Monsoon is the time of year many of us look forward to as active weather moves into the forecast! So what exactly is the “monsoon?" Below is a post we put together several years ago, to help people better understand the basics of the monsoon. Enjoy!

Before looking at how the monsoon works, one must understand the definition of monsoon and what it encompasses.

The word monsoon comes from an Arabic word “mausim” which means season (*2). This particular season does not correlate to the position of the sun but, instead, deals with a wind shift. In the desert Southwest, during the monsoon, the wind will shift from a normal westerly wind direction to a more southeasterly/southerly direction. This sets up a weather pattern that we will discuss in more detail later on, but for now, realize that it can import moisture into the state leading to widespread thunderstorm activity.

An important thing to remember is that a thunderstorm during this time is not a "monsoon." Thunderstorms that occur during the monsoon are called "monsoon thunderstorms" or simply thunderstorms. 

What signals the official start of the monsoon? Starting in 2008, the Arizona monsoon is officially defined by a finite start and end date. Under this new “system,” the start date of the monsoon is June 15th, and it ends on September 30th.

Before 2008, the season was defined to have started after a location had a daily dew point average of 55ºF or higher for three consecutive days. Using the old system, the average start date for Phoenix was July 7th, but it has started as early as June 16th in 1925 and it began as late as July 25th, 1987 (*6). Looking at Tucson data, the average start date was July 3rd. The earliest start date for Tucson was June 17, 2000, and the latest start date was July 25, 1987 (*11).

Every summer, the monsoon develops in the Southwestern United States. The area primarily affected by the monsoon can be seen in the image to the left, and it includes Arizona, Colorado, New Mexico, and a small portion of Texas (*3). Effects can go beyond this area, but for the most part, it is scientifically within this boundary. Phoenix is on the western edge of the monsoon area, which means the effects of the monsoon are a little less than that seen in Tucson and the higher elevations of Arizona and New Mexico.

The monsoon pattern usually begins to set-up in June and ends in September (*4). Phoenix, however, due to its location, usually begins to see the effects of the monsoon in the beginning part of July.

Looking at the June weather map, to the right, an area of high pressure develops over Mexico (*10). This setup allows Arizona to have the typical west to southwest wind pattern resulting in very little moisture being imported into the state. During this time as well, Phoenix and the surrounding desert locations experience scorching heat, which will eventually lead to the formation of a thermal low over the southwest portion of the state, which aids in the development of the monsoon pattern expansion.

Looking at the July weather map (left), a much different situation can be seen. High pressure moves northward toward the end of June to the beginning of July and centers itself over different regions throughout the season including West Texas, New Mexico, and the Four Corners region of Arizona. Also, as discussed earlier, a thermal low develops from the very hot temperatures around the Yuma area. Movement of this high pressure and the thermal low centered around Yuma changes the dominant wind direction in Arizona to the south/southeast. This change in wind direction allows moisture from Mexico, and the Gulf of California, to be imported into Arizona, New Mexico, and portions of Colorado. As high pressure drifts around, the moisture imported into the state will vary resulting in more thunderstorm development or less thunderstorm development.

The monsoon can be enhanced or not present for a couple of days at times during the season. These enhancements in moisture or lack of moisture to the area are called bursts and breaks (*9). Looking at the water vapor satellite image (right), it shows a surge of moisture resulting in numerous thunderstorms across the state. To read this water vapor image, the brighter the white, the more moisture available.

There are six main ways that moisture can increase during the monsoon:

Upper Air Disturbances:
As upper air disturbances move through the area, stability increases. This, in turn, allows the coverage and intensity of thunderstorms to increase (*10).

Gulf of California Moisture Surge Type One:
These types of surges will typically occur if the thermal low centered near Yuma is intensifying due to an increase in temperatures in the desert, and a large complex of thunderstorms is happening over central Mexico. The outflows created by strong thunderstorms in Mexico, as they collapse, can increase the moisture levels throughout the entire state leading to increased thunderstorm activity the next day (*6). 

Gulf of California Moisture Surge Type Two:
Another type of Gulf Surge is when a tropical system moves toward the tip of Baja or moves across Mexico from the Gulf of Mexico. This can import more moisture into the area increasing thunderstorm activity. (Image on left)

Thunderstorm Outflow:
Thunderstorm outflows are almost like the Gulf Surges, but on a more localized scale. Thunderstorms will develop in the higher terrain of Mexico and Arizona, and as the storms decay will send out a flow of cool/moist air. On the leading edges of these outflows, more thunderstorms can rapidly develop. (*8). 

