Sunday, July 6, 2014

Monsoonal Change Is Underway

The North American Monsoon System plays a major role in the warm-season meteorology of the western United States.  The development and strength of the monsoon varies some from year to year, but this year, an early July transition is pretty apparent over Arizona.

Precipitation analyses for the month of June show very little precipitation over Arizona over the month of June, with just some spits of precipitation in spots, including the southeastern corner of the state.

In contrast, over the past seven days, most of Arizona has observed measurable precipitation. Local accumulations reach as high as 1.5 inches.

Temperature and dewpoint time series from Tucson also show the transition quite well.  Monsoon moisture surged into the region late on July 3rd, the last day that temperatures reached or exceeded 100ºF.  Since then, dewpoints have been higher and maximum temperatures lower.  Overnight last night the dewpoint briefly reached 70ºF.  But it's a dry heat right?

Source: MesoWest
One of the curious characteristics of the climate of southern Arizona, southern New Mexico, and western Texas is that it features the earliest "warmest day of the year" in the United States (thanks to one of our readers for sending me the map below a few days ago).  On average, the warmest day of the year in this region occurs in late June due to the transition from typically hot, dry conditions to cooler, more humid conditions following the monsoon onset, as illustrated nicely this year.  

Source: NCDC

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