Interesting article on the North American Monsoon written by Kim Runk, Meteorologist in Charge at the
Las Vegas office of the National Weather Service:
The summer thunderstorm season across the Desert Southwest is sometimes referred to
as the “Southwest Monsoon” or “North American Monsoon”. The term “monsoon” comes
from an Arabic word meaning “season”. It is typically associated with a change in the prevailing
wind which persists for several months, bringing with it a change in the overall
character of the weather. The classic example occurs in India and Southeast Asia, where
winter is dominated by a cool, dry “northeast” monsoon flow, while in summer the
“southwest monsoon” leads to their rainy season.
In the southwestern United States the winds aloft shift from westerly in the winter to a more
southeasterly direction in summer. This shift is also accompanied by a lower atmospheric
wind reversal over the Gulf of California from the north during the winter months to the
south during the summer. These prevailing wind changes and the resulting altered character
of the weather qualify the general circulation as a monsoon system. Some areas are
moving toward defining the North American Monsoon as a more generic season (June 15 –
September 15), after the model of the “hurricane season”. This time frame does encompass
more than 95% of severe convective storms forming in the Southwest Monsoon regime,
and it establishes a consistent time frame for facilitating inter-annual monsoon season comparisons.
The Las Vegas forecast area (southern Nevada, northwest Arizona, and southeast California)
actually floats in and out of the monsoon moisture regime throughout the summer.
Therefore, the historical emphasis on identifying the onset and end time of the monsoon
has not been as high as it tends to be in southern Arizona. We have found it can even be
counter-productive, diverting attention away from more important issues, i.e., awareness
of, and preparedness for, the hazards associated with summer storms. We do use the
“monsoon” terminology and we actively track the location of the monsoon moisture boundary,
but the main purpose is to diagnose the character of thunderstorms which will form on
a given day. When we are outside the monsoon moisture boundary, and yet conditions are
sufficiently moist and unstable to support thunderstorm development, the primary threat is
associated with damaging winds. This is because the moisture is concentrated aloft. As
storms mature and collapse, precipitation falling through the dry air in the lower atmosphere
evaporates and accelerates, creating strong, erratic downburst winds. Within the
monsoon boundary the near-surface air is humid and the threat transitions into one of flash
flooding associated with heavy rainfall. Thus, exposure to thunderstorms poses dangers
regardless of whether those storms form within the monsoon flow regime. It is the nature of
the threat to life and property which is of paramount importance to us in issuing appropriate
warnings that will provide you, the public, with information you can use to make wise,
safe, informed decisions.
Source:
Spring 2008 Skywarn Spotter Newsletter
A new feature to monitor the status of the Las Vegas Doppler weather radar has been added to the site. When the radar is not operational due to malfunction or maintenance, a message will appear near the top of the radar page. This message will indicate the amount of time the radar has been down as well as the reason and ETA if available. Check it out at http://www.neondesertweather.com/lvradar.php
