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General Climatology in Oklahoma

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).