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General Climatology in Oklahoma

General Climatology - Ellen J. Cooter, Oklahoma Climatological Survey, taken from:
Tortorelli, R.L., 1991, Floods and Droughts: Oklahoma, National Water Summary 1988-89: U.S. Geological Survey, Water Supply Paper 2375, p. 451-452.

Oklahoma's climate is influenced by the State's geographic location on the leeward side of the Rocky Mountains. Average annual precipitation increases from west to east and ranges from about 16 inches in the extreme western panhandle to 56 inches in the southeastern corner of the State (National Oceanic and Atmospheric Administration, 1977).

In winter, Oklahoma lies in the southern range of the polar jetstream and the northern range of the subtropical jetstream. The result is extremely variable temperature and precipitation. January temperatures have ranged from daytime highs of 70 degrees Fahrenheit to nighttime lows well below zero. Winter precipitation commonly consists of a combination of rain, ice, and snow. At time, strong winds and large snowfalls cause severe drifting and blizzard conditions (National Oceanic and Atmospheric Administration, 1977).

The distribution of precipitation generally has two peaks during the year. The largest peak is in late spring, and the secondary peak is in early fall (Eddy, 1982). The primary source of moisture for precipitation is the Gulf of Mexico. The Pacific Ocean off the coast of Mexico is a source of moisture under certain airflow patterns.

Much of the spring precipitation results from large thunderstorms, many of which produce tornadoes and large hail. These severe storms occur as surface low-pressure and frontal systems develop when a transient upper-air trough approaching from the west interacts with warm, moist air from the Gulf of Mexico. Persistent flow in the upper atmosphere form the west combines with topography to establish a surface lee-trough just west of Oklahoma. The lee- trough is a common location for development of the dryline and thunderstorms that develop along it. Moisture from the Gulf of Mexico that feeds the thunderstorms flows northward east of the lee-trough.

In addition to the oceans, important moisture sources include local and upwind land surfaces, as well as lakes and reservoirs, form which moisture evaporates into the atmosphere. Typically, as a moisture- laden ocean airmass moves inland, it is modified to include some water that has been recycled one or more times through the land-vegetation-air interface.

Convective storms, which generally move individually from southwest to northeast, move eastward as complexes across the State and provide most spring and early summer rainfall. A typical storm system is about 10 miles wide by 25 miles long (Eddy, 1982). Flooding caused by convective storms tends to localized unless the storms fail to move quickly. If intense thunderstorms repeatedly develop over the same terrain for several hours or days, localized flooding can be massive.

A second large-scale feature in Oklahoma's precipitation delivery system is the Bermuda High. The Bermuda High is a semipermanent subtropical high- pressure cell in the North Atlantic Ocean whose circulation pattern is largely responsible for the warm and humid conditions that prevail in Texas and Oklahoma in summer (Bomar, 1983). Clockwise circulation around this airmass controls most of the surface-moisture supply received by Oklahoma from the Gulf of Mexico.

The location of the Bermuda high-pressure system substantially affects late summer and fall rainfall. If the system is south of its normal location, polar airmasses can move southward into Oklahoma. Moisture form the Gulf of Mexico, and occasionally from decaying tropical cyclones, combines with southward- moving polar air to cause rainstorms. Tropical cyclones, which include hurricanes, can originate either in the Gulf of Mexico or the Atlantic or Pacific Oceans. Widespread floods can result from storms produced by these conditions.

If the Bermuda High occurs north and west of its normal location, drought can occur. The air is hot and humid, but the upper air trough, along with disturbances necessary for the development of intense thunderstorms, stays north of Oklahoma. Without this triggering mechanism, thunderstorms can occur anywhere in the State, but none are large enough to produce intense rains and substantial runoff. Climatological data indicate that the absence of fall rains may result in prolonged less than average streamflow. Droughts also are characterized by a disproportionate lack of weekly rainfall that measures more than 0.5 inch (Eddy, 1982).

