The Spatial Distribution of Torrential Rain in the U.S. Southeast

 

                Abstract. Although other parts of the United States experience frequent torrential rainstorms, none are as large or as densely populated as the Southeastern United States. Torrential rain in the South is primarily a summer phenomenon. Low-latitude low-pressure systems that form over the Gulf of Mexico and the Atlantic Ocean, if they reach land, can produce very intense rainfall over a short period. Although the frequency of heavy precipitation is less during colder months, one part of the South experiences a significant number. In the winter the jet stream that crosses the continent dips into the South. Low-pressure systems that follow this stream may draw moist air onto the land from the Gulf of Mexico. The heavy rainfall that sometimes results from the arrival of these air masses on to the cool continent is largely confined to the western Gulf States. The intensity of torrential rainstorms during a 24-hour period usually diminishes rapidly once they reach the coast.

 

Introduction. In the United States loss of property from water damage is the nation’s most costly environmental hazard. Most in the Southeast were caused by both mid-latitude and tropical low-pressure systems that invade the region and frequently bring torrential rain.  These low-pressure systems may be as weak as depressions or as strong as hurricanes. In 1992 Hurricane Andrew devastated Dade County, Florida and   the Louisiana coast, causing an estimated 30 billion current dollars damage. More recently, in 2001, Tropical Storm Allison inflicted damage estimated at over 4 billion dollars, mostly in the Houston, Texas area, but also along its path through the Gulf Coast states and up the Atlantic Seaboard to Pennsylvania. Knowing the geographical distribution as well as the frequency of these intense rainstorms is important in risk management. This study has been undertaken to broaden an understanding of this phenomenon.

 

Data.  Weather data for this study was obtained from the National Climatic Data Center. The period 1948 through 2000 was chosen because in 1948 many stations were added to the nation’s weather reporting network, and 2000 was the last year in which complete records were available. The precipitation record of all but twelve stations was at least 95 percent complete, and two-thirds were at least 98 percent. The few exceptions that were less than 95 percent complete were included because they were located in areas where there were no better alternative stations.

Although arbitrary, a torrential rain episode had to have had at least three inches of rain during a 24-hour day. If the threshold were raised to four inches or more there would have been too few events for meaningful interpretation, and if lowered to two inches, too many. To further facilitate interpretation, all data for the 53-year period were converted to 100-year frequencies. The term “rain” here is used interchangeably with “precipitation,” since storms of the magnitude examined overwhelmingly produce only rain.

 

Interpretation.  That part of the Southeast that can expect an average of at least one torrential rainstorm each year is greatest along the Gulf of Mexico (Map 1).

  Here it extends far up the Mississippi River Valley into Oklahoma, Arkansas, and Tennessee. Also, virtually the entire Florida Peninsula can expect at least one rain event of this magnitude at least once a year, and Miami can anticipate almost two. The decrease in frequency of storms of this intensity is far more dramatic toward the interior of the Atlantic Seaboard than that of the Gulf. Deep within the region’s interior there is one anomaly, Highlands, North Carolina. Situated in the Appalachian Mountains, its weather station appears to be well exposed to moist air masses from both the Gulf and the Atlantic. On average it can expect almost three torrential rain episodes each year. In the less exposed locations in the Appalachians storms of this frequency are rare, as they are in the western part of both Texas and Oklahoma. In 53 years of weather records El Paso has never experienced one.

Where torrential rain is frequent they often contribute a substantial share of the annual precipitation (Map2). In Texas there is one station where the annual average share is 23 percent, but shares of at least 15 percent are identified with stations from Brownsville, Texas to Tallahassee, Florida. The share is somewhat smaller along the Atlantic Seaboard. Compared to the Mississippi River Valley, the contribution of torrential rain to the total in the Piedmont region from Georgia to Virginia is small, as it is in the Appalachians and West Texas.

Short but intense periods of rain not only may cause great property damage, including crops, but also, compared to rainfall of less intensity there is greater runoff, and less absorption into the water table. In dry years these rainstorms can contribute enormously to the annual total, but since they are so concentrated in short periods, their utility to farmers is less than if daily rainfall were both less in quantity and better distributed. In 2000 in Montgomery, Alabama, that year’s precipitation was 30 percent below normal, but 16 percent of it fell in one day. In 1953 that year’s precipitation in Bogalusa, Louisiana was 25 percent below normal but 18 percent of it accumulated in two days. Clarksville, Mississippi precipitation in 1981 was 28 percent below normal, but 18 percent of it fell in one day.  That city had almost normal precipitation in 1980 but 30 percent of it fell in four days. These are only samples of the concentration in annual rainfall because of heavy precipitation that can be experienced at weather stations throughout the region.  During these years, unless irrigation is possible, farmers can anticipate reduces crop yields and great stress on pasture grasses.  Forest fires also become more common.

The Gulf of Mexico and the Atlantic Ocean are overwhelmingly the major sources of water vapor for the torrential rain that falls in the Southeast. Since the warmer the air the greater its capacity to hold water vapor, the region receives more rainstorms of great intensity in the summer than the winter. The average station in winter (December through February) receives 2.96 storms in a century, while in the fall (September through November), it rises to 5.73. It also should be added that the intensity of the rainfall increases, there being a far greater chance of events of five inches or more of rain in the summer and fall than in the winter and spring.

