Meteorological data at the Portal LTREB has been recorded since 1980.
Annual precipitation is distinctly bimodal, with peaks in winter (November
to March) and summer (July and August).
Occasionally, in late summer or early autumn, dissipating tropical storms
affect the southwestern portion of North America, producing widespread
and intense precipitation. While such storms can affect the region several
times per decade, they are sufficiently severe to cause heavy flooding
approximately once every twenty years. In late September 1983, tropical
storm Octave generated record flooding throughout southeastern Arizona
and southwestern New Mexico. During a period of less than a week, the
study site received 129 mm of precipitation which was equal to about 50%
of the yearly average. This caused declines in several rodent populations,
especially bannertail kangaroo rats.
El Niņo events are thought to be associated with exceptionally heavy
winter precipitation in the Southwest desert region. there has been no
consistent relationship between El Niņo and winter precipitation at Portal,
however. Simiarly, rodent population increases or peaks are not related
to El Niņo events.