Diurnal Flow:
Diurnal flow is the daily flow of moisture from the Gulf of California into Arizona and Mexico. This does not have a large impact on Arizona, but it is one of the main contributors to the thunderstorms that develop in Mexico. This process does, however, have the possibility to increase the lower level moisture content throughout the area (*8). (Image on right)

Backdoor Front:
A backdoor front moving through Arizona can increase the thunderstorm activity as well. This occurs when a very weak cold front moves down from the northeast into the state. These systems can bring with them moisture in the lower levels of the atmosphere, resulting in an increase in thunderstorm activity throughout the state (*8).

Moisture Recycling:
Moisture recycling is a process of heat evaporating moisture on the ground created by rainfall from previous storms. This, in return, will increase the moisture content in the lower levels of the atmosphere providing more moisture for thunderstorm development. This occurs mainly in the mountainous areas of Arizona and Mexico but can occur in the valley as well under certain circumstances (*8). (Image on right)

Those are the six main ways moisture can increase in the state, but throughout the season, the moisture can decrease as well.

The water vapor image to the left shows a break in the monsoon. As can be seen, moisture has shifted to the east over western Texas. This results in Arizona and Northern Mexico being extremely dry with little to no thunderstorm activity (*9).

A final main component to discuss when dealing with the monsoon is rainfall. Rainfall throughout the state during the monsoon changes dramatically with elevation. This is mainly due to orographic effects caused by the mountains in Arizona’s high country (*5).

Highest average rainfall for Arizona during the monsoon is 11.46 inches, which is recorded in Greer. Lowest average rainfall is registered in Yuma with only 1.01 inches. Phoenix on average measures 2.71 inches.

As can be determined from this brief introduction to the monsoon, the season is very complex. If you have any questions, feel free to contact us or tweet @mattpaceweather.

Sources Used:
1. Arizona Climate Summaries, Western Regional Climate Center,
http ://www.wrcc.dri.edu/summary/climsmaz.html

2. The Arizona Monsoon, NWS Forecast Office in Phoenix, Arizona,
http ://www.wrh.noaa.gov/psr/general/monsoon/

3. Ellis, Andrew W., and Erinnanne M. Saffell, and Timothy W. Hawkins “A Method For Defining Monsoon Onset And Demise In The Southwestern United States.” International Journal Of Climatology 24 (2004): 247-265

4. Higgins, R. W. “Interannual Variability of the North American Warm Season Precipitation Regime.” Journal of Climate 12 (1999): 653-680.

5. Michaus, Jené D. “Spatial and Elevational Variations of Summer Rainfall in he Southwestern United States.” Journal Of Applied Meteorology 34 (1995): 2689-2703

6. Monsoon Information, NWS Forecast Office in Tucson, Arizona,
http ://www.wrh.noaa.gov/twc/monsoon/monsoon_info.php

7. Monsoon Statistics, ASU Geography Department,
http ://geography.asu.edu/cerveny/wxpart4.htm#monsoon2

8. Summer Thunderstorm In Arizona?, NWS Forecast Office in Tucson, Arizona,
http ://www.wrh.noaa.gov/twc/monsoon/az.php

9. What is the Mexican Monsoon, NWS Forecast Office in Tucson, Arizona,
http ://www.wrh.noaa.gov/twc/monsoon/mexmonsoon.php

10. What is the Monsoon?, NWS Forecast Office In Flagstaff, Arizona,
http ://www.wrh.noaa.gov/fgz/science/monsoon.php?wfo=fgz

11. Year-by-year monsoon statistics, NWS Forecast Office in Tucson, Arizona,
http ://www.wrh.noaa.gov/twc/monsoon/monsoon.php

If you thought the past few years have been hot, they have nothing on the year 1990!

To get a picture of the record-setting 1990 heat wave, it all started on June 20, when the high temperature in Phoenix reached 113º. The high stayed at or above 113º through June 24, until a high of 120º was measured on June 25. At the time, it was the highest temperature Phoenix had ever recorded since records began in 1896. Sadly, that record did not last long, as the very next day (June 26) the temperature topped out at a remarkable 122º! This temperature still stands as the hottest temperature ever recorded in Phoenix. This record stands just 12º cooler than the record high temperature ever recorded on Earth. That record was set on July 7, 1913, at Furnace Creek (Death Valley), at a scorching 134º.

