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.