Southeast Arizona Tropical Weather Page

Welcome to the southeast Arizona tropical page. This site was put together to give you, the user, access to tropical weather information that has affected Arizona.

Tropical cyclones in Arizona

Perhaps somewhat surprisingly, Arizona is occasionally struck by tropical cyclones. In fact, southern Arizona experiences flash flooding indirectly associated with a tropical cyclone about once every two years, while an intact, albeit decaying, tropical depression or tropical storm moves across southern Arizona about once every five years.

Until the 1920s, the mere existence of eastern Pacific hurricanes was disputed. Even when they were "discovered," eastern Pacific tropical cyclones were not routinely tracked until the 1940s, or named until 1960. When these tropical cyclones were detected, they were either bearing down on an unprepared coast, or were disrupting shipping and fishing. However with the deployment of weather satellites in the mid 1960s, tropical cyclones were finally tracked routinely and accurately. Forecasting these storms has improved immensely since the 1980s with the use of better computer weather models, Doppler weather radars, and a better observing system in both the U.S. and Mexico.

Tropical storms and hurricanes, because they thrive in warm water, weaken rapidly once they move inland. This is especially true in western Mexico and the southwest U.S. where several large mountain ranges severely disrupt landfalling tropical systems. However, given the right atmospheric conditions, these storms can survive far inland. Even if they dissipate hundreds of miles away from Arizona, heavy rain and flash flooding can still develop over the region.

Since 1965, eight remnant tropical storms or hurricanes have survived the trip over mountains and deserts to affect southern Arizona as an intact tropical storm or depression (Graphic 1).

They were, of course, in a weakened state by the time they arrived, but five (Katrina 1967, Joanne 1972, Kathleen 1976, Lester 1992 and Nora 1997) have actually made it into southern Arizona as a tropical storm, with maximum sustained winds of 39 mph or greater.

A map showing tracks of tropical weather system that hit Arizona since 1965.

Graphic 1: Track of the eight tropical cyclones which have struck southern Arizona since 1965. Data from the National Hurricane Center (//www.nhc.noaa.gov), Sellers et al. (1985), Smith (1986), and Garza (1999). Click on image for larger view.

Also since 1965, there have been 17 storms which, despite dissipating south or southwest of Arizona, triggered flash flooding as their remnant moisture spread into the state (Graphic 2). In fact, four of the five most serious floods in southern Arizona since 1970 were caused by the remnants of tropical cyclones

Composite map of tropical storms that had a direct or indirect impact on Arizona.

Graphic 2: Track of all eastern Pacific tropical storms and hurricanes which have produced flash flooding in southern Arizona since 1965. Click on image for larger view.

Historical Hurricane tracks

Local research has found that El Nino-Southern Oscillation (ENSO) has little effect on either increasing or decreasing the chance of a tropical cyclone impacting Arizona in any given year, although there are typically more tropical storms and hurricanes in the eastern Pacific during El Niño years. Although longer-range ENSO patterns may not offer much forecasting help, there is a critical weather pattern which tends to support tropical cyclone incursions into Arizona. About 70 percent of all eastern Pacific tropical storms and hurricanes track harmlessly west over the open waters of the eastern Pacific. The other 30 percent, though, are turned northward by the jet stream as it occasionally dips south along the west coast of the U.S. (Graphic 3). This tends to happen more frequently during the latter stages of the North American Monsoon from late August into early October. Once a tropical storm is forced to turn northward, or recurve, there is a much greater chance of it affecting a land area before dissipating, and for remnant moisture to be carried into northwest Mexico or the southwest U.S.

Graphic 3: Typical Arizona tropical cyclone weather pattern. An incoming upper level trough will either capture the deep moisture and funnel it into Arizona, or will steer the tropical storm or hurricane itself toward Arizona. This same trough will tend to weaken the tropical cyclone, but it also can accelerate it, and not allow it to weaken as quickly as usual as it heads toward the region. Click on image for larger view.

The turn north toward Arizona still does not guarantee a direct impact on the region. The interaction between the steering trough to the west and the weakening tropical cyclone is critical, and has to be exact for heavy rains to develop over Arizona, or for the tropical system itself to arrive in Arizona intact. If the trough moves into the west coast too fast, much of the tropical remnants can be suppressed to the south and east of Arizona. If the trough weakens or dissipates, either the tropical system may stall well to the south and dissipate, or there will not be enough atmospheric lift to work on the incoming moisture. In these examples, rainfall can end up spotty and limited, while clouds associated with both systems stabilize the atmosphere and prevent embedded thunderstorms from developing.

Tropical systems can have a significant impact for weeks and even months after the storm has passed. The heavy rains from these tropical systems can saturate the thin soils over Arizona in the fall, with the water remaining locked in the soil all winter due to lower evaporation rates. This makes it easier for subsequent winter storms to cause large-scale flooding.

Acknowledgement:
We are greatly indebted to David Roth at the NOAA/NWS Hydrometeorological Prediction Center, and the late Miguel Cortez at Servicio Meteorólogico Nacional, for their work in documenting tropical-cyclone rainfall in both the southwest U.S. and Mexico. The rainfall graphics shown for each tropical cyclone listed on our web page were produced by Mr. Roth. The extensive information exchange between the SMN and the NWS has greatly helped forecasters in recent years to better predict tropical cyclone rainfall in the southwest U.S. and Mexico.

References:
Garza, A.L., 1999: 1985-1998 North Pacific Tropical Cycles Impacting The Southwestern United States and Northern Mexico: An Updated Climatology. NOAA Technical Memorandum, NWS WR-258.

_____, 2008: National Hurricane Center. NOAA/NWS Tropical Prediction Center, Miami, FL. [Available on line at: //www.nhc.noaa.gov].

Roth, D., 2008: Tropical Cyclone Rainfall Data. NOAA/NWS Hydrometeorological Prediction Center, Camp Springs, MD. [Available on line at: //www.hpc.ncep.noaa.gov/tropical/rain/tcrainfall.html].

Sellers, W.D., R.H. Hill, and M. Sanderson-Rae, 1985: Arizona Climate: The First 100 Years. University of Arizona Press.

Smith, W, 1986: The Effects of Eastern North Pacific Tropical Cyclones on the Southern United States, NOAA Technical Memorandum, NWS WR-197.