Jarring temperature pullback in Chicago’s future second half of this week; jet stream shift to produce showery shift from summer warmth to an autumn chill



  • Phoenix, AZ baked by the greatest number of 110+ degree days on record—54 of them—has closed the books on its DRIEST MONSOON SEASON on record. The just completed summer saw half the next driest monsoon season which occurred nearly a century ago in 1924—0.15″ versus 0.36.” This came after welcome rains and snows swept California and the Southwest last winter—a development which was welcomed as its reservoirs, critical to the regions tens of millions residents, fell to worrisome record low levels. Last winter’s precipitation brought at least temporary relief but will the rains and snows keep coming in the cooler seasons to come?
  • Monsoons are big deals in the Southwest. They typically deliver 40 to 75% of the region’s annual precipitation.
  • Tropical Storm Hilary brought needed rains—but also flooding—to California, Nevada and extreme western Arizona north into Utah, Idaho and Montana.The summer season’s monsoon was disappointingly farther east.
  • El Niño conditions in the equatorial region are strengthening and past El Niño cold seasons have hosted regular storms which bring moisture to the region. Not all El Niños are alike—so you can bet precipitation trends over the coming fall and winter will be monitored closely.

September 29, 2023, Atlantic

  • The reference of course is in reference to the latest cloudburst last weekend and resulting horrific flooding covered in the Climate Nexus newsletter. These extreme rain events are occurring with increasing frequency. No surprise as our atmosphere and the planet’s oceans continue to warm. Here’s the Climate Nexus post Monday morning:
  • “NYC’s Latest Deluge Dropped a Month’s Worth of Rain in Just Hours: New York City came to a standstill last week after intense rainfall caused flash flooding across much of the city. Almost 8 inches of rain fell on JFK Airport on Friday, the most rainfall in one day since record-keeping began in 1948, while CNN reports that Brooklyn got a month’s worth of rain in just a few hours. Much of the city’s subway services were suspended, leaving the 2.4 million daily riders across the five boroughs stranded during morning commutes; while firefighters performed some rescues of people in basement apartments, no fatalities have yet been reported. More intense storms and higher rainfall totals are a sign of climate change, as warmer air can hold more moisture. Some important provisions that would keep New Yorkers living in illegal basement apartments safe—set in motion after 11 New Yorkers drowned during Hurricane Ida in 2021, trapped in their own homes—got stuck in the NY state Assembly earlier this year, and advocates say more needs to be done to keep the city safe during flash floods. Louise Yeung, the comptroller’s chief climate officer, told MSNBC in an interview that “Heavy rainstorms like the one we are seeing today are becoming our new normal as climate change intensifies…We are not fixing things at the pace our climate is changing and that’s going to continue to be a challenge every time we get one of these rain storms or hurricanes.”


  • Watch the spread of chilly nighttime lows into the nation’s mid-section between Tuesday morning and Sunday morning. Weekend lows will drop to the 40s Friday night, Saturday night and Sunday, and it wouldn’t be completely surprising to see a few 30s in the colder locations away from Chicago and Lake Michigan in the mornings this weekend.
  • Note too the upper air cold air pool forecast to set up over Chicago and the Midwest by Friday. Such cold pools aloft are often the harbinger of unsettled weather, i.e. lots of clouds and scattered instability showers.


  • The HUGE DOME OF WARM AIR which has delivered such beautiful weather to Chicago and the Midwest over the weekend—warm weather which has another two days to run—is shown breaking down on this animated UPPER AIR FORECAST off the National Weather Service’s GFS model.
  • What you are seeing here is a predicted sea change in the North American upper air pattern as a deep upper air tough develops over the nation’s midsection between NOW (Monday evening) and this coming weekend.
  • THE PATTERN CHANGE is to deliver Chicago’s coolest weather in nearly 5 months by Friday and the weekend with the 80s predicted Tuesday and Wednesday to crash to 60 Friday, 56 Saturday and 59 Sunday.


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