Monsoon 2013

Monsoon's on the way. Really.

It will rain soon, but when and how much is up in the air
(Headline in today's Arizona Daily Star - below are several confusing statements from the article.)

It's currently dry and hot to the north of us. That makes it easier for the high pressure system now parked over us, and delivering 100-degree-plus days, to move north as well, creating the vacuum that brings moist air from the Gulf of Mexico and Mexico's Sierra Madre.

 All I can say is: Huh?

Those conditions are similar to last year, when the area received its saturating moisture on June 24,(SIC) 

I think this sentence meant to state that the first significant rains last summer fell on June 24th ~ but that is not correct - see below.

... previous research in Colorado examined a link between the aromatic compounds emitted by pine needles and cloud formation. That continuing study indicates these terpenes, which leak out more quickly on hot days, "could have a significant effect on producing and thickening clouds and producing wetter storms," 

Again, all I can say is: Huh?

Several years ago the NWS decided to define a monsoon season that would run from June 15th to September 30th. This replaced the confusing definition based on average daily dewpoint temperatures. However, there is not much information available explaining exactly what "monsoon season" means. Nothing on the NWS Tucson webpage that I can find and only the statement below from the NWS Phoenix webpage.

Our "monsoon season" officially extends from June 15th through September 30th, the time during which associated hazards are most likely to occur. 

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Above is a figure I generated to take a look at the how the monsoon progressed during 2012. The integrated precipitable water (IPW) values are those measured from U of A GPS sensor data. I've highlighted in green values above 30 mm. We assume that days with values above 30 mm indicate the presence of maritime tropical (mT) air in low-levels at Tucson (i.e., the airmass that characterizes the monsoon, or summer thunderstorm season),  Zeng and Lu (2004) proposed this value of IPW as the lower threshold value used to compute a global monsoon index.

The plot above (top portion) shows clearly that the >30 mm monsoon period began on 24 June and continued until about September 13th. The intermittent periods that have IPW above 40 mm are associated with the most significant summer rains in southeast Arizona.

The lower part of the figure shows the period from June 20th to July 10th. The black numbers above the dates are a raininess index for eastern Pima County defined as the percentage of ALERT network stations that report measurable rainfall during 24-hour periods ending at 6 am the next morning. The rainfalls in June were mostly very light and affected only the far south and southwest parts of the network. The first widespread and important rainfalls occurred with the storms of July 3-4, associated with the first incursion of > 40 mm mT air into the Tucson area.

As a check on how things stand today (9 June 2013), the figure below shows 21 UTC IPW from CIRA at Colorado State Univ. - the area along the southern coast of the GoC (bright green) currently has IPW of 30 mm or a bit higher.

Reference: Zeng, X., and E. Lu, 2004: Globally unified monsoon onset and retreat indexes. J. Climate, 17, 2241-2248.