Thunderstorms in Arizona?|
Why do Thunderstorms
Occur in Arizona During the Summer?
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.
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.
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.
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
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
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.