To understand how intense this Phoenix heat was, let's take a quick look back at what those living in Phoenix would have felt on June 26, 1990.

First off, the low that morning was already 91º. By 8 am, the temperature had risen to 99º and was up to 112º by 11 am. No, that is not a typo; it was above 110º before noon! At 2 pm the temperature rose to the 120º mark and finally at 4 pm, the thermometer read the record-setting 122º! The temperature cooled to 120º by 5 pm and 119º an hour later. The heat continued through the night, with temperatures staying above 100º until midnight! Overall, the temperature remained above 100º for 15 hours, above 110º for 10 hours, and at or above 120º for 4 hours. Bottom line, it was HOT!

Did you live in the Valley during this record-setting heat wave? What were you doing? Tweet me your story: @mattpaceweather

If you stepped outside Monday morning you may have noticed a couple of things: clouds, humidity, and it was still warm!

The clouds and moisture that moved in overnight did not allow temperatures to fall very far during the overnight hours, with the low at Phoenix Sky Harbor only dropping to 87º. This breaks the record for the earliest 87º+ low temperature for the year. The old record was June 13, 2018.

It should be of no surprise that this also breaks the record for the warmest low temperature for the day, which was 82º and set in 2013.

In case you were wondering, the warmest low temperature ever recorded in Phoenix stands at 92º and occurred on both June 29, 1976, and July 6, 1983.

The first 100º temperature: it happens at some point each year in Phoenix and marks the unofficial start to summer. This year it happened on Sunday (April 26th), which is the exact same date as last year.

The average first 100º day is May 2nd. The earliest on record occurred on March 26, 1988, and the latest first on record was on June 18, 1913.

The high temperature on Sunday continued to rise until reaching 102º, which set a new record for the day. The old record was 101º and was set in 1992 and 1898.

Read More: Two Phoenix Milestones Just Days Apart

On average, there are 110 days with a high temperature at or above 100º in Phoenix, so the good news, only about 109 more to go!

It's that time of year when all the great Phoenix winter weather bragging begins to fade away, and we enter the sweat zone.

The forecast is calling for Phoenix to break the 90º mark for the time this year on Wednesday (April 22nd) and 100º on Sunday (April 26th).

If the forecast pans out, it will only be four days between the first 90º and 100º days. This would tie as the third shortest time-space between these two milestones. In 1994, they were just one day apart (April 15th and April 16th). And in 1988, they were three days apart.

The thirty-year average is just over a month apart, with the average 90º day being March 31st and the first 100º day on May 2nd.

On the other hand, the furthest apart these two events have been was 83 days which occurred in 1904 (Feb 24th & May 17th) and 1972 (Mar 5th - May 27th).

Stay cool!

Get ready for a significant drop in high temperatures! A storm system currently off the coast of California will move into Arizona Wednesday through the end of the week. This storm will result in a chance of rain for the Valley, but the big story will be the much cooler weather!

High temperatures by Thursday are forecast to be in the upper 60s! Keep in mind, the average for this time of year is 83º!

If the forecast pans out and the high temperature stays in the 60s, it will be the first time since 2012 that we have seen a high temperature below 70 degrees during the month of April! Since 2000, there have only been ten days in April with high temperatures below 70 degrees.

If you don't like the cooler temperatures, don't worry, highs are forecast to rise back into the 80s this weekend.

Did you catch a glimpse of the International Space Station as it soared high above the Phoenix valley tonight (April 5th)?

If not, our two weather cameras captured it as it moved from west to east (left to right in the loops).

The top image is from our camera in Gold Canyon and the bottom image is from Northeast Mesa.

Today (March 30th), the high temperature at Phoenix Sky Harbor was 77º, with a low temperature of 53º. If you asked what the high and low were yesterday, you would get the same answer. While nothing might sound too exciting with this, let's go to the numbers.

Since records began in 1896 to today, it has been 45,379 days. Out of those days, Phoenix has recorded the exact same high and low temperature on back-to-back days a total of 590 times. That is only 1.3% of all days on record!

So far this year, Phoenix has already recorded five of these days, and we are only three months into the year! The most recorded in a given year was ten, which occurred in both 2012 and 1930. The least is one in a year and occurred in 1989, 1980, 1978, and 1919.

Ok, that might still not sound exciting, but to a weather nerd... well, you get the point.