Monsoon Summary

A short-lived burst of monsoon activity began in southeast Arizona and southwest New Mexico on June 16. Several storms produced close to 0.4 inches of rain in parts of these regions (Figures 9a–c). Another series of monsoon storms produced some rain during the June 23 weekend (which is not reflected in the figures). The recent rain suggests the monsoon has begun earlier than average—historically, the monsoon begins in the southern parts of Arizona and New Mexico around July 1. The dew points, however, remain below temperatures historically used to define the onset of the monsoon. In 2008, the National Weather Service changed the definition of the onset of the monsoon to June 15 to facilitate clear communication of monsoon risks.

In most years, the monsoon forecast calls for equal chances that July–September rainfall will be above, below, or near average. This forecast is no better than flipping a coin. This year, however, signs are strong enough that the Climate Prediction Center (CPC), which issues the official forecasts, is calling for slightly increased chances for above-average monsoon rainfall totals (see The 2012 Monsoon Forecast). Confidence that the monsoon will begin early and be vigorous in July is driving this outlook. The CPC has more uncertainty in precipitation during August and September, in part because ENSO-neutral conditions may continue or evolve into an El Niño event (see El Niño Status and Forecast). If an El Niño forms, there is an increased chance that September could be wetter than average, in part because the frequency of tropical storms in the Pacific Ocean increases. The CPC forecast is also supported by methods that analyze past summers that most resemble current and expected conditions. Based on this analog forecast, wet conditions are also favored for July; it also suggests August will be near average and September will be wetter than average. Caution should be exercised when using monsoon forecasts, because predicting the monsoon is difficult; many climate and weather phenomena influence the convective storms.