Tucson's weather this winter will be exceptionally boring—one beautiful sunny day after another—thanks to a La Niña that is as certain as a Joe Biden win, according to outlooks from the Climate Prediction Center and The Economist online. Hopefully we'll get a few inches of rain this winter, not too far below normal. But that likelihood will be only half of what Tucson received in each of the last two winters.
So I'll be spending this winter revisiting the few exciting monsoon days from this past summer, and preparing and hoping for more of those days next summer. My most recent previous post here was on July 8. Three days later on Saturday, July 11, the monsoon made a dramatic appearance around sunset. A line of thunderstorms moved from east to west and resulted in several reports of high winds and damage in the Tucson area, including a 60 mph wind gust at Davis-Monthan Air Force Base. But the days immediately before and after July 11 had a similar weather pattern, and those days were just hot.
The selection of a "forecast problem of the day" as practiced by the National Weather Service amounts to wearing blinders. Typically the problem stated in a NWS discussion is confined to one element, for example "will the high temperature be a degree above or below the record" or "will there be damaging thunderstorm winds?" My gripe is not just that wearing blinders is a poor practice for a forecaster analyzing and monitoring the weather situation, it's that the fixation on a single element carries over to the public forecast, a communications disservice. The problem selected for attention on most summer days should be the underlying feature affecting multiple elements, "what is/will be happening with the cap?"
The cap of concern in the Southwest summer sits around 400-500 mb, as noted in the previous post here, or in terms of dry isentropic levels around 326-336 K. It persists most of the summer, yet can vary—in thickness and/or in height—from day to day and even hour to hour, and the variation can be on small as well as large horizontal scales. Let that stable layer subside several hundred meters, with mixed layer moisture overshooting into dry air at the base of the cap, and suddenly there is space for a record high temperature. Conversely lift that layer several hundred meters, and suddenly thunderstorms. Over the next several months I'll be examining available products that might help with what should be the mantra, "Analyze the cap."
