A SYNOPTIC CLIMATOLOGY OF EXTREME UNSEASONABLE FLOODS IN THE SOUTHEASTERN UNITED STATES, 1950–1990

Unseasonable floods are floods that occur in the season of lowest flood frequency, or dry season. Such floods pose a unique problem to flood planners and forecasters, yet little research has investigated the physical processes associated with unseasonable floods. The purpose of this study is to construct a synoptic climatology of extreme unseasonable floods for the southeastern United States. Results indicate that the types of storms creating unseasonable floods are location specific, with four unique regions across the study area: Carolina (tropical storms/hurricanes), Georgia Coastal Plain (Gulf depressions), Gulf-Atlantic (frontal), and Tennessee (frontal with upper-air enhancement). The precipitation created by these storms is low to moderate, rarely exceeding the 10-year 24-hour storm total. The precipitation levels suggest that a combination of meteorological conditions and land-surface conditions create the extreme events. A statistical analysis indicates that high soil-moisture levels combine with the moderate rains to produce extreme unseasonable floods. [Key words: unseasonable floods, synoptic climatology, land-surface conditions, southeastern United States.]     

A SYNOPTIC CLIMATOLOGY OF NORTHEASTERN UNITED STATES TORNADOES

This study documents the spatial and temporal characteristics of northeast United States tornadoes and the synoptic patterns associated with their development. Daily 1200 UTC surface pressure, 500 mb height and 850 mb temperature data are used in a compositing analysis to indicate the general conditions on tornado-producing days during four quasi-seasonal periods. Temporally, two-thirds of all northeast tornadoes occur between the hours of 1800 UTC and 0000 UTC. Annually, greater than 75% occur during the four-month period from May through August. During the period of study (1950 through 1986) the region had an average of 30 tornado occurrences per year. Spatially, three preferred areas of tornadic development are identified across the northeast region. These areas include western and southeastern Pennsylvania and north-central Massachusetts. The general synoptic patterns associated with tornadic events in the northeast United States remain consistent throughout the year. The composite analyses indicate that the presence of a strong surface low pressure system moving through the Great Lakes, coupled with significant upper-level divergence associated with a vigorous shortwave feature and a cold frontal boundary, are the synoptic features most common during the initiation of tornadic storms in this region. [Key words: synoptic climatology, tornadoes, northeastern United States.]     

Annual floods in New England (USA) and Atlantic Canada: synoptic climatology and generating mechanisms

New England and Atlantic Canada are characterized by mixed flood regimes that reflect different storm types, antecedent land surface conditions, and flood seasonality. Mixed flood regimes are known to complicate flood risk analyses, yet the synoptic climatology and precipitation mechanisms that generate annual floods in this region have not been described in detail. We analyzed a set of long-term annual flood records at climate-sensitive stream gauges across the region and classified the synoptic climatology of each annual flood, quantitatively describing the precipitation mechanisms, and characterize flood seasonality. We find that annual floods here are dominantly generated by Great Lakes-sourced storms and Coastal lows, known locally as ‘nor’easters.’ Great Lakes storms tend to be associated with lower magnitude annual floods (<75th percentile) and Coastal lows are more clearly associated with higher magnitude events (>75th percentile). Tropical cyclones account for few of all annual floods, including extreme events, despite causing some of the region’s largest and most destructive floods. Late winter/early spring is when the greatest number of annual floods occur region wide, and rainfall is the dominant flood-producing mechanism. Rainfall in combination with snowmelt is also important. Both mechanisms are expected to be impacted by projected regional climate change. We find little evidence for associations between flood-producing synoptic storm types or precipitation mechanisms and large-scale atmospheric circulation indices or time periods, despite upward trends in New England annual flood magnitudes. To more completely investigate such associations, partial duration flood series that include more floods than just the largest of each year, and their associated synoptic climatologies and precipitation mechanisms, should be analyzed.     

SYNOPTIC-SCALE VARIABILITY IN THE PROBABILITY OF PRECIPITATION FROM THUNDERSTORMS IN THE UNITED STATES

A synoptic-scale climatology of precipitation amounts from thunderstorms was developed by analysis of amounts from individual storms for 220 stations in the conterminous United States for the period 1948-1977. The probability of having a thunderstorm without rainfall was assessed for each station. For storms which did produce precipitation, the probability distribution of amounts was found to be well summarized by the incomplete gamma distribution. Sets of seasonal maps of the probability of receiving any measurable amount, less than 4 mm, and more than 20 mm of precipitation are presented. Consistent spatial patterns are found. Thunder without precipitation is most likely in the west. The greatest probability of heavy precipitation occurs along the Gulf Coast, extending in the summer throughout the mid-section of the nation. Topographic effects are apparent, with mountainous areas generally having less intense precipitation than surrounding regions. [Key World: thunderstorms, precipitation probabilities, gamma distribution.]     

