Have The Temperatures Since May 15 Really Been That Cool?

 

My cucumber plants are loving the frequent showers. Took a risk planting on May 7th this year. Thankfully, we survived some of the colder mid May nights.

Yet the bigger question remains. Have temperatures since mid May really been that "cool"?

Before we take a deep dive into years past, put the earlier spring weather especially April and early May aside.  April and most of May for that matter was governed by the stratospheric warming in the higher latitudes in mid March. A winter pattern that developed late in the season. We felt the effects in spring with above normal rainfall and cooler than normal temperatures. That will alter your perception significantly.      I wrote about this last month. LINK HERE. This major driver was fading by mid May. Since we are into the last half of June, looking back to mid May is more appropriate. All temperature maps and charts are from May 15 to June 15-17 and give the temperatures vs the 30 year averages.

First, the average temperatures for Cleveland vs normal since May 15. Obviously a big drop off this year yet pretty much in line with 2023!  Most of the last 20 years have featured warmer than normal late springs. 2025 overall has been more in line with the 1980s and early 1990s.

How about just the average HIGH TEMPERATURES? 

Obviously a big drop off this year. Again the last 20 years featured warmer than normal high temperatures in late spring.

Now the average LOW TEMPERATURES. 

2023 was cooler. The early 2000s and 2010s warmth definitively featured here similar to the above charts.

How about the number of days at/above 80 degrees?

This is where this year stands out. We've only had FIVE days at/above 80 since May 15th. Only three other years had less:  2003, 1997 and 1948.  

Another way of showing this is to look at the number of days UNDER 70:

2025 is more in line with the late springs of the 1990s and early 2000s.

The extremes are what we remember, not so much the averages!

Since we are comparing year to year, how about rainfall between May 15 and June 17th?

Plenty of years similar in recent memory.  Remember not to think about April and early May's rainfall. That was due to a completely different set of weather drivers. LINK HERE. 

A few things to note, we are only looking at ONE location (Cleveland) over a select 30 day period. This late spring has been very similar across the southern Great Lakes, Ohio Valley and the entire mid-west!

Average Temperatures VS Normal - May 15 to June 15

Rainfall rates are starting to slow especially across the Ohio Valley over the last 30 days.

Rainfall Rate VS Normal Since May 15th

The take home message is that this late spring has been "cooler" than in recent years here in northern Ohio AND much of the central US. While the rainfall is closer to normal since mid May, our average temperatures whether you look at the highs or the lows are more in line with what we would have experienced in the 1990s/early 200s during the same period. Even the number of days in the 50s and 60s since May 15 is similar to the 1990s. The fact that we've only had 5 days above 80 degrees since May 15 makes it seem that much cooler.  If it weren't for the Stratospheric Warming in mid March, more than likely our spring weather (April and May) would have been much different!

The pattern as outlined earlier this month is starting to switch to summer. Get ready!

What's With The Cool Lake Erie Water?

Check out this chart. It shows the water temperatures off Cleveland for each year since 1995 from May 1 to June 15. I dimmed out every year except last year (2024) and this year for easier reference.

Note that last year's water temperature in May was close to the warmest on record. This year, the temperatures were below normal. Lake Erie water temperature responds to the air temperature.  Notice how last year the temperatures around the Great Lakes were well above normal in May.  This year, below normal.

May 2024 Temperatures VS Normal

May 2025 Temperatures VS Normal

Given the pattern for the remainder of June through mid July, the Lake Erie water temperature will rebound nicely!

Summer Heat Emerging

Interesting temperature anomaly solution by the European AI model for the week of June 19 through the 25th back on June 9th. My idea of a slow start to summer (June 1 starts the clock) is still looking good.

June 9 afternoon solution

Here is the same model late June 10th.  Extreme heat projected in the middle of the US.

GFS wasn't showing any heat on June 9 model output...

GFS did a complete 180.  Now its showing the heat the EURO was showing.

High temperatures Sunday June 23 via Euro AI:

GFS same day:

What's interesting is that the heat is developing over extremely wet ground especially across southern Kansas, Missouri and Kentucky.  Northern Illinois, Iowa and Indiana rainfall is running well below normal since mid May.

How much will these dry conditions feed the heat if this continues into July/August?

Notice the Sonoran High (southwestern US) and the SE ridge beginning to merge by the end of the month. This should be enough to keep temperatures above normal across the Ohio Valley/Central US the final week of June, through the 4th of July and beyond.

Why Has Spring Been So Wet/Cool?

The overall pattern this spring has lacked substantial warm periods. Yes we've reach 80 degrees a few times earlier in May but nothing since. Add the high frequency of rainfall and this spring has been anything but pleasant. It's not only across northern Ohio but a large portion of the Great Lakes, Ohio Valley and central US.

Below are the temperatures compared to normal over different time periods since early April. Shades of blue represent below normal temperatures. Notice that most of the extremely "cool" air occurred in the last 2 weeks of May.

May 23 to May 30

May 15 to May 30

May 1 to May 30

April 1 to May 30

The percentage of days with high temperatures above normal is low since April 1st.  One of the biggest drops year to year. April/May of 2024 featured 74% of the days above normal.  This year only 36%.  We forget that 2020 we only had 28% of the days above normal

Rainfall vs normal across the US over different time intervals:

May 23 to May 30

May 15 to May 30

May 1 to May 30

April 1 to May 30

Interestingly, the Ohio Valley has had well above normal rainfall from Ohio, western PA south along the Ohio River including the entire deep south. Yet northern Indiana, Illinois and all of Iowa is running 2-5" below normal since early April.

