What Drove the COLD End of April/Early May?

Seven Hills, April 20, 2021

The first 2 days of April featured snow and cold. We forget about how warm April the second full week of April was (highs in the 70s and 80s). 

Here's what most of us said on April 10th: "Winter is over. Spring is here!"

Then the bottom fell out.

Late April/early May cold was significant across the northern half of the US.  Slushy snow fell (which melted off fast) on May 9 along with 2-4" of rain Mother's Day.  Temperatures for most days were running well degrees below normal from April 15th through last week. 

So what happened?

The MJO (Madden Julian Oscillation) headed into a high amplitude phase 7,8,1

This connects well to colder than normal temperatures across the eastern US.

You can see how the surface temperatures & 500mb (upper level pressure pattern) vs normal evolved over the period. I circled the Greenland High Pressure/Block. This was responsible for locking in the pattern set up by the MJO.

April 24-30 temperatures vs normal

Upper Level Pressure Pattern vs normal

May 1-9 temperatures vs normal

Upper Level Pattern vs Normal

May 10-14 Temperatures vs Normal

Upper Level Pattern vs Normal

Notice how the Greenland Block started to weaken last week. As it weakened the cooler pattern started to break down.

Here's what the general public was say on May 9-10th: "What is going on here? Snow in May. Seriously? I can't remember the last time this happened this late."

In reality, we had a COLDER early May last year (2020) with even later snowfall.

In late April we started to see signs of warmth the last 2 weeks of May.

The pressure pattern still strongly suggests the Bermuda High to pulsate north through the end of the Month/early June as the MJO amplitude goes down/heads into phase 3,4,5,6.

Temperatures from May 17 through May 27

This WARMER pattern is a harbinger of what is to come this summer!

I can hear the cries of viewers saying "The weather is going from winter to summer again".  

(In reality this has happened many times before in weather history but I digress). Our spatial and recency bias is strong when it comes to weather.