Tortorelli, R.L.,Floods and Droughts: Oklahoma, National Water Summary 1988-89: U.S. Geological Survey, Water Supply Paper 2375.

Table 1. Chronology of major and other memorable droughts in Oklahoma, 1923-88

[Recurrence interval: The average interval of time within which streamflow will be less than a particular value for droughts. Symbols: >, greater than; <, less than. Sources: Recurrence intervals calculated from U.S. Geological Survey data; other information from U.S. Geological Survey, State and local reports, and newspapers]

DateArea AffectedInterval (years)Remarks
1929-41Statewide10 to >50 Regional, Length and severity were less in south and east.
1951-57Statewide>10 to <50 Regional. One of the most severe on record. Municipal water-supply problems critical
1961-72Statewide>10 to >25 Discontinuous in many locations.
1975-82Statewide10 to >25 Least severe of recent long-term droughts. Discontinuous in many locations. Longer duration in northeast.
1984-86Southwestern10 to >25 Local and south-central Oklahoma.

Major droughts in Oklahoma, as determined from streamflow records collected since the early 1920's, have occurred during four periods: 1929-41, 1951-57, 1961-72, and 1975-82. A significant but more localized drought occurred during 1984-86. Drought analysis is more subjective than that for floods. For example, records for some gaging stations indicate an essentially continuous less than average discharge throughout a given drought, but records at other stations have one or more years when flows were average or greater. Such short-term reversals in trend may indicate two or more separate droughts or a temporary, local wet period within the longer drought. In most instances, the longer period was used to compute recurrence intervals for the droughts in table 1.

The drought of 1929-41 was regional in extent and had a recurrence interval of greater than 50 years in large areas of Oklahoma. The length and severity of the drought were greatest in the central and western parts of the state. This drought was one of the most noteworthy for Oklahoma because of its adverse effect on landowners and the agricultural industry. Major soil damage from wind erosion affected most of the State-the phrase "Dust Bowl" was coined during this time-and caused a mass exodus of people from farms in the panhandle and western Oklahoma (Nace and Pluhowski, 1965). The total expenditure by the American Red Cross for drought relief in Oklahoma in 1930-31 was the third largest in the Nation (Hoyt, 1936).

The 1951-57 drought was severe nearly statewide but generally less intense than the 1929-41 drought, having a recurrence interval of less than 50 years (table 1). By this time, a network of operating gaging stations permitted a more refined determination of the areal extent of droughts. The records for the Baron Fork at Eldon, and Blue River near Blue indicate that the period of less than normal annual flow began slightly earlier in the eastern part of the State. During this period, rural population decreased but not to the same extent as during the 1930's. Wind created major soil erosion in large areas of western Oklahoma, but damage was not on the statewide scale experienced in the 1930's (Nace and Pluhowski, 1965). This drought motivated many Oklahoma communities to expand their public water-supply sources. Oklahoma City constructed a dam forming Atoka Lake in southeastern Oklahoma and as of 1988 imported 19.5 million gallons per day through a 100-mile pipeline from the lake. The devastating floods of spring 1957 ended the drought.

The entire State was affected by the 1961-72 drought, which was less severe than the two previous droughts; nonetheless, in most areas, the drought had a recurrence interval greater than 25 years. The duration and severity of the drought differed across the State. Records of the Cimarron River at Perkins indicate that the onset of the drought at that gaging station was delayed but that there was less than average annual discharge through the 10 years that followed. Records of the Arkansas River at Ralston and Elk Creek near Hobart also show a delay in the onset of the drought and there were short-term reversals in the trend of deficit flow. In east-central Oklahoma, the 1961-72 drought had two deficit periods separated by more than 3 years of surplus from the late 1960's to early 1970's, as illustrated by Baron Fork at Eldon. In southeastern Oklahoma, the pattern was similar, but the period of low streamflow began as early as 1959 and was interrupted by about 2 years of more than average flow during 1968-69.

