Arizona, tropical cyclones, and floods: A story
Background image: Phoenix flooded streets after remnants of Hurricane Rosa (2018) passed through
When the monsoon season comes to an end, the Eastern Pacific Ocean can create a new type of threat: tropical cyclones (hurricanes/tropical storms/tropical depressions).
Some of these tropical cyclones can move towards the north and influence rainfall events in Arizona. How frequent are these events? Which part of Arizona are they most likely to impact?
The University of Arizona (UArizona) and the UArizona/NASA Space Grant , in partnership with the Tucson National Weather Service (NWS), has developed this page as a resource for the community to learn more about these rare events and the flood risk associated with them.
¡Para la version en español, haz click aquí !
Image on the right is a satellite image from Hurricane Rosa in 2018 (from: goes.noaa.gov), which ultimately weakened, but the remnants produced widespread flooding in parts of Arizona.
Background on tropical remnants, rainfall, and floods in Arizona
Arizona's Annual Average Rainfall
Arizona's annual average rainfall ( right ) is NOT uniform. In fact, it tends to vary according to abrupt elevation changes (see figure below) such as those in the Mogollon Rim in the Central Highlands as well as in the so-called Sky Islands in the Southeastern Basin and Range.
General Arizona regions (from: Paretti et al., 2014 )
There are two distinct wet seasons: the Monsoon (June 15-September 30) and the Cool Season (November 1-April 1).
Tropical cyclone remnants tend to influence rainfall events from September through November.
How can tropical remnants influence rainfall?
Although tropical cyclones start to weaken long before they reach Arizona, the highly moist remnants can be transported towards the northeast into the southwestern US (southern California, Arizona, New Mexico and even west Texas).
You can follow the track of Hurricane Rosa from the tropics (September 25, 2018) to Arizona (October 2, 2018).
However, there needs to be some sort of atmospheric disturbance before it can rain!
How much rain can we expect?
It varies from event to event!
The remnants of Hurricane Rosa produced rain three days, but they varied in amount. For example, the map on the left shows the rainfall totals for September 30, 2018-October 1, 2018.
A day later, the map on the left shows much more intense rainfall totals for October 1-2, 2018!
Rainfall = Runoff?
Sometimes!
The hydrologic system in Arizona is very complex and depends on many components. Storms related to tropical cyclone remnants can last a very long time (few hours to days), which produce large floods.
The figure on the left shows the hydrologic response ( Tonto Creek near Roosevelt : a relatively dry river northeast of Phoenix) related to the remnants of Hurricane Rosa. You can see the river reacts quite fast to rain!
Generally, these types of floods carry a lot of water, take a long time to develop, and tend to last from hours to a few days!
How frequently are rivers flooded due to tropical cyclone remnants?
As seen in the map on the right, floods caused by tropical remnants are uncommon, but not rare in the state of Arizona.
Click on each circle to get information about the US Geological Survey streamflow station!
How big are these types of floods?
Tropical cyclone remnant-related floods have the potential to cause widespread damage. The largest floods on record (up to 2011) in the USGS stations marked with green diamonds (right) are related to tropical remnants.
Click on each station to get more details!
Examples of Major Tropical Cyclones that have affected Arizona:
Note: In the US, we measure the volume of water flowing in a river in cubic feet per second (cfs). The easiest way to imagine this is 1 cubic foot being the size of a basketball.
One cubic foot is about 7.5 gallons, and 1 gallon weighs approximately 8 pounds. So, 1 cubic foot per second is the equivalent of a basketball weighing 60 pounds passing by every second!
Why does this matter? How are Hurricanes related to the Monsoon Season?
Recent example: Hurricane Lorena 2019
The 2019 monsoon season was on the way to become one of the driest on record. From July to the end of August, most of Arizona had below average rainfall.
The map on the left shows that most of Arizona had less than 25% average rainfall during the month of August!
However, from September 22-25, 2019 the remnants from Hurricane Lorena interacted with an upper atmosphere disturbance and brought much needed relief, but ample flooding as well.
Flooded street filled with debris in Apache Junction (from: Tom Tingle, The Republic)
Flooded street as seen from Arizona Department of Transportation (ADOT) camera
Satellite imagery showing Hurricane Lorena
Image taken from GOES-16 as it approached the Mexican west coast.
Track of Lorena
Forecasted track of Hurricane Lorena a few days before it dissipated is shown on the right.
Actual track (from: noaa.ncep.gov)
How much water did Hurricane Lorena carry?
A lot! About twice as much moisture was available during this event than on the average September day.
Atmospheric disturbance?
The upper atmosphere had a disturbance that stalled over southern California for several days (low pressure system). This set up created the perfect conditions for water vapor to move into the area and provide a trigger for storms.
How much did it rain?
The loop on the right shows the daily rainfall totals. You can see that in:
September 22: NO RAIN!
September 23: Light rain in central Arizona
September 24: Widespread rainfall especially in central and southern Arizona
September 25: Heavy rainfall across southern Arizona
September 26: Light rain through western, central and southern Arizona
September 27: Light rain in western Arizona
Did it help?
Depending on location, the total rainfall ranged from 0.5 inches to over 5 inches!
Unfortunately, the 2019 monsoon season totals were still below normal for most of Arizona locations as seen on the map on the right (bar graph below shows Southern Arizona stations).
from: NWS Tucson
How does this affect you?
Flooding!
Remember that since it can rain a lot and very fast during these events, arroyos and low-lying roads will fill with water.
Flooded arroyo from rainfall caused by remnants from Hurricane Lorena in 2019, from: Vail Preservation Society
Tornadoes!
Very rare, but they can be produced by these events!
Tornado spotted near Willcox, from: Laurin Richey
Significant damage!
Not all events are the same, but houses, roads, utility poles can be damaged during and after these events.
Uprooted tree in Tucson, from: tucson.com
Saguaro blown over, from: tucson.com
