What the dew points say about Monsoon 2017

While the first week of July historically is the true onset of our Summer storm season, 2017’s version of the Monsoon have been abnormally quiet.

As of 2008, the Monsoon is defined with a start date of June 15th, ending September 30th. Prior to 2008, the National Weather Service (NWS) determined the Monsoon start date when Tucson’s average daily dew point was 54°+ for three straight days. The start date would then be the first of the 3 consecutive days.

What do the dew points tell us about Monsoon 2017? As of July 7th, Tucson has failed to record even one day with an average dew point of 54°.

For perspective, the average Monsoon start date using the dew point method is July 3rd. Based on dew point data, the Monsoon started on June 25th the last two years. The latest Monsoon start date for Tucson using the dew point data was July 25th, back in 1987.

Why did NWS abandon the dew point method? For starters, public safety. It allows NWS & its media partners to start getting people in the Monsoon mindset prior to the true onset of storms.

Weather record keeping is another reason for the shift to defined dates for Monsoon. By using June 15th through September 30th, NWS and other meteorologists can have a better apples to apples comparison of each Summer storm season.

Defined dates eliminate the dew point method’s major flaw – false starts to the Monsoon. There were several years where daily dew points would fall below the 54° mark after three straight days. This flaw made weather record keeping tedious.

Finally, the three days of dew points at or above 54° was a good measuring stick for Tucson. However, it was not representative for the rest of Southern Arizona. Places like Nogales, Sierra Vista and Douglas usually have much higher dew points during Monsoon. Thus, the threshold for the dew point method would need to be altered to determine the start in these towns.


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