SPECIAL TOPIC NUMBER 1:
MONSOON DEFINITION AND CRITERIA
MONSOON: A seasonal reversal of pressure and wind patterns, energy and mass
transfers, with characteristic regimes of rainfall and temperature. Monsoons may be
hot or cold and/or wet or dry depending upon the circumstances involved in the
seasonal change. Though thunderstorms may be a part of the characteristic regime,
it is incorrect to call them Monsoons-instead the correct terminology is Monsoon
MONSOON SEASON/START DATES: With the recent change in NWS philosophy
regarding the onset of the monsoon, the following criteria no longer applies to
determining the onset of the monsoon but rather will alert people as to when the
monsoon flow regime is present.
1. The pre monsoonal heat burst has occurred as the 500 mb High shifts to its
monsoonal position and begins to initiate the windshift.
2. The 500 mb windflow over Arizona shifts from SW to NE-SE with the 700mb
windlfow becoming E-SE or S with mean windspeeds at both levels of 10 kt or more.
3. The average dewpoint in Sierra Vista (Tucson) is equal to or greater than
52F(54F)/11C(12C) for three consecutive days.
4. The monsoon moisture boundary at 850mb (with the northern boundary defined
by the 850 mb dewpoint of 8C) moves north of Sierra Vista (Tucson).
5. A well defined temperature break from the pre monsoon heat burst occurs.
National Weather Service defines the official Monsoon "Season" as running from
June 15th through September 30th.
The northern extension of the North American Monsoon into the SW US and
especially Arizona is a unique circumstance that gives a character to our weather that
is often unexpected and unappreciated by those who have not experienced it and
especially with respect to our higher humidity and Summer rainfall that comes as a
surprise to those who consider Arizona to be a bonedry desert.