Is This late Summer Cool Period Unusual?

Summer is ending on a cool note across northern Ohio and surrounding states after a very humid start to summer through July (see previous post).  August ended up being far less humid across the Ohio Valley vs the June/July period. It was still very humid especially the first half of the month but not historic.

June and July was the most humid on record for northern Ohio. Yet the June through August period ended up being the 6th most humid in northern Ohio.

Between August 20th and September 9, much of the eastern half of the US has been well below normal.

Since August 15, we've only had six days at/above 80 degrees. Five of those occurred before August 23rd. The last time we had six or less days at/above 80 degrees between August 15 and September 10 was in 1997!

When was the last time we had overall average temperatures this cool this time of year?  How about 43 years ago...1982!

Notice how quickly our temperature fell after August 15th.

On top of the cooler pattern, rainfall is also well below normal after a very wet spring and first half of summer.

A large portion of Ohio is in a slight/moderate drought. Interestingly, compare this late summer/early fall to 2024 and you can see drought similarities.

Most of the Ohio Valley is running more than 4 inches below normal since mid August.

What does all of this mean for the rest of fall and into winter 2025-26?  Details in the weeks ahead.

What is A Drought? How Does Summer 2024 Compare to Others?

Rainfall over the last 24 hours as of this writing (August 3) has been the most widespread in quite some time across Ohio and surrounding states. We needed it!

Yet each summer, it seems as if we go through the same process:  Rain at the beginning in May and early June with a gradual drying out with limited rain across northern Ohio by late June/July and into August. The grass gets brown. We mow every two weeks or longer versus every other day in early May. Most of us will mix in the word "drought" in  everyday conversation describing the conditions as we exclaim "this happens every summer."  At least that's how we perceive it. But is this really the case?  What does the data show?

First what is the definition of a "drought"?

Drought is generally defined as "a deficiency of precipitation over an extended period of time" resulting in a water shortage. This is per the National Integrated Drought Information System on drought.gov (a great resource). However drought can mean different things to different regions. If you are a farmer, a drought means below normal rainfall resulting in damage to crops. So not enough water means low crop yield for the season. 

Scientifically, drought can be measured through comparing precipitation departures over a period of time along with soil moisture, evaporation, temperatures among others.  A great way to compare year to year is to use the US Drought Monitor.  This is a blend of around a dozen different variables including the ones listed above. This is combined along with potential drought impacts and other expert insight. 

The US Drought Monitor index is on a scale of zero to four. One being low on the drought scale up to four meaning highly extreme drought conditions as defined above.

Just looking at precipitation changes since April you can see how the conditions in parts of Ohio/southern Ohio, the Ohio Valley and into the Carolinas/SE US have dried out significantly.

Interestingly, the corn belt and most of the Great Lakes, conditions have not been dry at all. In fact, soil moisture has been well above normal.

Back to the original question. What about the comment that we are always in a drought every summer here in northern Ohio.  Is this true?

Look at the Drought Monitor history for Ohio as a whole. Here are the years since 2000 (data only goes back that far) with portions of the state in at least a D2 drought with areal percentages.  The last time ANY part of Ohio was under a D2 drought was 2016.

Can we go back further?  Absolutely, but we need to use a different set of data.  It's called the Palmer Drought Severity Index. This data goes back more than 100 years. The PDSI is good at determining long term drought using temperature and precipitation data to estimate dryness.  So a +4 shows extremely wet conditions. A -4 is extremely dry. 

Here are the PDSI conditions across the US as of late July:

Notice that the conditions across Ohio are not as severe when looking at the LONG TERM drought conditions.

Let's go back in time just looking at the summer months of June, July and August combined. To do this, we can examine specific climate divisions versus state wide conditions. Here in northern Ohio, we have four climate divisions that encompasses our viewing area. Two, three, six and seven.

I highlighted some of the memorable drought summers for Division two. The dust bowl years of the 1930s, the early 1950s and 1960s were long periods with summer drought. Of course 1988, 1991 and the early 2000s also featured very dry summers.

Here is climate division 3:

Climate division 6:

Climate division 7:

As you can see, there are much drier periods over the last 100 years across northern Ohio.  There are also (green bars on these charts) with above normal summer wet conditions.  In fact in the last 30 years, the summers with above normal wet conditions have outnumbered the abnormally dry ones. 