Maps 3 through 14 display by month the frequency of heavy precipitation. As previously mentioned, during the cooler months intense rainstorms are less frequent than in warmer ones. Nonetheless, it can be seen that in January, in most of Louisiana and Mississippi as well as parts of Arkansas and Alabama, there are stations that can expect at least three inches of rain in a day once or more every ten years. It is here that mid-latitude low-pressure systems, many within the winter jet streams that cross the nation, are most effective in drawing maritime moist air from the Gulf of Mexico onto the continent’s cool surface. As these air masses reach land they are cooled, frequently to the dew point, and precipitation may occur. Infrequently this rainfall can be very intense. In January (Map 3) the highest frequency of heavy precipitation in the region is in Highlands, North Carolina, much of it in the form of snow. Neighboringstations, though also located in the mountains, are not so well situated to face these moist air masses, and as a consequence receive far fewer such storms, not only in January but also throughout the year. 

The core area in which there is a high frequency of heavy rain in February (Map 4) continues to be in the Mississippi River Valley, but by then it has extended up into Tennessee, into coastal Texas and western Georgia. The cause of these rains is much the same as in January, as it is in March. However, by March (Map 5) the Gulf of Mexico has warmed sufficiently so that the air above it can now hold more water vapor than in mid-winter. Consequently, rainstorms become more frequent in the Gulf Coast states east of Texas as well as the Florida Panhandle. However, they continue to be infrequent along the Atlantic Seaboard, and throughout Peninsular Florida.

Peninsular Florida and the states farther north along the Atlantic Seaboard continue to experience few severe rainstorms in April (Map 6), but in May (Map 7) they begin to be more frequent. The most important controlling factor inhibiting the development of these storms is the Bermuda-Azores High Pressure system, which usually extends over this part of the South in the spring. In Peninsular Florida in both the winter and the spring this pressure system brings a prolonged period of drought and even farther north it inhibits the formation of intense storms.

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By May, however, days of intense rain become more frequent within Peninsular Florida and along the Atlantic Seaboard farther north. However, the expansion of the frequency of these storms into both East Texas and most of Oklahoma is far more general. Many of these storms continue to result from mid-latitude low-pressure systems moving in from the west, but by then low-pressure systems develop more frequently over the Gulf of Mexico. The most devastating example of a low-pressure storm that developed over the Gulf in the spring was Tropical Storm Allison, which in 2001 caused approximately 4 billion dollars of damage in the Houston area. It began to form in late May, but reached shore on June 5^th.

During June (Map 8), July (Map 9), and August (Map 10), when the jet streams move northward and rarely pass through the South, lower latitude low-pressure systems that move onto the land become the principal source of heavy precipitation in the region. Most of these pressure systems are classified as mere depressions, and seldom are capable of causing rainfall of three inches or more in one day. Depressions sometimes develop into tropical storms, and less frequently one of these storms may attain a wind velocity to be classified as a hurricane. Rain from these tropical storms and hurricanes make up a large percentage of the total torrential rain that falls in the Southeast.

Tropical storms and hurricanes are most frequent in the western Gulf of Mexico during spring and early summer. By August they are more evenly distributed throughout the region’s coasts, but are slightly more frequent along the Atlantic Seaboard. September (Map 11) is the peak month for low-latitude storms throughout the region except in Texas, where they peak in August. They are especially common in Florida in that month. By October (Map 12), as mid-latitude low pressure systems begin to reappear, a storm frequency distribution throughout the Southeast more similar to that of May begins to emerge, although the east coast of Florida continues to sustain short periods of intense rain from storms that have developed in lower latitudes. By November (Map 13), a month when few low-latitude storms develop, mid-latitude low-pressure systems overwhelmingly govern heavy rainfall. These mid-latitude low-pressure systems continue to be the major cause of torrential rain in December (Map 14). And will continue to be so until the following May. 

Commonly the rainstorms of high intensity rapidly lose their strength as they penetrate into the interior of the Southeast. The median one-hundred year frequency of storms of 3 to 3.99 inches in a day for stations from the coast to 50 miles into the interior was 91, while for those more than 50 miles from the coast it was 63, or 31 percent less. However, for storms of 5 inches or more the percentages were 29 and 11. Compared to the coastal stations, interior stations had 31 percent less 3 to 3.99 inch storms in a 100-year period, while for those 5 inches or less the interior stations had 62 percent less. This would indicate that many storms  that bring  5 inches or more of rain in a day lose there strength rapidly. This can be seen cartographically when storms of 3 to 3.99 inch rainstorms are compared with those that produced 5 inches or more (Map 15).

  The percentage drops dramatically not far from the coast. That decline is most noticeable along the Atlantic Seaboard and even can be seen when coastal Peninsular Florida stations are compared with those in the state’s interior. The decline is less rapid along the Gulf coast, and in southwest Texas it is almost nonexistent