|The North American Monsoon
The North American Monsoon typically affects Arizona, New Mexico, Southern California, Utah, and Colorado. For
the purpose of this paper, I will focus on Arizona. The North American Monsoon is a regional-scale circulation
that develops over southwest North America during the months of July through September. It is associated
with a dramatic increase in rainfall that occurs over what is normally an arid region of North America. The
NWS officially signifies the start of the monsoon when 3 consecutive days have passed with the dew point
above 54 degrees F. Typically the southwest will see airmass thunderstorms develop well before the
NWS "official" start of the monsoon season.
The monsoon is a seasonal change in upper level winds from the polar westerlies to tropical easterlies
and a change from very dry west winds aloft to moist winds aloft from the east or southeast.
The monsoon is an annual occurrence as the sub-tropical ridge shifts poleward during the summer months. As
this occurs, the ridge elongates to the west and ushers in warm moist air into the region. A thermal
low also sets up over the Desert Southwest due to the intense surface heating providing an optimal
pressure gradient from the east-southeast.
The source regions for the moisture influx are the Gulf of Mexico, southern plains, Sonoran Plateau,
and the Gulf of California.
The worst days for activity across Arizona occur during a "Gulf Surge". A Gulf of California moisture
surge is a low-level influx of moist, relatively cool air that moves northward over the Gulf of
California and into Arizona. Surges of moisture from the Gulf of California provide the fuel
that feeds the wide spread and intense storms that form across Arizona. A Gulf Surge can
occur whenever the surface based thermal low over the southwest intensifies at the same
time large thunderstorm complexes are forming over northwest Mexico. Many of the severe
and heavy rain producing thunderstorms that form across Arizona during the monsoon are
directly related to Gulf of California moisture surges. The most common
large-scale pattern observed before a Gulf of California surge is an easterly
wave passing over central portions of Mexico at the same time mid-level high
pressure and very high surface temperatures are plaguing Arizona. Not all Gulf
of California surges are created equal. Some surges are shallow and weak with
virtually no affect on Arizona except to increase the humidity. Surges of
moderate depth and strength can penetrate all the way into the Mogollon Rim
country with a distinct upswing in convective activity across Arizona. Occasionally,
maybe once or twice a monsoon season, a strong and deep surge will find
its way into Arizona. This is when the real fireworks begin!
As this moisture moves into the southwest, a combination of orographic lift caused by the mountains
and intense surface heating causes thunderstorms to develop across the region.
The monsoon season typically begins early July when we see east/southeast winds and an
increase in moisture aloft. Storms will develop initially over the mountains with some
development in the valleys with the rain never reaching the surface (virga). The problem
with these dry storms is significant downbursts and dust storms due to evaporational
cooling with storm bases around 12,000 feet. By mid-July we begin to see a rapid increase
in surface moisture and thunderstorm coverage. In southeast AZ, downbursts become
less of a problem as storm bases lower to around 8,000 feet, however; large downburst
potential remains high across the Phoenix area. Thunderstorm timing shifts into the
early afternoon throughout southeastern AZ and evening hours across Phoenix. By late
July-mid August, the flash flood threat across the state is at it peak with storms
continuing into the nighttime hours. Microbursts are less common, but large scale
downbursts remain a challenge as storms collapse across the region. We do see
occasional breaks in thunderstorm activity especially following a very active day. Around
late August-early September, we begin to see an increase in number of days between
thunderstorm activity. The flash flood threat remains high, especially if tropical
cyclones get in the mix. This time of the year the hail and tornado threat
increases as cells tend to take on a more supercell-like structure. By
late September, we see the easterly flow gradually transition back to
the westerlies as the sub-tropical ridge retreats equatorward. The
lingering moisture begins to interact with incoming cold fronts as we start the
transition to the fall/winter season. The hail and tornado threat is at its
peak during this time as cold air starts creeping its way into the region and
cut-off lows start to develop off the coast of southern California.
Forecasting the onset of the North American Monsoon is not difficult with knowledge of the synoptic-scale
pattern. A knowledgeable forecaster can monitor computer models to determine when the atmospheric
conditions are in place for the monsoon season to begin. The biggest challenge for forecasters is
when the monsoon season is in effect. Powerful winds, lightning, and heavy rains are the biggest
challenges forecasters in the southwest face when dealing with thunderstorm development. One
of the most useful tools to use is hands down, the skew-T. The skew-T diagram can tip off
the seasoned forecaster to impending doom! Looking at the cold (morning) sounding and
forecasted soundings throughout the day can give the forecaster clues to see how bad
your day is really going to be. Thunderstorms are not likely unless CAPE/unstable
LI are present or develop during the day. Thus, the biggest
challenge in estimating a reasonable lifted index and/or CAPE for Arizona locations is
correctly forecasting the afternoon depth, temperature, and moisture profile of the boundary layer.
Here's an example of a really busy day! A 1200Z sounding with a CAPE value of 5180 j/kg and LI of -10.2:
Arizona forecasters face numerous challenges during the monsoon season. However, armed with a strong
working knowledge of skew-T, models, and radar the forecaster will be able to warn the public and
government sectors to potentially deadly hazards in a timely manner.
Erik Pytlak, NOAA/NWS Tucson, AZ, "The North American Monsoon: Impacts on the Desert Southwest"
National Weather Service Forecast Office Tucson, AZ, "What is the Mexican Monsoon", retrieved 15 Mar 08
National Weather Service Climate Prediction Center, "Reports to the Nation: The North American Monsoon", Aug 04
Wikipedia, "Monsoon", retrieved 15 Mar 08