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CLIMAS SW Monsoon Outlook - June 2017 | CLIMAS


CLIMAS SW Monsoon Outlook - June 2017

Friday, June 16, 2017

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.

Thirty-year averages for daily dewpoint and precipitation demonstrate the gradual increase in dewpoint temperatures during the monsoon season, as well as the variability of precipitation observed over the same window (Fig. 4).

The updated definition of the monsoon identifies a season that lasts for 108 days with defined start and end dates of June 15 and Sept 30. In the Southwest, however, the majority of monsoon storm activity occurs in July and August (Fig. 5), with some lingering activity into September (occasionally augmented by eastern Pacific tropical storms). Dewpoint and precipitation may provide a more granular assessment of monsoon activity, but the seasonal designation allows for easier comparisons between years, and focuses planning activities on a discrete monsoon season (see NWS video about Monsoon Awareness Week).

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