Tuesday, July 24, 2018

A Beastly July Ridge Dominating Southwest Weather

Take a gander at the GFS 10-day forecast below, especially the top frame, which illustrates the evolution of the winds on the dynamic tropopause (jet-stream level).  Note the slow westward movement, or retrogression, of a very strong upper-level ridge across the southwest US and northern Mexico during the period. 

The intensity of this ridge over the southwest US and northern Mexico is quite unusual for this time of year.  For example, the NAEFS mean 500-mb height forecast for 1800 UTC today shows values that are outside climatological values for the 1979-2009 period.  In other words, we are dealing with unusually strong ridging at those latitudes for July.

Source: NWS
In general, people associate high pressure with fair weather, but the #1 weather-related killer nationally is heat and a strong ridge this time of year over the southwest makes for extreme heat.   As can be seen in the NWS forecast below, excessive heat warnings extend across much of southern Arizona and California, and up the lower Colorado River Valley to portions of Nevada and southern Utah. 

Source: NWS
I'll provide the NWS infographic below as an example of forecasts for today's max temperatures, which include everything from 123˚F in Thermal to 96˚F in Los Angeles. 

Source: NWS
With the ridge moving so slowly, this is going to be a multiday event.  Riverside, for example, is currently 102˚F (1:15 PM PDT), with a high expected to be near 110˚F today, tomorrow, and Thursday.  Riverside is inland enough that it is typically warmer than Los Angeles, but still, the average high this time of year is about 94˚F, comparable to the average for Salt Lake City.  

Source: NWS
We should all feel fortunate that we're on the periphery of this ridge and only running in the mid 90s here in Salt Lake.  

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