Lower Tropospheric Warm Air Advection Patterns Associated with Heavy Rainfall over the Appalachian Region*

A synoptic climatology of warm season heavy rainfall is developed from patterns of 850 mb thermal advection over the Appalachian region. Heavy rain events are categorized according to the position and orientation of a warm air advection (WAA) ridge, a feature found in nearly two-thirds of the events. Numerous study events occur within the conditionally unstable region of the WAA ridge. In fact, numerous occurrences of heavy rainfall are tied to a superpositioning of a WAA and air mass instability ridge in the vicinity or upstream of the heavy rain area.     

MOISTURE TRAJECTORIES ASSOCIATED WITH HEAVY RAINFALL IN THE APPALACHIAN REGION OF THE UNITED STATES

Synoptic data associated with a sample of 554 heavy rainfall events is utilized to carry out a trajectory analysis that identifies the movements of moisture towards regions of heavy rain. Both seasonal and regional variations are found in the moisture trajectories associated with heavy rain events occurring in three regions in the Appalachian study area. Numerous events in the region west of the mountain range are tied to westerly and south-southwesterly circulations that bring moisture from the Mississippi River Valley and the Gulf of Mexico during the warm and cool seasons, respectively. Many events southeast of the mountain range are associated with southerly to southeasterly circulations that advect moisture from the Atlantic Ocean. Because of orographic precipitation enhancement and a good exposure to the Gulf of Mexico and the Atlantic Ocean, the southern and southeastern slopes of the Appalachian mountains display high frequencies of heavy rainfall, particularly during the cool season. The interior portions of the mountain range and the adjacent plateau to the northwest, on the other hand, are sheltered from moisture source regions and, therefore, exhibit low heavy rain frequencies. [Key words: precipitation, synoptic climatology, Appalachia.]     

An All-Season Synoptic Climatology of Air Pollution in the U.S.-Mexico Border Region*

The potential exists for widespread air quality problems in the U.S.-Mexico borderlands. Climate and weather are major factors governing the behavior of air pollution, and thus there is a need for greater understanding of border-region air pollution climatology. This paper presents a synoptic climatology of the 850 mb atmospheric circulation for the U.S.-Mexico border region, and an accompanying analysis of relationships between synoptic conditions and ground-level ozone. The synoptic methodology employs high-pass filtering to enable comparisons of all seasons, and it uses modified multiple k means clustering to identify six characteristic circulation patterns. The climatology succinctly summarizes important spatial and temporal complexities of border region circulation, including various pressure configurations, the seasonality of those patterns, and associated weather conditions across the region. These results are linked with ozone data for four border-region cities, and the subsequent findings highlight systematic seasonal and region-wide variations in ozone pollution corresponding to patterns of controlling climatic factors. Three high-ozone scenarios are identified, each of which selectively affects a different area or time of year.     

Preliminary Analysis of the Temporal Patterns of Heavy Rainfall across the Southeastern United States

This paper examines trends in heavy rainfall events across the southeastern United States over the past century. Time series of the number of annual storm events over a 76.2 mm threshold were used for analysis. Significant trends that had some spatial coherence were identified. Increasing trends were detected in a region extending from northeastern Texas to the Appalachians, with a weaker signal of decreasing events along the East Coast. These patterns may be linked to fluctuations in the strength and migration of the Bermuda High.     

A Synoptic Climatology of Severe Storms in Virginia*

A synoptic climatology is developed for Virginia using 21 years of late spring and summer surface and upper air observations. The climatology is produced by applying a combination of principal components analysis and cluster analysis such that each day is classified into one of a distinct number of synoptic situations. Days on which at least one severe storm occurred in Virginia are merged with the synoptic climatology. A majority of severe storms are associated with one synoptic situation distinguished by moderate instability and a high moisture content.     

A SYNOPTIC CLIMATOLOGY OF COOL-SEASON DERECHO EVENTS

Synoptic-scale environments favorable for producing derechos in the cool season (September through February) are examined with the goal of providing useful techniques for identifying commonalities within derecho activity corridors. Fourteen derechos were identified from two activity corridors located in the southeastern United States and Atlantic seaboard regions between 1986 and 1995. The synoptic environment at the initiation and midpoint of each derecho was then evaluated using surface, upper-air, and the NCAR/NCEP reanalysis data sets. Models are provided in order to illustrate the synoptic-scale environment and to assist meteorologists in recognizing conditions favorable for cool-season derecho formation. Marginal instability and strong synoptic-scale forcing characterized the environments of events in both corridors. The overall synoptic patterns associated with cool-season derecho-producing mesoscale convective systems (DMCSs) resembled environments found with cool-season tornado episodes. Recognition of key elements in this environment could lead to improvements in cool-season severe weather prediction. [Key words: derecho, windstorm, mesoscale convective system (MCS), synoptic climatology, southeast United States.]     

