September Cool To Mid- September Warmth - Part II

Let's go back to the first 10 days of the month. Temperatures were below normal. We had a 5 day stretch of highs in the 60s/70s and lows in the 40s in spots. Then the pattern flipped. Starting on 9/11, the temperatures reached 80 degrees. The first of 12 straight days. 

To have 

Here is an animation of the upper level pattern starting September 6th.  Cool colors are low pressure.  Warmer colors are high pressure. Notice the cool colors over the Great Lakes at the start. See how the ridge of high pressure replaced it. It developed over western Canada, moved east and deepened between Hudson Bay and the Great Lakes. Low pressure stalled in the western US. Southern systems undercut the big ridge as indicated by the cool colors across southern states.

The overall temperatures vs normal starting September 6th:

The difference in average temperature from September 1-10 and September 11-20 was huge.  I compared each year to see where 2024's September temperature flip ranked.  Sure enough, the temperature change was the second biggest jump ever!

Here is the last 30 years for comparison. 1985 is on the left.  2024 is on the right.  2017 had the biggest jump.

The warmth (80 straight days with highs in the 80s) ended up lasting 12 days.  Here are the years that had similar stretches of September warmth.

What was unusual about this year's 80 degree stretch was the OVERALL lack of high humidity.

Here are the daily temperatures (line) in the years above during each stretch with the average dew point for each year:

You can better see the differences when you plot the daily dew points in each year. Notice that this year was lower overall.

Here is a plot of the daily dew points for each year in September since the early 1970s. Cool colors indicate comfortable dew points. Any day with a dew point UNDER 60 is shown with a blue color. Right side is the second half of the month. Notice the frequency of less humid days is much higher after September 15th (black line).

Are Days With Oppressively High Dew Points More Common Now?

In June, days with dew points higher than 70 were more common in the 1950s and the 1990s/early 2000s than in the last 15 years.

THROUGH JULY 27, 2017

In July (two graphics above) 12 years since the late 1940s had at least 15 days with 70 degree dew points. The peak in the data period was over a 25 year period from the late 1970s through the early 2000s. We have trended below normal since the early 2000s in August.

In August, spikes of 70 degree dew points in August in the 1980s through the early 2000s. Small peaks in the late 1950s, 1980s and late 1990s in September with a small peak in the last 7 years.

How about 80 degree dew points?

Its only happened three times since the late 1940s. Twice in 1995 and once in 1998