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Most Recent Forecast Calls for Drier-Than-Average Monsoon

August 2, 2010
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Crackling thunder and pounding rain finally pelted my tin roof in downtown Tucson for the first time this summer on July 21, more than two weeks after the historical start date of the monsoon season in Tucson. The slow beginning to the season begged the question, Why?

I recently spoke with Bob Maddox to get his take on the first half of the monsoon season. Maddox is a retired meteorologist who was formerly the director of the National Oceanic and Atmospheric Administration’s (NOAA) National Severe Storms Laboratory and actively blogs on daily weather events in Tucson. In his words, the monsoon to date has been characterized by weak winds aloft and a more stable atmosphere. This means that clouds have not frequently been blown off the mountains into the valleys, and even when they have, the stable atmosphere has prevented moisture over the valleys from rising, condensing, and causing the thunderous storms for which the monsoon is famous.

Maddox could not speculate on whether this pattern would continue into August. The NOAA Climate Prediction Center (CPC), however, is in the business of forecasting. Their most recent seasonal outlook calls below-average precipitation for the August–October period (Figure 1). This is a slight downgrade in monsoon precipitation from outlooks issued one month ago—in June the CPC stated that precipitation during the July–September period had equal chances of being above-, below-, or near-average. To understand why the CPC changed their forecast to a drier outlook, I emailed Dan Collins who authored the most recent seasonal forecasts. Dan highlighted four reasons for increased chances that the second half of the monsoon season will be drier than average.

<i>Figure 1:<i/> August-October precipitation forecast from NOAA-CPC (Issued July 15).

 

  • The North American Monsoon (NAM) has been less active than average as of July 14—the publication date of the most recent seasonal forecasts—and short-range models indicate that these conditions will continue.
  • Historically, conditions tend to be drier than average when La Niña events are present during August–October periods, which is currently the case.
  • The equal chances forecast issued in June for the monsoon period was based in part on not knowing how long the dissipated El Niño event would impact the atmosphere. By July, forecasters better understood that the El Niño would not have lingering effect on summer rain.
  • There has been a drying trend during the August–October period during the last 15–20 years in southern parts of the NAM region and the Great Basin.

 

 

If this outlook holds up, the Southwest could experience expansions in drought conditions. To make matters worse, the La Niña event, which has gained a firm foothold in the tropical Pacific Ocean, will likely accentuate the dry conditions because La Niña events tend to decrease in winter precipitation.

Interesting to hear that La

Interesting to hear that La Nina conditions portend a drier monsoon in August, but I'm a bit puzzled.  Wasn't the development of El Nino conditions last summer also responsible for the dry monsoon?  Or is this Aug-Oct outlook less about the monsoon and more about diminished fall tropical storms in the Pacific (which can, of course, impact the end of the monsoon)?

That's a good question. My

That's a good question. My understanding of the forecast is that El Nino affects the tail end of the monsoon season via reducing the number of tropical storms which sometimes waft into the Southwest and deliver copious rains. The forecast for August issued by the NOAA-Climate Predictions Center (CPC) in mid-July states that there are equal chances for precipitation to be above, below, or near-normal. Therefore, I don't think the emerging El Nino has a significant impact on rainfall in August. Just to be sure, I have sent an email to the lead forecaster for the CPC to get their take on your question, and I will post the response as soon as I receive it.

The CPC forecaster who issued

The CPC forecaster who issued the August–October season forecast last month, Dan Collins, had this to say about your question in an email: "Though August may lean slightly towards dry in a La Nina [year], the signal is very weak.  The impact of La Nina on fall tropical storms in the Eastern Pacific is where most of the signal is found... the [August–Ocober] forecast reflect[s] this."

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