Is Late August/Early September Heat More Frequent Nowadays?

Today the temperatures reached the lower and mid 90s across not only Ohio but much of the lower Great Lakes and Ohio Valley. The question of whether or not late season heat is becoming more frequent in northern Ohio always surfaces during heat like this.  

High temperatures August 27th:

Heat Index reading on August 27th:

So how do we answer this question?  First what time period are we looking at when we say "late August/early September"?  I like to use the last week of August and first week of September. A fourteen day period that is a decent duration sample size.

August 25 to September 7

What about temperature?  First, let's take look how many instances each year the high temperatures reach 90 degrees or higher during this 14 day period? (Given the forecast we could conceivable hit 90° four times by the end of the week).

Contrary to some of our recollections, its been 6 years since we had a late August stretch of 90+ similar to this year.  This actually occurred the most frequently in the 1950s. The early 2000s are a close second with high occurrences in 2010, 2012, 2016 and 2018.  With the exception of a few years like 1973, 1983, 1991 and 1993 it rarely happened in the 1960s, 70s, 80s and 1990s.  

What was interesting about the late August/early September of 1953 period was that we were above 90 degrees eleven of the fourteen days. 

The top number is the high temperature. The bottom number is the highest heat index that day.  Notice the extreme humidity didn't accompany the heat initially. In fact, dew points were very dry the first 6 days of that stretch from August 25 through the 30th. It wasn't until later in the period -- day 7 of the 11 day stretch -- that the heat index rose above 100° for 4 straight days.

Now compare 1953 and its 10 days above 90° to the 7 day stretch in 2018.

What about the overall average high temperatures during this 14 day period.  Are there any changes locally?  Here is a look each year since 1980. 2016 and 2018 were well above normal in high temperatures as was 2010 and 2012.  

Were these warm, late season stretches just relegated to northern Ohio?

Notice that most of these late season hot periods were centered over the Great Lakes, Ohio Valley and portions of New England and eastern Canada while the western US experiences well below normal temperatures.  Below are the temperatures above normal (warm colors) and below normal (cool colors) across the US and Canada.

The bottom line is that long hot stretches don't happen every year in late August/early September like many remember. 

  

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

Where Does This Recent Tropically Humid Stretch Rank?

Our recent stretch of 90+ degree heat was accompanied by high humidity.  It doesn't always work out that way but in most instances here in northern Ohio and surrounding states, 90 degree heat and high humidity go hand and hand.

Where does last week's summer humidity rank?

Before we answer this, let's set some ground rules.  Since last week's hot and humid stretch was 7 days total (90+ degrees with a dew point of at least 70), I initially used this as the benchmark then dropped it to 6 days in length. 

After downloading the average dew point for EACH DAY since 1950, sorting them only highlighting dew points at/above 70 degrees (considered tropical), here is the list of the most humid 6+ day stretches since 1950:

So where was this past week's high humidity ranked?  TIED FOR 16TH HIGHEST among all 6+ consecutive day stretches with dew points at/above 70.

The last time we had a stretch like last week was in the summer of 2022!

Late June Heat...For How Long?

Our first heat wave in northern Ohio since 2021 continues. The long range outlook picked up on this pattern change a few weeks prior to the start.

June started out dry but with "cooler" daytime highs in northern Ohio. June 10 was 61 degrees with low 40s for overnight lows. Then a two day "teaser period" of warmth then the first of a weeks worth of 90+ degrees starting on June 16.

You can see how the high pressure ridge (Bermuda High) expanded across the eastern seaboard to near record high levels starting Monday June 17. By the weekend, the heat ridge (dome) gradually dissipated as the ridge flattened in the deep south.

The position of the ridge east of Ohio allowed a few "weak fronts" (disturbances) to clip the southern Great Lakes with small but potent storms with heavy rainfall. Future radar from Thursday June 20 to Saturday June 22.

Rainfall totals from June 16 to June 19:

Heat Advisory was issued from Monday through Saturday across northern Ohio.

Heat Advisories and Excessive Heat Warnings extended into Michigan and New England.

Tuesday high temperatures were running well above normal eastern US with well below normal temperatures across the west.

Northern Ohio heat index forecast was showing 100+ in spots where storms didn't develop.

The last time we had a heat index 100+ was in 2022. We measure the heat index in total hours.  Here are the totals since 2012 with the warm summers of 1995 and 1988 for reference.

Dew points at/above 75 degrees only occur in small periods this far north. We reach this level June 19. We average around 2 days with humidity levels this high each summer. 2019 was the last summer with above normal of days with dew points at/above 75°.

Anytime we experience extreme weather, people will compare the current conditions to past events they remember. Stretches of heat like this elicit memories of the summer of 1988.  However, there are big differences between 2024 heat and 1988 heat.

The 2 week period where we reached 100° degrees (this was the last time) featured some cool overnight lows. In fact, we broke four low temperature records starting three days AFTER we broke the all-time high temperature record of 104°

Look at June 20, 1988 high temperatures across the US.  

Now high temperatures on June 19, 2024

Another note about the summer of 1988 in northern Ohio.  We reached 90° 35 times. Only 14 of the 35 days at/above 90° had a dew point at/above 70° (tropical) and only 5 of the 13 days at/above 95°.  So plenty of 90s with relatively lower humidity that summer.

Will this heat continue?   Expect a break as the heat shifts to the central US.