Good Morning Folks another hot day expected in South Florida today. But I can say that from Mid-April to October sometimes even longer. I have been reading weather stories throughout our planet, yes I do read weather stories that cover the globe and I am seeing all the flooding that is devastating India, Sri Lanka, Pakistan and South Asia. The Asian Monsoon is the strongest on Earth because of the ITCZ – Intertropical convergence zone goes the farthest North anywhere on the planet in this location. In fact the ITCZ goes to Northern India near the Himalayas near 35 degrees LAT, where the subtropical high belt is usually located. Also the Thermal Low that develops over the Tibetan Plateau and a strong onshore flow from the Indian Ocean. These factors contribute to this Monsoon being the strongest on Earth and ITS in overdrive now. The regions affected by NAM is Arizona, New Mexico, Utah, Southern Nevada, and Southwest California.

But I wanted to blog about the North American Monsoon, its not as strong as the Asian Monsoon but similar processes during occur here for initiation. There is a lot to cover so lets get started. On the news we see scenes of water rescue, flooded out cars and roads as well as homes being carried away by floods. This is a summer time phenomenon caused by the North American Monsoon. I am going to discuss the : Definition, Stages, and Processes that cause it.


The North American Monsoon is not as strong as the Asia counterpart because the Mexican Plateau is not as high or large as the Tibetan Plateau. The NAM(North American Monsoon) share the same characteristics as the Asian Monsoon. There is a shift in wind and barometric pressure, because Monsoon means seasonal shift in wind and pressure not heavy rain. There is a shift in wind patterns in Summer which occurs as Mexico and the Southwest U.S> warms under intense Solar heating under the Subtropical High and the Thermal Low. As this happens, the pressure and wind flow reverses from dry land area to moist areas, In the NAM, the low level moisture is transported primarily from the Gulf of California, SSTs here can reach great than 90 degrees and the eastern pacific and as far as the Gulf of Mexico due to the counter-clockwise flow around the Thermal Low. This Thermal Low is a shallow feature in the atmosphere but very effective. The Gulf of California and Eastern Pacific are responsible for low-Level Moisture and the Gulf of Mexico is responsible for Upper Level moisture carried by Easterly winds aloft. The Sierra Madre Occidental green up from initial Monsoon rains which leads to evaporation and transpiration and more moisture for the atmosphere.


The are 5 stages of the North American Monsoon

A. RAMP UP – This occurs during Mid-June to Early July.

By Mid-June the flow of tropical moisture has begun into the Sierra madres in Southern Chihuahua, Durango and Sinaloa, Mexico. During the Ramp up stage the Monsoon Ridge has formed as pictured above and is expanding North into the Rio Grande Valley. This causes upper level winds over Southeast AZ to turn from the southwest to west to the Southeast to Easterly direction. The shift begins to spread upper level subtropical moisture from the GOM into NW Mexico and into the Desert Southwest(AZ,NM). Isolated thunderstorms at first develop due to heating of the day. The lowest level of atmosphere remains dry, so t-storm rains mainly affect mountain regions. In the hot valley strong, damaging winds and dry lightning(without rain) can ignite wildfires on the parch desert landscape. This can be considered the highest fire danger of the year. Winds from these downdrafts can reach 70+mph.

B. ONSET –  This occurs during Late June into Mid July

As moisture increases over Northern Mexico and t-storms become a daily occurrence, over the Sierra Madres just South of the Arizona border, dramatic changes take place over the mountain landscape. Areas that were brown and sun parched now have become green with subtropical trees and plants due to the monsoon rains. The trees sprout canopies of leaves, these trees and plants as well as the wet soil through evaporation/transpiration(moisture evaporated through plant matter) put considerable moisture into the atmosphere. Meanwhile the “Monsoon Ridge” continues to strengthen and move northward into the Southern plains of the U.S. By now winds in SE Arizona are firmly aloft out of the East/SE. Moisture spreads from aloft and the surface. At this satge Deseerts are still very hot and dry but t-storms are becoming more numerous and are producing more rain. They start developing earlier in the afternoon and grow and organize into the early evening, Severe weather becomes a threat with damaging winds and microbursts(strong downdraft of winds) the primary threats. Dust storms named Haboobs can occur at this stage.

C. PEAK – This occurs in Mid July – Mid August

This is Monsoon “prime time in SE Arizona. The “Monsoon Ridge” is at its strongest and Northern most extent in the 4 corner regions. This High can even meander into the Great basin area. Steering flow is persistently out of the East over Southern Arizona and into Northeast AZ as well.  There is two particular phenomenon that can take place and can cause heavy ‘bursts’ of rain.

  1. GULF SURGES – There are occasional low level moisture surges from the Gulf of California which at this point has SSTs over 90 degrees. These surges travel up the Colorado River Valley and low deserts of AZ sometimes reaching southern to central Utah.
  2. UPPER LEVEL SHORTWAVES –  Upper level disturbances rotate around the base of the Monsoon ridge or Subtropical High that pass regularly across Southern AZ, and the Sonoran Desert. These create the instability for t-storms to develop. these storms can produce hail and torrential rainfall up to 4 inches/hour. Flash flooding is a major concern here.

At 4 a.m. this is when the Gulf Surge begins its March north.

D. LATE MONSOON –  This occurs from Mid August to Early September

It is unusual for a Monsoon to end abruptly. By Mid-august, the increasing low sun angle in the Northern Hemisphere becomes less effective in maintaining the “Monsoon Ridge”. The High Pressure Ridge begins its retreat Southeast into the Southern Plains and even breaks into pieces as the polar Jet stream begins to shift South from Canada. Winds become more variable, so any t-storms that develop will be slow movers and can dump heavy rains and cause flash flooding.

E. DECAY STAGE – This occurs from Early September to Late September.

By September, the “Monsoon Ridge” has weakened considerably and the Polar Jetstream continues to strengthen and shift south with all general circulation features of the Northern hemisphere. Winds aloft now are blowing from the SW and work their way down to the surface. this flow disrupts moisture both in the upper and lower levels in AZ. Soils to begin to dry out, and temps begin to cool at the surface, but remain warm aloft. Instability begins to fade and t-storm activity dies out. In the Decay phase, weather can still be active, moisture lingering from the peak phase can initiate storms but rarely. At this time, if a Mid-Latitiude Travelling Cyclone or MLC from the Pacific comes along, they can tap into that moisture laid behind by the Peak Monsoon phase, and Supercell t-storms can develop. This occurs because lapse rates are steep due to much colder air aloft. Every type of Severe Weather can occur here even Tornadoes(rare though).

This is the Peak Stage of the North American Monsoon(NAM). Where the Easterly flow is firmly entrenched.

Well I hope I made a clear understanding of the North American Monsoon. I hope this was educational and entertaining as well. I appreciate all my viewers and comments. If there is something you want me to write about please let me know in the comment section. Stay well everyone, and many Blessings.

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