Tuesday, July 11, 2017

Was Yesterday Really Cooler?

Chances are you thought that yesterday afternoon was comparatively pleasant compared to the previous several afternoons.  However, was yesterday really cooler than the previous several days?

Well, it depends.  If we're talking maximum temperature, it was cooler.  Yesterday's maximum temperature was 96ºF, the lowest maximum temperature since July 1st when we reached only 94ºF.  However, if we're talking minimum temperature, it was warmer.  Yesterday's minimum temperature was 79ºF, the highest minimum of the past 10 days.

Source: NOAA Regional Climate Centers
With a low of 79ºF and a high of 96ºF, yesterday's mean temperature was 87.5ºF, rating as the fourth highest of the month behind the 5th (90.5ºF), 8th (90.5ºF), and the 9th (88.5ºF).  So, from this perspective too, yesterday was pretty warm.  

You can blame all of this on the cloud cover, which acts to reduce the diurnal range of temperature, or the difference between the minimum and maximum temperatures.   Further, thanks to the cloud cover and higher humidities, swamp coolers were a bit less effective than we've seen thus far this summer.  

The lowest temperature to which air can be cooled by evaporation is known as the wet-bulb temperature.  Plotted below is the temperature and wet-bulb temperature at the Salt Lake City International Airport over the last 14 days.  Both exhibit daily ups and downs, reaching a maximum typically during the day and a minimum at night, although the wet-bulb is more damped. Yesterday, however, the wet-bulb temperature was nearly flatlined and remained near 65ºF overnight.  That's the highest sustained value we've seen probably all summer, although I haven't gone back through June to confirm this.  

Some people have asked me if this is the start of the monsoon.  That is a difficult question to answer.  "Monsoon" is often used to describe a seasonal reversal in the wind that is accompanied by rain.  In northern Mexico, for example, the flow during summer typically shifts from westerly to easterly, with a pronounced increase in precipitation.  For example, over Mexico, the climatological flow at 500 mb (about 18,000 feet) is westerly in May, but shifts to easterly in July.

Source: Douglas et al. (1993)
Source: Douglas et al. (1993)
This results in a very pronounced rainy season over northwest Mexico and portions of Arizona and New Mexico.

Monthly rainfall at selected sites. Source: Adams and Comrie (1997)
From the standpoint that we have had and will continue to have an upper-level ridge centered in the four-corners area and easterly large-scale easterly flow over Mexico and portions of the American Southwest, we are already "in" the monsoon.  Such a circulation is quite consistent with the North American Monsoon.  However, in such a pattern, moisture transport into northern Utah is quite fickle.  Really, northern Utah is in the monsoon surge region, meaning we need some help from a well positioned upper-level high and/or troughs in the easterlies to the south and westerlies to the north to draw in abundant monsoon moisture to give us wide spread showers and thunderstorms.  We really haven't had those key ingredients come together yet.  Thus, we've had some scattered thunderstorms, but nothing to truly soak the region and cool us down.

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