North American Monsoon Forecasting Difficulties



The North American Monsoon (NAM) is a yearly event that receives comparatively scant attention outside of atmospheric researchers and residents of the southwestern portions of the United States. Though the NAM rather pales when considered next to its better known relations in Asia and Australia it is not only a fascinating atmospheric occurrence but also absolutely critical to the arid and semi-arid regions that prevail in Arizona and New Mexico. Because of this fact forecasters located at the various NWS offices in Phoenix, Tucson, Flagstaff, Albuquerque and El Paso watch the southern skies as June approaches July awaiting the frequently sudden arrival of the NAM and the end of the typical spring drought.


The NAM occurs each year traveling from southwestern Mexico in early June and arriving at the US border usually by the beginning of July. Though its arrival date is fairly predictable and the duration of its stay is reasonably certain, the day to day changes are far more difficult than meets the eye. Whereas other portions of the US that are prone to summertime thunderstorms are concerned with convection temperatures, the cap, depth of moisture and various instability indices the southwest has additional variables which complicate forecasting the intensity, coverage and duration of storms.


The southwestern portion of the US is home to some of the most complex and diverse terrain in the country. Elevations can vary thousands of feet in mere miles. Mountains run north to south as well as east to west. An altiplano covering tens of thousands of square miles at an elevation over six thousand feet occupies the Four Corners region.

Dry washes, called arroyos, crisscross the desert like so many abandoned roads and present a real hazard during the summer monsoon. They can fill with swiftly running water in minutes and yet not witness a drop of rain, being swamped by the immense runoff from heavy thunderstorms over nearby mountains or foothills.

In addition, this quarter of the US has three distinct moisture sources that produce the NAM. For many years it was thought that the sole source of moisture was the Gulf of Mexico. However, additional research has shown that whereas New Mexico storms are indeed largely fed by the Gulf of Mexico the Arizona storms are not. The primary sources here are the Gulf of California and the tropical Pacific. Ergo, the complex and varied terrain coupled with multiple moisture sources equals additional headaches for forecasters as the risk of busted forecasts is heightened.

Further, the elevation of the terrain and moisture plays a crucial role in the development of storms. Substantial mid-level moisture (around 700mb) is actually low-level Boundary Layer moisture for the higher terrain. Thus this moisture couples with afternoon heating to produce significant instability over the mountains. However this does not necessarily translate into storms on the desert floor even if the steering flow is favorable unless Boundary Layer moisture at the desert elevations (typically below 2000 feet) is present. The intense daytime heating at these lower elevations produces a remarkably deep Boundary Layer that can extend to 800mb or higher and, if dry, quickly mixes out any moisture at the mid-levels during the course of the day leaving the deserts rainless.

There is yet another difficulty presented by this unusually deep Boundary Layer. It tends to render the typical LI or CAPE calculations inadequate. This means that morning soundings must be modified to realistically reflect anticipated conditions. Among the necessary modifications is that the mean mixing ratio, normally calculated from a 100mb depth from the surface, must be estimated for 200mb or more on some days when intense heating creates deep vertical mixing. Hence, part of the greatest challenge for estimating a rational LI is in correctly forecasting the afternoon depth and vertical temperature and moisture structure of the Boundary Layer.

Additional complications are presented with the standard synoptic scale charts. Due to the fact that a very weak environment establishes itself over the southwest during the summer, typical synoptic analysis and forecasting are of no essential use aside from the detection of easterly waves that cross the region. Numerical and statistical models and their products also have little skill in this environment and are notoriously unreliable. Hence, analysis at the mesoscale level and below is critical.

Lastly, difficulties all too frequently arise due to errant or tardy data from Mexico. Moisture surges from the south are initially detectable, naturally, at surface and upper-air sites in Mexico. Data from these sites routinely do not make it in time for NMC analysis and sometimes the data is erroneous. In fact, a forecaster for a private firm in southern California told me that there are times when he believes that the sounding out of Guaymas, located in the Mexican state of Sonora on the coast of the Gulf of California, appears to have been concocted out of whole cloth. This has lead on more than one occasion, he said, to forecasters being surprised at summer convection development as the Guaymas sounding did not reflect any significant moisture.

These peculiar dilemmas all converge on the southwest and create a scenario wherein forecasters need to be diligent, keenly aware and also learn from experience, their own and others. Naturally there are some techniques that have been developed to help the forecaster stay on top of the situation. First, technological progress made with remote sensing has been remarkable. Contemporary satellites have the capability of creating a general sounding of a cloud-free area. When dealing with absent or questionable data from south of the border, satellite data is considered reliable. Today's satellites can provide PW, LI and CAPE values that are indispensable to the monsoon forecaster.

Another crucial method is the tracking of 24 hour "deltas" or changes. Because, as previously stated, most disturbances that enhance or suppress the monsoon are of a sub-synoptic nature one must make abundant use of 24 hour change charts. Many of the anomalies in the sub-tropical easterlies originate in the southeastern US or Gulf of Mexico. These features can only be detected due to leading/trailing pressure fall/rise couplets associated with them. Hence the need to analyze the difference between today's pressure and heights and yesterday's at the same time.

Surges of moisture from the Gulf of California occur frequently throughout the monsoon season. Depending upon the depth of the moisture it can mix out with afternoon heating and have little impact or, if deep enough - typically 9000 feet or more - produce widespread convection accompanied with heavy rain and flash flooding.

Major surges can be detected the previous day via Mexican soundings and surface observations along the Gulf of California or through the aforementioned satellite sounding. Surges that are less evident require hour to hour monitoring of northwestern Mexican and Arizona data in an effort to locate 24 hour pressure fall/rise couplets. Studies indicate that a 5mb gradient from Sonora to the lower Colorado River valley will initiate a surge within 3-6 hours. Thus, experience with local conditions plus a well maintained local forecast techniques handbook are of paramount importance.

Another anomaly that impact local monsoon conditions and has to be watched is the Madden and Julian Oscillation (MJO) that originates in the western Pacific. These disturbances travel across the Pacific, Mexico and Atlantic enhancing tropical development. Taking some 30-60 days to make the journey, should one translate just south of the southwest US during the monsoon season it will initially suppress convection upon its approach then enhance convection while nearby then suppress it again upon departure. Obviously it behooves local forecasters to keep one eye on the tropical Pacific in anticipation of an MJO.


In sum then, the southwestern portion of the US is deceptive. Known for its mild winters that attract snowbirds it produces some of the most volatile and difficult forecasting situations during the three month, July to September, monsoon season. With exceedingly complex terrain creating chaotic wind flows, mountains and ridges abruptly rise from the desert floor near sea level to over 13000 feet. Three different moisture sources from three different directions take aim at the region. Subsynoptic and mesoscale disturbances traverse the region requiring a meteorological magnifying glass to locate. And finally, not always reliable data lurks to the south thus making a difficult situation worse. To combat this forecaster have, through experience, created unique techniques peculiar to the area. These, combined with improving satellite abilities, have greatly evened the odds between Mother Nature and the forecaster.