Saturday, July 01, 2006

Monsoon "officially" arrives at Tucson

The 2006 monsoon has "officially" arrived at the NWS surface observation site at Tucson International Airport (TUS) as of Wednesday 28 June. Has it arrived at other places in southeastern Arizona, and if so when? There is no clear answer to that question, assuming anyone is actually interested. I can not resist pointing out several things.

Using the NWS Tucson definition for the official start of the monsoon, it appears that the monsoon arrived at Davis-Monthan Air Force Base one day earlier than it did at TUS. These two surface observing stations are less than 10 km apart.

If one used other definitions of the "start" of the monsoon, the dates of arrival would vary considerably.

For example, if the start at TUS were defined as the first time that the running three-day average surface dewpoint temperature exceed 50F, then the monsoon would have begun on 6 June.

If we defined the start of the monsoon in Arizona as the first day on which the count of cloud-to-ground lightning strikes across Arizona exceeded 10,000 (a fairly significant number of strikes!), then the 2006 monsoon would have begun on 8 June.

Enough said about the Arizona monsoon conundrum for summer 2006!


  1. Anonymous5:30 PM

    It seems widely agreed that no clearly definable ‘start time’ exists for the monsoon season in the southwest U.S. And, further, even if a unique starting date could be identified it is also widely acknowledged that the monsoon pattern is irregular rather than steady. Despite this fuzziness, the public (and professional meteorologists) would like some guidance as to whether the rains associated with the summer season have gotten underway in earnest or whether only a ‘false start’ has been experienced. This question is likely to arise at the beginning of each monsoon season.
    A useful criterion for selecting a ‘start date’ should not be triggered at the first sign of a shift in the mid-level winds to southeasterly or southerly. Instead, one would want the monsoon pattern to have been established for some number of days. Secondly, although strict usage of the term monsoon involves only wind pattern, I think it is fair to say that relatively few people would be concerned about a seasonal wind shift that didn’t bring with it a change in sensible weather. Thus, a useful start date criterion should, in addition to a seasonal wind shift, require a significant shift in atmospheric moisture content.
    There seems to be wide criticism of current dew-point criteria [e.g., 3 consecutive days in which the dew point averages 55F or greater at Phoenix, etc.] because they represent arbitrary step function thresholds and because they do not involve wind ---and wind, after all, is what the monsoon is all about. But, I would suggest that this type of criterion may have greater merit than generally recognized. First, it is desirable that the ‘start’ criterion have a threshold that is not immediately triggered upon the first appearance of a seasonal wind shift or scattered thunderstorms. But, of course, neither should the criterion be so difficult to trigger that the rainy season is well underway before it is attained. This 3-day dew point criterion seems to satisfy this requirement. Secondly, although dew point has no direct dependence upon wind direction, it is certainly fair to say that the dew points in Phoenix will not average 55F unless the summer monsoon wind pattern has been established. That is, a shift in the seasonal wind direction over an extended period of time is implicitly integrated into the dew point criterion. Finally, unlike a strictly wind-based criterion, which would say nothing about moisture, one can be relatively confident that there is sufficient moisture to trigger convection with the dew point criterion.
    I am not saying, nor do I mean to imply, that a better, more scientifically sound, criterion can’t or shouldn’t be developed for the start of the summer monsoon. However, despite its simplicity, I do believe that some of the merits of the current technique outlined above may deserve a second look.

  2. Per the start of the monsoon, I have put my 2cents worth under the blog for July 2nd with a possible new type of definition that would use lightning detection data and radar rainfall estimates.