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The summer thunderstorm season in Arizona is the northern extension of the Mexican Monsoon. As a result summer rain in Arizona is more variable than across northwest Mexico. Even so, storms in July, August and early September are usually frequent enough to provide a welcome change from the hot, dry days of June.

Where does the moisture for summer thunderstorms in Arizona come from?

There are several moisture sources for thunderstorms in Arizona during the monsoon. The following discussion summarizes the most significant moisture sources for thunderstorms and rain over Arizona.

Mid-level moisture is often the earliest moisture to arrive over Arizona during the monsoon. The mid-levels are roughly defined as the layer between 10,000 and 30,000 feet above sea level.

The source for mid-level moisture during the summer is mainly thunderstorms that occur in surrounding regions. The moist remnants of thunderstorms can move into Arizona from the northeast, southeast or south depending on the large-scale flow pattern.

There are several potential sources of low-level moisture over the southern half of Arizona. They include; diurnal flow, Gulf of California moisture surges, thunderstorm outflows, moisture recycling and backdoor fronts.

Diuarnal Flow - The moisture input due to diurnal flow is realtively small in Arizona but increases dramtically further south across northwest Mexico. In general the diurnal surface flow is about 5 to 10 knots from the Gulf of California towards Arizona and the mountain foothills of west and northwest Mexico. This source acts to moisten the low-levels in Arizona but seldom provides enough moisture for significant convective outbreaks. In Mexico the diurnal contribution from the Gulf of California is higher and can enhance developing thunderstorms.

Gulf Surges - Gulf of California moisture surges are a major source of low-level moisture for thunderstorms in Arizona. A Gulf of California surge is a push of low-level moist and cool air that moves northward over the Gulf of California and into southern and Central Arizona. The "Gulf Surge" is such an important player in Arizona monsoon weather a more in-depth presentation is located on the Monsoon Basics Page.

Thunderstorm Outflow - Outflows from thunderstorms also contribute to increasing moisture in Arizona during the monsoon. Each day, during an active monsoon period, thunderstorms develop over the mountains of northwest Mexico and/or Arizona. As these storms release their storage of rain and hail evaporation occurs. This creates cool and moist pockets of air that descend to the ground and move out from the parent thunderstorms. If they are strong enough they can generate new thunder storms in front of the existing storm. Over a series of days the increase in moisture due to outflows can produce a threat of thunderstorms.

Moisture Recycling - Recycling primarily moistens the low-levels but also provides some moisture at mid-levels due to the depth of desert heating. Moisture recycling occurs when daytime heating causes evaporation at the surface and plant transpiration. The moisture released is mixed upward into the atmosphere. The recycling moisture source is usually in addition to moisture already present and is part of the reason the mountain foothills receive so much more rain than the deserts.

Backdoor Fronts - On occasion, when strong and deep cold fronts form over the High Plains of the United States and move southward along the front range of the Rocky Mountains, they can move across New Mexico and into Arizona. By the time these fronts reach Arizona they are weak but can transport moist low-level air from the High Plains into Arizona. Occasionally enough moisture is present to enhance thunderstorm development over Arizona.

What is the character of thunderstorms observed during Arizona's summer thunderstorm season?

The following examples below depict the different types of thunder storms obsevered on a daily basics during the monsoon. The storms occurance and strengh are highly dependant on the available moisture and the strengh of steering winds that can drive the storms off the mountains and into the deserts.

The strengh of thunderstorms and the amount and coverage of Monsoon thunderstorms can range from low grade, typical or active patterns. On ocassion a tropical connection can enhances the thunderstorms and can lead to wide spread severe weather.

Low Grade Monsoon: - This ocurrs when there is mid-level moisture present across Arizona the high terrain of northern and eastern Arizona can thunderstorms. Storms that form are usually rather benign (except for cloud-to-ground lightning!) and produce little if any significant weather. However, rain amounts of up to half an inch can occur in the mountains. Over a number of days the rainfall totals in the mountains can really add-up. When only mid-level moisture is present it is very difficult for thunderstorms to venture into the desert areas.

Typical Monsoon - Condtions occur when mid-level moisture is present along with an increase in low-level moisture storms will build over the mountains. These storms, while in the mountains, are likely to be stronger and produce more rain than their mid-level moisture counterparts. Since there is more low-level moisture storms are able to move off of the mountains by mid-afternoon and make their way into the high desert. But once again, as these storms move towards the low desert, the air is too dry and stable and they diminish. On days such as this a thunderstorm or two will usually manage to make into Phoenix or Tucson but they then to be weak with only a trace to half inch of rain. On these days Southeastern Arizona and the mountains will see the majority of the thunderstorms.

Active Monsoon - Is When ample mid-level and low-level moisture is located across Arizona widespread thunderstorm activity is possible. Storms form over the mountains first but head towards the desert area by mid-afternoon. With ample low-levemoisture present storms can become very strong producing a large area of strong wind, heavy rain and frequentcloud-to-ground lightning. Storms can move across the low desert when plenty of low-level moisture is present. In cases of extremely high levels of low-level moisture thundestorms can develop all night, and form into meso- scale convective complexes and move all the way to the California/Arizona border.

Tropical Connection - Is a less frequently observed pattern for a Gulf Surge is when a tropical storm or hurricane is located near the southern tip of Baja California. The realtively cool and moist tropical air mass is essentially pushed northward through the Gulf of California. At the same time Arizona is usually experiencing very hot daytime temperatures with a strong thermal low at the surface. This feature is most likely to occur in late season (August or September).

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