-min intensity. A group of precipitation characteristics was subjected to a cluster analysis that identified nine regions of the conterminous United States with homogeneous seasonal cycles of mean storm precipitation characteristics. Both mean and extreme statistics were derived for each characteristic and season for each zone. Continuous probability density functions were generated that appropriately fit the empirical distributions of storm total precipitation and maximum 15-min intensity. The storm characteristics, in turn, were a function of seasonal water availability from source regions, atmospheric water vapor capacity, and storm precipitation mechanism. This is the first time that such an extensive climatology of storm precipitation characteristics has been produced. A preliminary trend analysis of the 1972–2002 storm characteristic data by zone showed substantial changes that tended to be geographically coherent, with noteworthy differences between the western and eastern United States. The western United States displayed a trend toward decreasing storm total precipitation and storm duration in most seasons, while storm intensity increased. The eastern United States experienced a general pattern of increasing storm total precipitation and storm duration during winter, as well as a tendency for maximum 15-min precipitation intensity to increase.

The 1993 record-breaking summer flood in the Upper Mississippi River Basin resulted from an unprecedentedly persistent heavy rain pattern. Rainfall totals for the Upper Mississippi River Basin were, by a large margin, the largest of this century for the 2-, 3-, 4-, and 12- month periods encompassing the 1993 summer. The totals for these periods are estimated to have a probability of occurrence of less than 0.005 yr⁻¹ In addition, the number of reporting stations receiving weekly totals in excess of 100 mm (events identified in a previous study as being closely correlated with floods) was the largest in at least the last 45 yr. Other conditions contributing to the flood include above-normal soil moisture levels at the beginning of June 1993; large-sized areas of moderate to heavy rains; occurrence of rain areas oriented along the main stems of major rivers; a large number of localized extreme daily rainfall totals (greater than 150 mm); and below-normal evaporation. The large-scale atmospheric circulation patterns during the summer of 1993 were similar to the patterns associated with past heavy rain events, although much more persistent than past events.

The Midwestern Climate Information System (MICIS) is a near real-time system which provides access to a wide variety of climate information products. These include current temperature and precipitation data for several hundred midwestern United States stations, historical temperature, and precipitation for about 1500 stations, climate summaries, long-range predictions, regional soil moisture estimates, and crop yield risk assessments. The region covered includes the states of Illinois, Indiana, Iowa, Kentucky, Michigan, Minnesota, Missouri, Ohio, and Wisconsin. Because agriculture is a major sector of the Midwestern economy and is sensitive to climate fluctuations, some products have been oriented to the needs of agriculture. However, many other products have general applicability. Users of this system include agribusinesses and researchers.

MICIS has several unique features: a) regional coverage provides climatic information for a major part of the United States corn and soybean belt; b) daily temperature and precipitation data are obtained daily from an average of 500 stations providing an up-to-date assessment of current climatic conditions; c) process models provide an estimate of potential impacts on soil moisture and corn and soybean yields.

Abstract

The NOAA National Climatic Data Center maintains tables for temperature and precipitation extremes in each of the U.S. states. Many of these tables were several years out of date, however, and therefore did not include a number of recent record-setting meteorological observations. Furthermore, there was no formal process for ensuring the currency of the tables or evaluating observations that might tie or break a statewide climate record. This paper describes the evaluation and revision of the statewide climate-extremes tables for all-time maximum and minimum temperature, greatest 24-h precipitation and snowfall, and greatest snow depth (the five basic climate elements observed on a daily basis by the NOAA Cooperative Weather Network). The process resulted in the revision of 40% of the values listed in those tables and underscored both the necessity of manual quality-assurance methods and the importance of continued climate-monitoring and data-rescue activities to ensure that potential record values are not overlooked.