The least severe of the recent long-period droughts was during 1975-82; this drought had recurrence intervals greater than 25 years in about one-half the State and 10-25 years in the rest of the State. The length and severity of the 1975-82 drought differ considerably among the gaging stations. Records of the Arkansas River at Ralston, Cimarron River at Perkins, Walnut Creek at Purcell, and Blue River near Blue illustrate an almost continuous annual discharge deficit. Records of Baron Fork at Eldon and Elk Creek near Hobart indicate longer and shorter drought periods, respectively. During 1976-77, the drought extended over much of the United States (Matthai, 1979).

The most recent drought identified in figure 4 (1984-86) was local. It affected areas in southwestern and south-central Oklahoma.

Selected References

Bergman, D.L., and Tortorelli, R.L., 1988, Flood of May 26-27, 1984 in Tulsa, Oklahoma: U.S. Geological Survey Hydrologic Investigations Atlas HA-707f.

Bingham, R.H., Bergman, D.L., and Thomas, W.O.,Jr., 1974, Flood of October 1973 in Enid and vicinity, north-central Oklahoma: U.S. Geological Survey Water Resources Investigations Report 74-27.

Bomar, G.W., 1983, Texas weather: Austin, University of Texas Press, 265p.

Buckner, H.D., and Kurklin, J.K., 1984 Floods in south-central Oklahoma and north-central Texas, October 1981: U.S. Geological Survey Open-File Report 84-065, 112 p.

Corley, R.K. and Huntzinger, T.L., 1979, Flood of August 27-28, 1977, West Cache Creek and Blue Beaver Creek, southwestern Oklahoma: U.S. Geological Survey Open-File Report 79-276.

Eddy, Amos, 1982, A rainfall climatology for Oklahoma - Operational weather modification: Oklahoma Climatological Survey, v.5, 120p.

Hauth, L.D. 1985, Floods in central, southwest Oklahoma, October 17-23, 1983: U.S. Geological Survey Open-File Report 85-494, 21 p.

Heimann, D.C., and Tortorelli, R.L., 1988, Statistical summaries of streamflow records in Oklahoma and parts of Arkansas, Kansas, Missouri, and Texas through 1984: U.S. Geological Survey Water-Resources Investigations Report 87-4205, 387 p.

Hoyt, J.C. 1936, Droughts of 1930-34: U.S. Geological Survey Water Supply Paper 680, 106 p.

Matthai, H.F., 1979, Hydrologic and human aspects of the 1976-77 drought: U.S. Geological Survey Professional Paper 1130, 84 p.

Nace, R.L., and Pluhowski, E.J., 1965, Drought of the 1950's with special reference to the midcontinent: U.S. Geological Survey Water-Supply Paper 1804, 88 p.

National Oceanic and Atmospheric Administration, 1977, Climate of Oklahoma: Washington D.C., Climatology of the United States no. 60, 18 p.

Oklahoma Planning and Resources Board, 1934, Washita River, Hammon flood, April 3-4, 1934: Oklahoma City, Division of Water Resources report, 112 p.

Tortorelli, R.L., and Bergman, D.L., 1985, Techniques for estimating flood peak discharges for unregulated streams and streams regulated by small floodwater retarding structures in Oklahoma: U.S. Geological Survey Water Resources Investigations Report 84-4358, 85 p.

U.S. Department of Commerce, 1958, Rainfall and floods of April, May, and June 1957 in the South-Central States: Weather Bureau Technical Paper 33, 350 p.

U.S. Geological Survey, 1986, national water summary 1985-Hydrologic events and surface-water resources: U.S. Geological Survey Water Supply Paper 2300, 506 p.

U.S. Geological Survey, 1990, national water summary 1987-Hydrologic events and water supply and use: U.S. Geological Survey Water Supply Paper 2350, 553 p.

Prepared by: Robert L. Tortorelli, U.S. Geological Survey

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