A Beastly Ridge

Forecasts for the next several days show the development of a strong ridge of upper-level high pressure over the north-central United States.  The GFS forecast valid 0000 UTC 14 July (1800 MDT Monday) shows 500-mb heights above the dreaded 6000-m level centered over Minnesota with anomalously high heights across most of the continental United States.  

Source: pivotalweather.com

This is a recipe for extreme heat to affect a good chunk of the continental United States, beginning in portions of the interior western US today and this weekend.  Currently, extreme heat warnings are in place for an area spanning from Utah to North Dakota and I suspect we will see warnings spreading farther east across the northern tier of the continental United States as the ridge fully develops. 

For northern Utah over the next few days I'll invoke my 20/20 rule here for recognizing temperature extremes.  The 20/20 rule says that anything above 20C at 700-mb (10,000 feet) is exceptionally warm, whereas anything below -20C is exceptionally cold.  In the Salt Lake City upper-air sounding record, 700-mb temperatures of +20C have been observed three times (soundings are only taken typically at most twice a day, so this is a somewhat unsampled record) on 13 July 2002, 14 July 2002, and 16 June 2021.  The high temperatures at the Salt Lake City International Airport on those days were 107F, 101F, and 97F, so the correlation between that level and the surface on any given day isn't perfect, but the 107F on 13 July 2002 is tied with four other days for the all-time record at the airport, the 14 July 2002 observation was preceded by a 107 on 13 July 2002, and the 16 June 2021 observation was preceded by a 107 on 107 on 15 June 2021.  So, all three times we reached +20C at 700 mb we tied the all-time high at the airport at some point during that heat wave. 

Yesterday afternoon we were only 14.4C at 700mb at the airport, so not in exceptional territory yet.  But let's look at the next few days.  I'll focus on GFS forecasts valid 1800 MDT, roughly the hottest part of the day.

1800 MDT Friday 10 July: 18C
1800 MDT Saturday 11 July: 20C
1800 MDT Sunday 12 July: 21C
1800 MDT Monday 13 July: 19C
1800 MDT Tuesday 14 July: 14 C

So the GFS is really pushing it into rare territory this weekend, especially on Sunday, before a cooling through Tuesday.  The ECMWF deterministic is also at 20–21C for Sunday afternoon. 

Whether or not Sunday can tie or break the all-time record at the airport will depend on factors such as cloud cover and local winds (those can make a difference of a few degrees in the maximum temperature, but the National Weather Service recognizes the potential for all-time records on Sunday, with guidance (forecasts from computer models and statistical approaches) putting the odds of meeting or exceeding the all-time high of 107 at the Salt Lake City airport at 30%.  

Their summary is below.  

This is all a reminder that July is a four-letter word.  

weather.utah.edu

The servers we use to produce graphics for weather.utah.edu and to allow access to the web site are really having a devil of a time right now.  The 700-mb temperatures for the GFS and ECMWF above were pulled by eye from pivotal weather as even I can't access that information on our system at the moment.  I apologize for all of the problems with the web site.