Rainfall Rates, Atmospheric Moisture & Humidity - Part IV

In the previous three posts, I examined WHY it was so dry and warm, the temperatures during and prior and the lack of rain mid month. This post looks at the influence the Rex Block had on the amount of moisture in the atmosphere and the rainfall rates as systems approached from the west and the two tropical system from the south as well as the overall moisture in the atmosphere.

Precipitable Water (PW) is a measurement of the amount of moisture in the atmosphere at a specific location. Roughly, anything under 1" is considered a low amount. Above 1" and especially 1.5 to 2" is considered moderate to high. We use PW to determine whether or not the precipitation event will be significant . The high the PW number, the stronger the downpours.

Here is the animation of PW from September 6 to September 21. Cool colors are very low PW values and warmer colors are high PW values. Focus on the Ohio Valley. See how the ridge (Rex Block) suppressed higher atmospheric moisture movement from the south even as the remains of Francine moved inland. 

A close up of the rainfall rates told a similar story. Warm colors indicate low rainfall rates. Cool colors indicate above normal rainfall rates. Watch the two tropical system head north and either get deflected west and/or fall apart. It was around the 21st when the ridge started to weaken allowing a front to pass from the west. (See purple colors)

Rainfall amounts were only in areas where the Rex Block did not have a pronounced effect.

Humidity levels across northern Ohio were relatively low compared to the overall temperatures especially when compared to other long stretches of 80 degree days in September.

Cloud cover across northern Ohio was well below normal during this time.

What's interesting about this is that the frequency of clear skies with temperatures 80+ is barely 20% in September?

Last year's cloud cover (September)

2022 September cloud cover

2021 September cloud cover

2020 September cloud cover

Why Was It So Dry and Warm? Part III

The answer has many elements. We'll work from the more direct and near-by causes to the antecedent ones.

The pattern across North America was dominated by a "Rex Block" named after Meteorologist Daniel Rex in 1950 who discovered it. You can read about it HERE.

The configuration resembles a backward "S" with high pressure north of low pressure.  These are more common in the western US then over the Great Lakes. The result was a dominant easterly/SE flow which kept the temperatures well above normal not only across Ohio but much of the Great Lakes and Midwest with little rainfall. READ PART I HERE and  READ PART II HERE

Why did this Rex Block form?  I believe the answer lies first in the changing Equatorial Pacific. We have been teetering with a La Nina since last winter/spring.  Most of the summer has been in neutral territory. In the last month, the upwelling of cooler water to the surface is lowing sea surface temperatures especially west.

The pressure changes between Darwin station and Tahiti have been quick.

SOI changes have gone from positive to negative in recent weeks.

Further north in the higher latitudes, the North Atlantic Oscillation also went through changes at roughly the same time. Notice how the jet stream across North America and Europe was strong. Follow the black line across the US, northern Atlantic and Europe.

HUGE low pressure north of the Arctic circle and northern Atlantic with high pressure over Canada and eastern Europe.

As the NAO went negative, the jet stream shifted into southern US and Mexico and east into northern Africa. Little jet stream influence across Canada and the northern Atlantic.

HUGE high pressure ridge developed in eastern Canada north into the Arctic. This extended into the northern Atlantic and into northern Europe.

These changes in the tropics as well as the pattern shift in the higher latitudes among others all contributed to the changes across North America in early/mid September.

What about other years that had similar long stretches of 80+ degree warmth?

Here were the patterns across the US/Canada in east instance: Notice in most instances, the configuration is very similar.

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).

Our First Rain in 17 Days! - Part I

The first rain since September 6th moved through late yesterday and earlier today. 

Radar loop above is from 5PM Sunday, September 22 through 12:30PM Monday September 23.

The top amounts were around 1/4" with most well under 0.1".  A small bulls-eye in southern Cuyahoga county from late evening showers on the 22nd.

Here is the total rainfall between September 7 and September 21.

This was the NE Ohio rainfall on September 6th. I superimposed the drought monitor information over the rainfall contour for reference.

Rainfall vs normal between September 7th and the 21st:

Midwest was just as dry.

Even if you extend this map out to early August, rainfall has been well below normal.

Early summer (June 1 to Auguts 1) was well above normal in the midwest/Central US. Ohio was overall below normal since June 1.

The frequency of storms was also way down over the last 3 weeks. Lightning strikes were scarce across northern Ohio.

Last year was September storms were a bit more prevalent.