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In 2008, the National Weather Service changed the definition of the start of the Southwest monsoon from a variable date based on locally measured conditions to a fixed date of June 15. Prior to 2008, the start date reflected the seasonal progression of the monsoon (Fig. 1). This is based on larger seasonal atmospheric patterns and the establishment of the ‘monsoon ridge’ in the Southwest (Figs. 3a-b, also see sidebar for link to NWS pages).

In Southern Arizona, the start date was based on the average daily dewpoint temperature. Phoenix and Tucson NWS offices used the criteria of three consecutive days of daily average dewpoint temperature above a threshold (55 degrees in Phoenix, 54 degrees in Tucson) to define the start date of the monsoon. As shown in Figure 2, the dewpoint temperature criterion produced start dates ranging from mid-June to late July over the period of record (1949-2016). The average daily dewpoint temperature is still a useful tool to track the onset and progression of conditions that favor monsoon events, and the National Weather Service includes a dewpoint tracker in their suite of monsoon tools. (read more)

Precipitation & Temperature: May precipitation was variable across the Southwest, ranging from average to much-above average in Arizona and below to above average in New Mexico (Fig. 1a). Similarly, May temperatures were average to above average across Arizona and ranged from below to above average in New Mexico (Fig. 1b). Taking a longer view, spring (March-May) precipitation was mostly below average in Arizona, while New Mexico ranged from below average in the southwestern region to above average in the northeast (Fig. 2a). Spring temperatures were much-above average across most of the Southwest (Fig. 2b). So far in June, temperatures have ranged from 0 to 8 degrees above normal across much of Arizona and New Mexico, with extreme heat forecast for the week of June 19. June precipitation has been sparse in most of Arizona, with infrequent storm activity mostly in southern and eastern New Mexico. (read more)

Precipitation & Temperature: April precipitation was average to above average in New Mexico, while most of Arizona was below average, including much-below average and record-dry conditions in the southwestern corner of the state (Fig. 1a). April temperatures were above average in nearly all of Arizona and New Mexico, with much-above average temperatures in southern Arizona (Fig. 1b). May has been dry in southern Arizona and New Mexico, while parts of northern Arizona and northern and eastern New Mexico have picked up decent precipitation relative to the normally dry May climate (Fig. 2). May temperatures in Arizona and New Mexico have ranged from 4 degrees below to 4 degrees above normal, while temperatures in higher latitudes and upper elevations (e.g. Upper Colorado River Basin, California Sierras, etc.) have been generally warmer than average, ranging from 0 to 8 degrees above normal. Water year precipitation has been normal to above normal across most of Arizona and New Mexico aside from a small pocket of dry conditions along the Arizona-Mexico border (Fig. 3). (read more)

Oceanic and atmospheric indicators are still within the range of neutral (Figs. 1-2), although sea-surface temperatures have more consistently hinted at El Niño compared to atmospheric indicators. Outlooks and forecasts generally agree that ENSO-neutral conditions are likely to remain through the summer, but by mid-to-late 2017, chances of an El Niño event emerging become approximately equal to the chances of continued ENSO-neutral conditions. (read more)

March 2017 turned out to be the warmest non-El Niño March on record and the second warmest March overall (Figs. 5a-c).

(read more)