Importance off monsoon ridge placement!

During the North American Monsoon, there is a lot of forecasting emphasis on where and how the “monsoon ridge” (a subtropical high pressure circulation) evolves over the course of the summer season! The subtropical high can be tracked at the middle and upper levels of the atmosphere on 500mb weather charts representing an altitude around 20,000 feet MSL. You’ll often see centers of high pressure denoted on weather graphics with a “H” (usually in blue color). It is possible to have multiple areas of high pressure in the region to help complicate prevailing wind flow patterns in the atmosphere. Critically, air flows clockwise (anticyclonic) around high pressure centers.

Important forecast considerations for anticyclones aloft are that their outer periphery, which can extend outwards hundreds of miles, tends to steer both moisture and thunderstorms (for an example check out a previous post on “ridge rider” thunderstorms found in the Great Plains). Being positioned near or under the anticyclone center on the other hand often precludes cloud and thunderstorm development due warm and dry sinking air called subsidence. During the summer, the resultant “heat dome” under high pressure centers may bring dangerous heatwaves at the surface! Focusing on rain chances, though, the placement and geographic persistence of the monsoon ridge are key drivers for who gets subtropical moisture directed to them or deflected away and for how long!

There are four ideal monsoon ridge placements documented to bring active thunderstorm periods into the Desert Southwest, such as Arizona (shown in the introduction of the animation). A vital initial monsoon ridge pattern for the Arizona region starts with the Four Corners setup (junction of AZ, UT, CO, and NM) on the Colorado Plateau as such a south/southeasterly flow pattern taps and brings northward subtropical moisture from Mexico, Gulf of California, and Gulf of Mexico. Once moisture is introduced and entrenched by daily recycling from ongoing regional thunderstorms, the other three shown patterns can interact with existing moisture to enhance a thunderstorm and heavy rain threat!

Unfortunately, you can see in the video animation of 2023 daily high pressure weather charts we have lacked a sustained Four Corners pattern into mid-July, leaving most areas west of New Mexico without deep monsoonal moisture and, hence, deprived of daily thunderstorms. Note how the daily monsoon ridge evolution through this period is similar to the “non-soon” (very dry) 2020 summer season in general where you see the general absence of a persistent and strong anticyclone in proximity to the Four Corners zone (strong high pressure are indicated by yellows, oranges, and reds in the animation). This was not an issue with the wet 2022 summer monsoon!

The goals of this discussion were to 1) give an update on Monsoon 2023 and why it has been rather dry and hot to start for Arizona, 2) show a daily monsoon ridge comparison between a historically “wet” and “dry” summer rainy season in the southwestern U.S., and 3) highlight how complex and variable the monsoon environment is based only on observing daily monsoon ridge behavior!

Learn more about the monsoon!

WHAT IS A MONSOON? (PART II)

HOW DO WE PERCEIVE A MONSOON SEASON?

CAN A WET WINTER LEAD TO A DRIER MONSOON?

CAN A WET WINTER LEAD TO A DRIER MONSOON? (PART II)

MONSOON FLASH FLOODING: IT CAN HAPPEN IN A FLASH!

WHY DO MONSOON THUNDERSTORMS FAVOR THE MOUNTAINS?

MONSOON AIR QUALITY: AN INTRODUCTION

DO THUNDERSTORMS HAVE FAMILY TREES?

HOW TO SPOT A SEVERE THUNDERSTORM?

WHAT IS THAT “CAP” LOOKING CLOUD ABOVE A THUNDERSTORM?

WHY ARE THUNDERSTORMS "LUMPY" LOOKING?