Chapter 2. Hydrology
23
Convective Thunderstorms
Summer convective thunderstorms in the southwestern U.S. are the result of
land surface heating and an influx of moist tropical air from the Gulf of Mexico,
Gulf of California, and eastern tropical Pacific. This influx of tropical moisture is
associated with the northwestward migration of the Bermuda high pressure ridge
and the northward displacement of the Pacific high pressure cell (Bryson and
Lowry 1955, Hales 1974, Carleton 1985, Douglas and Englehart 1990) (Fig. 6).
This so-called "summer monsoon" is characterized by scattered convective
thunderstorms that are triggered by intense surface heating and enhanced by
orographic effects.
Summer precipitation totals associated with convective thunderstorms are
greatest in Arizona and New Mexico, as these states are the closest to the tropical
moisture sources. Uncertainty still exists as to the source of the tropical moisture
fueling the monsoons, but it appears to be a combination of low-level moisture
originating from the Gulf of California and eastern tropical Pacific and higher-
level moisture from the Gulf of Mexico rising over the Sierra Madre mountains
in Mexico (Adams and Comrie 1997). The amount of summer precipitation
exhibits great interannual variability. Summer precipitation is highest in Arizona
and New Mexico when a subtropical high pressure cell develops over the Four
Corners region (Carleton et al. 1990). When this high pressure cell is depressed
farther south, precipitation values are much lower. Small, steep watersheds are
the most responsive to convective thunderstorms, as rainfall intensities are high
and a single isolated storm cell can cover an entire small watershed. The isolated,
short-lived nature of the thunderstorms means that runoff to larger rivers is being
contributed from only a small portion of the total watershed area.
North Pacific Tropical Storms
Tropical cyclones that form off the coast of southern Mexico normally
migrate northwestward over the Pacific Ocean, but occasionally the cyclones
recurve toward the northeast and make landfall, usually over Baja California or
western mainland Mexico (Fig. 7). Moisture from tropical cyclones is, however,
occasionally drawn into Arizona, New Mexico, southern California, and southern
Utah, resulting in intense precipitation and floods (Ely 1997, Farfan and Zehnder
2001), with the Tucson Flood of 1983 being a prime example (Kresan 1988).
Nearly every year the remnants of tropical cyclones move into a position where
the upper-level winds transport moisture and then produce showers and thunder-
storms in the Southwest (Farfan and Zehnder 2001). The frequency of tropical
storm recurvature is greatest in the fall (September and October), when the
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