Notes from an Applied Climatologist: Tropical Storms and the Southwest Q&A

How do (Pacific) tropical storms affect the weather patterns of the SW? What type of tropical storm paths affect weather in the SW?

The eastern Pacific Ocean along the west coast of Mexico and central America are an active area of tropical storm formation through the summer and early fall (typically between June and September) (Figure 1). Warm water and light winds create the perfect conditions in this region for thunderstorms to flare up, and eventually organize into tropical storms and hurricanes.

Surprisingly, the landlocked southwest U.S. is impacted by these storms in a variety of ways. The most common way is by inducing surges of moisture up the Gulf of California into Arizona that can fuel widespread outbreaks of monsoon season thunderstorms.

Clusters of storms or tropical systems that pass near the mouth of the Gulf of California can produce a cool outflow of winds and create a pressure differential up the Gulf of California (high pressure near the cool thunderstorm outflows and low pressure near the hot conditions and rising air typical in the lower Colorado River valley at the north end of the Gulf). This pressure differential leads to the surge of moisture traveling from south to north, up into the low deserts of Arizona, and sometimes southern California (Figure 2).

Tropical storms and hurricanes in the eastern Pacific can sometimes actually impact the Southwest through a direct hit (Figure 3). Easterly winds will typically carry eastern Pacific storms west out to sea, but storms that take a more northwesterly track along the Mexican coast can eventually track north or even northeast. This typically happens later in summer or in early fall, as the subtropical high pressure system retreats south giving way to very light winds aloft in no particular direction or stronger westerly winds advancing from the north, the sign of fall and winter approaching. When tropical storms move north and aren’t steered away from the coast of Mexico they can continue to wander north or can get caught up in westerly flow aloft and directed inland towards southern California and Arizona.

Hurricanes weaken quickly when they encounter land and break away from their energy source of warm ocean water, so only weak tropical storms or ‘remnant low’ pressure systems actually make it to inland locations like Arizona. Still, these systems are often potent rain producers bringing with them abundant moisture and a source of lift to produce widespread thunderstorms and can produce very dangerous and widespread flash flooding events. The infamous flash flooding disaster that occurred in October of 1983 was caused by tropical storm Octave where much of Arizona observed record precipitation and catastrophic flooding (Figure 4).

Michael Crimmins is an Associate Professor and Climate Science Extension Specialist in the Department of Soil, Water and Environmental Science

Figure 1: Tropical Storm Formation by Date - Source: NOAA
Figure 2: Pacific Moisture Flow into Southwest - Source: NOAA
Figure 3: Figure 3: Historical Tropical Storm Tracks in the SW - Source NOAA
Figure 4: 1983 - Hurricane Octave and Flooding in the SW - Source NOAA