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Tucson AZ tropical page
| Southeast
Arizona tropical weather page |
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| 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. |
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| 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. |
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| 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. |
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| 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. |
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| 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). |
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| 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. |
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| Graphic
1: Track of the eight tropical cyclones
which have struck southern Arizona since 1965. Data from the
National Hurricane Center (http://www.nhc.noaa.gov),
Sellers et al. (1985), Smith (1986), and Garza (1999). Click
on image for larger view. |
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| 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 |
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| 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. |
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| 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. |
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| 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. |
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| 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. |
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| 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. |
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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. |
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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: http://www.nhc.noaa.gov].
Roth, D., 2008:
Tropical Cyclone Rainfall Data. NOAA/NWS Hydrometeorological Prediction
Center, Camp Springs, MD. [Available on line at: http://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.
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National Oceanic and Atmospheric Administration
National Weather Service
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Tucson, AZ 85719
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