Number of Abnormally Dry summers (JJA) Since 1995

Climate Division 2:  9 of 30 summers

Climate Division 3:  12 of 30 summers

Climate Division 6:  9 of 30 summers

Climate Division 7:  9 of 30 summers

While our lawns dry out each summer to some degree, is it true that every summer or even most summers in northern Ohio we end up with drought conditions?  Not even close

2016 Drought Conditions vs 2012

The dry conditions in northern Ohio are getting worse as the daytime temperatures reach the lower 90s coupled with higher humidity levels--heat index between 100 and 105--this weekend.  Many people are comparing summer to 2012 both in temperature and in degree of dryness. Is it a valid comparison? How do our dry conditions compare to other years in mid July?  When was the last time we had conditions this dry in mid July in northern Ohio?

The index that's used in determining the degree of dryness or wetness historically is the Palmer Drought Severity Index. It gives us a good barometer in comparing drought events of the past.

According to the National Drought Mitigation Center:  "The PDSI is calculated based on precipitation and temperature data, as well as the local available water content (AWC) of the soil. (Link here) From the inputs, all the basic terms of the water balance equation can be determined, including evapotranspiration, soil recharge, runoff, and moisture loss from the surface layer." 

The more negative the number, the more severe the drought.  Whereas higher numbers indicate wetter conditions. Each week the Climate Prediction Center updates the drought conditions nationwide for each climate division. I circled Ohio for reference.

Based on the PDSI, this summer shows little similarities to the summer of 2012 nationally. The PDSI is falling to 2012 levels in some spots in northern Ohio but the overall degree of dryness in 2012 was far more severe. You could argue that the summer of 2007 is a closer match to 2016. I threw in the Super El Nino summer of 1998 at the end since we are at the end of the Super El Nino of 2015-16.

2016

2015

2012

2007

1998

What Is Driving This Cool Pattern? 2014 vs 2009

This unseasonably cool pattern across the Great Lakes and upper mid-west hasn't been completely unexpected. Back in April and early May, we talked about how this summer would feature periods where temperatures would drop below normal. Take a look at the 12 day stretch from July 15th to July 26th. The area of the US that experienced below normal temperatures this year is exactly the same as 2009.

2009 was one of our years we used as a guide in formulating our summer outlook. Remember, this does not include this week's cool-down. I'll redraw this anomaly map at the end of this week.

The upper level (18,000 foot) trough is a bit stronger this year but in the same location as 2009.

Notice where the ridge (high pressure) is located: The western US where the present drought is the most extreme. Drought drives temperatures higher due to the dry ground. The dry ground and heat creates a high pressure ridge. This ridge keeps the dry conditions going further which drives the temperatures higher....and on and on. 

Water temperatures in the Gulf of Alaska are off-the-charts above average this summer (as it was last winter). 

The above normal Gulf of Alaska water temperatures have enhanced the western high pressure ridge by pushing the jet stream further north. The movement of the jet stream is indicated with the red arrow. The strongest portion of the jet stream (highest winds) is shown with the warmer colors over Alaska. The jet on the east side of the ridge dives southeast in the center of the country and bottoms out over the Ohio Valley where the trough is located.

The PNA index is used to measure these pressure patterns across the Northern Hemisphere. When the PNA is positive, the ridge is very strong out west and the trough/low pressure is strong east as it has been since last winter. You can read about it more HERE. 

The southeasterly jet becomes the conveyor belt for frequent rain and thunder events leading to above normal rainfall across much of the Great Lakes, Midwest and Ohio Valley. This also leads to more frequent periods of below normal temperatures.

For perspective, July in Cleveland, Ohio finished the 6th coolest since 1900.

The average high temperature in Cleveland from July 15th to July 31st was the the second coolest on record!

So the warmer Gulf of Alaska water and the drought out west have significantly boosted the high pressure ridge out west which is reinforcing the trough/cool pattern further east across the midwest and Ohio Valley.  This is not a complete list of the drivers of this summer pattern. The weak El Nino, which garnered a lot of attention during the spring, has had an influence too but not to a major degree. The weak El Nino will factor into our winter outlook which I'll talk about in the fall. Let me know of any other major contributors that I've missed.

Bottom line, it doesn't look like this pattern is going to change anytime soon!