Thursday, October 01, 2015

When Did Summer Monsoon Really End?

Russ Scott emailed me last week and asked when I thought the monsoon ended. I ended up replying that it didn't seem very clear to me. I was inspired to take a closer look when I noted one of the headlines in this morning's newspaper:

Dry, triple-digit day marks end of monsoon - The official monsoon “season” ended Wednesday with no rain for the day and a high temperature of 101 degrees.
I have just parsed through all of the skew-T plots for the upper-air soundings taken at TWC for June through September. Russ's question remains very difficult to answer. 

The only thing that ended yesterday was the arbitrary, calendar-based definition of the "monsoon season" that was adopted by the Arizona NWS Forecast Offices several years ago. The definition has nothing to do with the actual day-to-day, large-scale circulation patterns affecting our part of the state.

Although PW values remained high for much of the summer (thanks to the influences of several named tropical storms/hurricanes) the tropospheric wind profiles were highly variable through the summer. If one considers a wind profile that varies from east-southeast at lower levels to south-southwest at upper-levels typical for the summer monsoon circulation, then there were surprisingly few days with "typical" winds aloft.

Here are some sounding plots (from the University of Wyoming upper-air page) for the last part of the summer. The sounding above for August 16th has high PW but northerly to northwesterly winds through the troposphere. This was the first day of an extended period with westerly to northwesterly winds aloft and would be my candidate date for the end of the monsoon - based on circulation patterns. 

The sounding below, from near the end of August, shows a situation with almost no winds through the entire troposphere - a situation that seemed to plague us (especially at 500 mb and below) for an unusually large number of days.

The sounding at the bottom is from early September, when light winds below 500 mb were overlain by very strong, baroclinic southwesterly winds - a hybrid (or mixed-mode) situation of which there were quite a few - note the unusually strong upper-level jet with speeds of almost 100 kts. 

So, my subjective summary is that it was an usual summer with many days of hybrid circulation patterns accompanied by PW amounts that stayed high through much of the summer. Here at the house I noted the first summer thunderstorm on June 24 with the first measurable rain the next day during a brief, severe thunderstorm. (thanks to early moisture associated with Hurricane Blanca). The last rains and storms of September (21st to 23rd) were associated with the remnants of TD-16E, interacting with an upper-tropospheric trough in the westerlies.

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