Preliminary Analysis of the Temporal Patterns of Heavy Rainfall across the Southeastern United States

This paper examines trends in heavy rainfall events across the southeastern United States over the past century. Time series of the number of annual storm events over a 76.2 mm threshold were used for analysis. Significant trends that had some spatial coherence were identified. Increasing trends were detected in a region extending from northeastern Texas to the Appalachians, with a weaker signal of decreasing events along the East Coast. These patterns may be linked to fluctuations in the strength and migration of the Bermuda High.     

Spatial distribution of tropical cyclone rainfall and its contribution to the climatology of Puerto Rico

This study explores two different tropical cyclone rainfall (TCR) problems: first, the identification of areas where TCR is highly concentrated and, second, the contribution of tropical cyclones (TCs) to the climatology of Puerto Rico for the period 1970–2010. A total of 86 storms within a 500-km radius of Puerto Rico were analyzed. Daily and monthly rainfall data from 32 weather stations were used to generate interpolated surfaces. Two geostatistical interpolation techniques were implemented: ordinary kriging and ordinary cokriging. Results show that rainfall from most TCs tends to be clustered in the eastern, southeastern, and central regions of the island, with a decrease in values toward the west. TCs closer to Puerto Rico (≤230 km) and embedded in high moisture environments (≥44.5 mm) exhibited the highest rainfall values, with most concentrated in the high elevation areas of the southeastern region of the island. Months with the largest TC contributions to rainfall (20–30%) were August and September, while the lowest contributions were found in June and November. For August, stations in the southern and eastern portions of the island had TCR contributions of >20%, with some stations in the southern coastal plains exhibiting 30%.     

A rain on snow climatology and temporal analysis for the eastern United States

ABSTRACT

Rain-on-snow (ROS) has the potential to produce devastating floods by enhancing runoff from snowmelt. Although a common phenomenon across the eastern United States, little research has focused on ROS in this region. This study used a gridded observational snow dataset from 1960–2009 to establish a comprehensive seasonal climatology of ROS for this region. Additionally, different rain and snow thresholds were compared while considering temporal trends in ROS occurrence at four grid cells representing individual locations. Results show most ROS events occur in MAM (March-April-May). ROS events identified with rainfall >1 cm are more frequent near the east coast and events identified with >1 cm snow loss are more common in higher latitudes and/or elevations. Decreasing trends in DJF (December-January-February) ROS events were identified near the coastal areas, with increasing trends in the northern portion of the domain. Significant decreasing trends in MAM ROS are likewise present on a regional scale. Factors playing a role in snowpack depth and rainfall, such as movement of storm tracks in this region, should be considered with future work to discern mechanisms causing the changes in ROS frequency.     

Spatial and Temporal Characteristics of Tornado Path Direction*

Common perception is that tornadoes travel in paths from the southwest quadrant of directions toward the northeast. This study examines path directions for 6,194 tornadoes that occurred in the eastern two‐thirds of the United States during the twenty‐three‐year period 1980–2002. At the national scale, nearly 70 percent of tornadoes included in the study propagated from the west, west‐southwest, and southwest, with west‐southwest being the highest frequency origin direction. Nevertheless, distinct seasonal and regional variations were found. In central and northern areas of the country, a more westerly or northwesterly path origin prevails during late spring and summer. The midtropospheric flow, convective typology, and synoptic patterns of tornado outbreaks are thought to contribute to the distributions observed in the climatology.     

A SYNOPTIC CLIMATOLOGICAL ANALYSIS OF SUMMERTIME PRECIPITATION INTENSITY IN THE EASTERN UNITED STATES

The spatial patterns of precipitation frequency and intensity over the eastern United States for summer from 1961 to 1990 are analyzed using a recently developed continental-scale air mass-based synoptic classification. This procedure, the spatial synoptic classification (SSC), is based on “seed” day identification of synoptic events and discriminant analysis to group together days that are within the same air-mass type. Results show differences in the types of precipitation associated with different air masses. Two air masses in particular-Moist Tropical and Moist Temperate-appear to be highly correlated with a majority of the precipitation, particularly in the southeast. The synoptic characteristics, daily intensity of rainfall, and radiosonde soundings during prevalence of these two air masses suggest that convective rainfall is common during Moist Tropical, and stratiform-type rainfall occurs during Moist Temperate. A simple stratification scheme based upon a synoptic-based air-mass delineation may be useful for studies that need to divide days into stratiform or convective regimes.     

AVALANCHE CLIMATOLOGY OF THE CONTINENTAL ZONE IN THE SOUTHERN ROCKY MOUNTAINS

The avalanche hazard in the United States is most severe in the continental zone of Colorado, where property damage and deaths exceed those in any other state. The continental zone is normally characterized by a shallow snowpack, faceted crystal growth, and relatively fewer avalanches as compared to the coastal and intermountain zones farther west. This study illustrates that variations in the avalanche character in the continental zone may, at times, resemble some less continental characteristics that are found farther west as a result of anomalous atmospheric circulation patterns. Results from cluster analyses show that some sites in the southern portion of the continental zone generally represent a less continental character. Anomalies of 500-mb heights explain the variability of avalanche climates for selected sites, particularly for Berthoud Pass, within the continental zone. Negative heights over the southwestern United States during early winter correspond with less continental conditions, but the zone of negative heights tends to shift westward over the eastern Pacific Ocean during February and March. However, generalizations of how synoptic patterns govern avalanche climate variations also vary between different locations as a result of smaller-scale climatic controls that operate over the region. [Key words: avalanche climatology, continental zone, 500-mb heights, Rocky Mountains.]     

中国暴雨区划初步研究

本文根据近三十年水文站和气象站实测和调查24h最大暴雨均值和极值,将中国暴雨进行了初步区划。分区原则是:综合考虑暴雨强度及其分布特点、发生季节、暴雨天气系统以及地理诸因素(包括地形、拔海高度和海陆性质)。首先采用天气气候学方法对全国暴雨进行初步划分;然后,根据全国气象站25年汛期(5—9月)雨量和年雨量资料,算出各区域面雨量指标,以某区面雨量指标为变量,分别对其周围各站雨量求相关,如果相关系数通过显著性水平α=0.05,即将该站划归为同一暴雨气候区,否则,划归另一气候区。 最后将全国分为十个暴雨气候区和四个副区,并简述了各区暴雨洪水成因及特征。     

南极海冰与我国夏季天气的关系

南极各区海冰变化不同,这种不同的变化必然对各区局地大气环流产生不同的影响,进而可能对全球气候系统产生不同的作用。本文利用Hadley中心提供的1968年1月-2002年12月全球海冰密集度格点资料,结合中央气象台的北半球500hPa、100hPa高度场资料以及中国160站降水、温度资料,利用诊断分析方法,分别对南极Ross海区和Weddell海区海冰与我国夏季天气的关系进行了研究。研究表明,南极Ross海区和Weddell海区海冰对我国夏季天气均具有指示意义。Ross海区是影响我国夏季东北地区降水的海冰关键区,若该区上一年9月海冰偏多,则来年6月我国东北降水偏少;Weddell海区是影响我国夏季东北温度的海冰关键区,若该区上一年9月海冰偏多,则来年6月我国东北温度偏低。     

REGIONAL-SCALE TROUGHING OVER THE SOUTHWESTERN UNITED STATES: TEMPORAL CLIMATOLOGY,TELECONNECTIONS, AND CLIMATIC IMPACT

Regional-scale middle- and upper-tropospheric troughing over the southwestern United States represents a departure from the modal circulation pattern for western North America. Once developed, southwestern troughs often are associated with positive vorticity advection aloft, surface cyclone formation, and moisture advection over areas of the western Great Plains and Intermountain West. These trough systems may play an important role in the precipitation climatology of the western and central United States. However, very little work has focused on the temporal climatology, developmental characteristics, or climatic impacts of southwestern troughs. This study provides a detailed climatology of southwestern troughing that focuses on: (1) the temporal frequency of these events; (2) the teleconnective circulation changes that are associated with their development; and (3) the importance of these systems in the precipitation climatology of the western and central United States.

The temporal climatology of southwestern troughs reveals that most of these systems occur during spring and autumn, with somewhat fewer events in winter and very few events in summer. An examination of 500-mb geopotential height and 24-hr height change composites during trough development shows that much of the wave-train activity that accompanies trough onset is limited to the North Pacific and North American regions. These changes are characterized by the amplification and eastward movement of a ridge/trough couplet over the eastern Pacific, which is preceded by synoptic-scale transient wave activity over the western and central Pacific. While southwestern troughs occur less than 30% of the time, southwestern trough-derived precipitation comprises over 60% of the monthly totals for some sites.