Five Things To Look for During The Southwest Summer Monsoon

Tom Moore
Published: July 16,2017
The "wet phase" of the summer monsoon has arrived as it does, like clockwork, around this time of year.
Many residents of the Southwest U.S. know that they should prepare for weather events related to the summer monsoon, but here are five weather impacts of this pattern.
(MORE: Why Pop-Up Summer Thunderstorms are Among the Hardest Weather to Predict)

1.  Slightly Lower High Temperatures

Some of the highest temperatures of the year can be recorded just before the onset of the summer monsoon. Humidity levels are low, so the sun's energy can be used to heat the air rather than being absorbed by water vapor or used for evaporation.
Cities like Phoenix can record high temperatures of 115 degrees or hotter. The highest recorded temperature in Phoenix was on June 26, 1996, when the mercury soared to 122 degrees.
During much of the summer monsoon period, high temperatures can be around 10 degrees lower, in the 100- to 105-degree range.

2. Increased Humidity

Prior to the onset of the summer monsoon, dew point temperatures during the day can be in the 20- to 25-degreee range across the Desert Southwest. This may translate to relative humidity levels of 5 to 15 percent due to the excessively hot temperatures.
During the monsoon period, the dew point temperatures can rise to the 55- to 65-degree level across the Desert Southwest and the relative humidity can exceed 40 percent. The dew point is a measure of how much moisture is in the air. For example, dew point temperature of 10 degrees is very low.
However, when the dew point temperature exceeds 55 degrees, it begins to feel more uncomfortable. Residents of the Southwest experience uncomfortable humidity levels like some of their neighbors east of the Rockies.
When the temperature is at or above 100 degrees, it can be extremely uncomfortable, even with a relative humidity of 40 percent.

3. Dust Storms (Haboobs)

The increase in humidity levels during the summer monsoon means that there is more moisture in the air to produce thunderstorms. Thunderstorms across the Desert Southwest are more common in the higher elevations, but can occasionally move into the desert floor.
Quite often, these thunderstorms have very little rain with them, but they can produce a haboob.
A haboob is an intense, widespread dust storm. The word comes from the Arabic word "habb," which means wind. As a thunderstorm collapses, air is forced rapidly downward on the front end. The wind rapidly picks up dust and debris, and the result can be blinding and almost choking dust in the air.
The most intense haboobs can lift dust over 5,000 feet into the air. The storm can continue on for up to 150 miles in extreme cases. If you are driving, these dust storms can appear in a short period of time and visibilities can be reduced to zero.
The National Oceanic and Atmospheric Administration (NOAA) has issued some dust storm safety tips:

• If dense dust is observed blowing across or approaching a roadway, pull your vehicle off the pavement as far as possible. Stop, turn off the lights, set the emergency brake and take your foot off the brake pedal to be sure the tail lights are not illuminated.
• Don't enter the dust storm area if you can avoid it.
• If you can't pull off the roadway, proceed at a speed suitable for visibility, turn on your lights and sound the horn occasionally. Use the painted center line to help guide you. Look for a safe place to pull off the roadway.
• Never stop on the traveled portion of the roadway.

(MORE: It's Prime Time for Damaging Winds in the United States)

4. Flash Flooding

Heavy monsoon rains flooded I-10 at 43rd Ave. in Phoenix, Arizona, on September 8, 2014
(AP/The Arizona Republic, Michael Chow)

Thunderstorms across the Southwest can sometimes produce torrential rain in a short period of time. In these situations, rivers, creeks or dry streams (arroyos) can quickly fill and produce high water on roadways, or even in cities like Phoenix and Las Vegas.
Be very careful driving across the Southwest if a thunderstorm produces heavy rain. The Arizona Emergency Information Network has issued some flash flood safety tips for drivers.

• Do not drive into a flooded area.
• If floodwaters are near your car, abandon it and move to higher ground immediately.
• Know that 6 inches of water can cause loss of control of a car and possible stalling.
• Cars can float in 12 inches of water and 24 inches can sweep your vehicle away.

5. Dangerous Lightning

Summer thunderstorms across the Southwest can put on quite a lightning show, especially with a flat desert floor and mountains in the background. Also, many of these storms have very little rain and relatively dry air, which also assists viewing in this part of the country.
Lightning can be deadly, however, so be careful if a thunderstorm develops in your vicinity. The National Weather Service has the slogan, "When thunder roars go indoors."

Setup for the Summer Monsoon

Typical Setup For The Summer Monsoon.

Although the dates for the summer monsoon may vary a bit across the region, the National Weather Service's Phoenix office has established some guidelines. One example is if the dew point temperature is 55 degrees or higher over a three day period. Also, there are occasional breaks in this pattern during the season.
The time period established for the summer monsoon is from June 15 to Sept. 30. The average onset of the monsoon in Phoenix is July 7.

Prior to the onset of the monsoon (generally late May and early June), temperatures across the Desert Southwest are extremely hot and humidity levels are quite low. That changes as we move into July and August, and into September.

In late June and early July, a ridge of high pressure commonly sets up over the Four Corners area (where Colorado, New Mexico, Utah and Arizona come together). Low pressure (a thermal low) develops across interior areas of Southern California, and a southeasterly flow of moist air is transported into the region.
MORE: Haboob Slideshow

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