Here is a close up look at Ohio's rainfall since early May:

Why has this spring featured above normal rainfall and a high number of days with below normal temperatures?  The answer is multi-faceted. First, the weak La Nina continues to weaken in the tropical Pacific. Second, the upper atmosphere over the North Pole went through a warming in mid March. (stratospheric warming). This resulted in the Polar Jet stream to buckle south resulting in periods of unsettled weather across the middle of the US and southern Canada this spring. It's been a slow developing process over the last 2 months. (If this "warming" would have occurred in January, the chances would have been much higher that this winter would have featured more extreme cold!)

The stratospheric warming is identifiable by looking at the high pressure over the North Pole. Notice that it descends down into the middle of the atmosphere. It tool a long time to "drip" deeper into the atmosphere after the initial warming in mid March.  (graphics below courtesy Judah Cohen)

The trickle-down continued well into May.  

Another way to look at this is to chart the temperature increase. The area in black shows the rapid rise to almost record levels at the top of the atmosphere in mid March.

The rise was accompanied by a reversal in the upper level winds. The red line which dropped below the blue dashed line shows this perfectly.  This reversal weakens the polar vortex which allows more frequent periods of "cooler" air and an active storm track.

The upper level pattern in April shows the storm track across northern Canada, a ridge of high pressure across the eastern US/New England and the low developing in the SW US.

By May, the SW low deepened and became a permanent fixture across the central US, Ohio Valley.  This matches up with the cooler temperatures in the graphic at the top of the post.

Here's a close up image of the upper level pattern in May

Remember that the cooler and unsettled influences are kind of like tentacles off an octopus. They extend and retreat. When they retreat, you get periods of warmth, which we've had similar to late last week when we reached the lower 80s. When they extend you get periods of rain/highs in the 50s and 60s when the average highs are above 70.

We've had more than 30 days with measurable rainfall since April 1.

8th wettest April and May thus far.

Number of days with measurable rainfall - April 1 thru May 31:

1953                 41

1973                 39

1947                 38

2011/2002         37

1989-05-31         36

1956/1943         35

2025/2019/2004 34

Cloud cover has been well above normal this month.  When we look at the April/May cloud cover vs normal for each year over the last decade we notice several things.  May 2025 is the cloudiest May since 2020 with April cloud cover is only slightly above normal.  The biggest takeaway item about this chart is the April-to-May cloud cover increase vs normal is more than any year in over a decade!

This pattern extends to most of the mid-west, Great Lakes and New England. This has also influenced the multiple severe weather events.  Luckily the Ohio tornado count way down from last year.

The Best Way To Measure Winter As A Whole

April is winding down.  I was sitting on post this until the end of the month.  After all, we've had light snow in late April even early May in recent years so I wanted to be sure we could put a bow winter and store it away.  

How do we accurately measure winter as a whole?  More specifically how do we define a harsh winter? No one gold measurement standard exists.

Over the years, I've been toying with different ways to appropriately answer this question. I've posted this question on social media several times.  What I found out was that everyone remembers winter in their own way, much of which is strongly influenced by personal events that occurred coincident with specific winter weather events.  All of which seems highly subjective and personal.

We all see the weather through our own personal preset lens. Highly personal memories can alter our baseline perspective on weather (strong recency effect) especially when comparing current weather to past weather.  I remember posting follow up questions in response to many comments about winter. Some took the responses personally.  Their experiences were strong. These helped reinforce an already established weather vantage point. Their perspective was steadfast. Good luck changing it. I quickly found that the entire project slowly became an exercise in recognizing cognitive bias and recency effect.

Knowing all of this, I compiled several standard metrics (average temperatures and snowfall) used by the National Weather Service and NOAA with a few that I like to call "non-traditional metrics" that attempt to take into account public perception, recency effect and overall bias as outlines above. 

The result was a 21 different metrics that measure different components of the winter as a whole.  There is no 100% completely objective way to determine the harshness of winter but I think this is a great move forward. 

Here is the list in no particular order:

1. Snowfall vs normal
2. Average temperatures vs normal
3. Days with snow on the ground between December and February
4. Days with 1" snowfalls November through April
5. Days with 4" snowfalls November through April
6. Days below normal - DJF
7. Days below normal - January/February
8. Longest stretch highs under 30
9. Days with Highs under 30
10. Days with highs above 40
11. Days with highs under 20
12. Nights below zero
13. Days Wind Chill below zero
14. Days Wind Chill below -15
15. Longest stretch below normal temps DJF
16. Longest stretch snow on ground 1"+
17. Days under 30 degrees & wind gusts 20+ mph
18. Longest stretch between 40 degree days
19. March snowfall
20. April snowfall
21. Days between first and last snowfall

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I calculated all 21 metrics for each winter going back to the late 1940s.  Some of the wind chill data had to be reconstructed using temperatures and wind speed prior to the early 1970s. Once the numbers were found, I calculated a 15 year average for each one working back in time starting with this winter.  So the first 15 year period was 2011-2025, the second was 1996 to 2010 and so on.  My goal was to compare winters to the average of the winters close to it.  Each winter metric that was "harsher" than the 15 year average, I colored the block blue.

First chart is colored blue with the actual numbers.  The second is without the actual numbers and looks cleaner.

You'll notice that most of the extreme winters we remember are represented.  While this past winter (2024-25) was by most measures more "winter-like" than recent years, historically it was in the middle of the pack with 10 of the 21 metrics checked.

Here is the entire chart for reference: DOWNLOAD it since its a large image.

Chart without numbers.  DOWNLOAD it since its a large image.

The winter with a high number of blue boxes represents a harsher winter.

Here are some notable harsh winters:

Now notable not-so-harsh winters: