When Will This Cool/Unsettled Pattern Change?

It's not uncommon for April to feature stalled low pressure with long stretches of "cool" air across the Great Lakes/New England.  Our long stretch of warmth a few weeks ago makes this cooler/cloudy/rainy pattern that much harder to deal with.  What does the long range (2-3 week outlook) say?

The southern oscillation index went through some significant changes over the last week and a half. The greater these day-to-day changes the stronger the effect across North America, Here is the peer reviewed research:  PAPER HERE

SOI analog forecast show the Great Lakes/Eastern US trough backing off by mid month.  How quickly will the Southeast ridge build back is the big question.

Another element going forward this summer and fall will be the building El Nino. I'm not going to go into the specifics of that here. Perhaps a blog post on this in the weeks and months ahead. But as La Nina decays, the overall effects will be cascading across the Pacific augmenting the jet stream and the weather across North America.

Watch the decaying La Nina as shown through the weekly sea surface temperature anomalies from December 2022 to mid April 2023:  Watch the blue area disappear slowly.

Sea Surface Temperature Anomalies

The big drivers of the pattern change have been the ridge of high pressure in the Gulf of Alaska and the western corners of North America along, the deep low pressure over the Aleutian islands and the stalled ridge over Greenland (negative North Atlantic Oscillation). The position of these high pressure ridges have allowed low pressure to undercut them through the middle of the US. This has significantly suppressed the Bermuda high which brought us the warmth a couple weeks ago locking in below normal temperatures across the eastern 2/3 of the US.

Based upon all of these factors, I don't foresee any long stretches over warmth across the Great lakes and Ohio valley through at least the middle of May.  

The reinforcing front Sunday/Monday of next week could produce rain/snow mix.  Here was my forecast tweet from April 20 on the early May rain/snow mix potential.

As of this writing (April 25) the new EURO and GFS models show the Greenland Ridge & western North America ridge (red areas) holding strong until the second week of May then gradual weakening. This flattens the trough across the central US bringing an end to the unsettled pattern. These solutions were indicated on the SOI analogs more than a week ago.

Temperatures per the GFS also corroborate what the SOI analogs indicated 2 weeks ago.  That is gradual improvement across the central US and Ohio Valley but still no big signs of any long stretches of warmth.

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.

Progression of April Cold

After an extremely mild winter, the pattern across the North American continent shifted significantly colder around the second week of April. 

Remember that April started off above normal across the eastern US. The first 9 days of April the average high temperaure was close to 60 degrees in northern Ohio! We reached 72 on April 7th.  Slowly colder air settled in especially across a large portion of the country. Each graphic below illustrates the gradual progression of the cold (above/below normal temperatures) in 20,15,10,7,5,3,1 day periods prior to April 15th. The last graphic is the 5 day period from April 15-20th.

20 day temperature anomalies - 20 days leading up to April 15

15 day temperature anomalies - 20 days leading up to April 15

10 day temperature anomalies - 20 days leading up to April 15

7 day temperature anomalies - 20 days leading up to April 15th

5 day temperature anomalies - 20 days leading up to April 15th

3 day temperature anomalies - 20 days leading up to April 15th

1 day temperature anomalies - 20 days leading up to April 15th

5 day temperature anomalies - 20 days leading up to April 20th

Why these early spring changes? Let's go back to the winter for a moment. The pattern was dominantly mild. There was no "blocking" over Greenland.  The Arctic Oscillation stayed STRONGLY positive. The Polar Vortex was strong with no big perturbations south. 

The next three slides are the 500mB heights for each month January through March 26th.  High pressure over the northern Pacific. SE ridge across the US. Low pressure over the poles/North Atlantic.

By the last week of March, notice the changes. The northern Pacific high shifted west. A strong ridge developed in the North Atlantic! The SE ridge from March was still intact.

These pressure changes were a significant sign that April would more than likely feature some significant changes in areas that were used to above normal temperatures especially the eastern US and possibly the Great Lakes. (April 5th I posted a tweet noting that the next 10-14 days would feature some cold air/below normal temperatures) 

By the first week of April, the ridge across the eastern US was breaking down. High latitude blocking was present.

Between April 6 and 10, the high started to pop back in the southern states. Northern Pacific ridge was far weaker. The trough in New England was building in response to the blocking

By April 11-15th the trough/ride wavelength were much larger. The central US/eastern US trough had grown (blue colors) and was locked in.

April 16-19th featured the dominant trough across much of the US and Canada with the exception of Florida.

Watch how the colder air bled southeast throughout early/mid April in response to the upper level changes. From April 14 to the 23nd we had 5 days in the 30s and 40s with wind chills in the 20s in spots!

Is this "cold mid-April" unprecedented?  Here are some years in the past with cold mid Aprils similar to 2020. (Temperature anomalies from April 12-21 for the years 1875, 1943, 1953, 1983, 1997 and 2007 pictured below. I am sure I missed a few)

How about here in northern Ohio. Where does this cold mid April rank?  Here is the list: 2020 rank 33rd.  Also keep in mind that 32 of the 40 years listed occurred BEFORE 1960. The last year similar to this year was 2018 then 2007, 1975.

Here's a more recent analysis: Below is a graph showing the number of days with below normal temperatures through April 2nd since 1990 (last 30 years) in northern Ohio (Hopkins Airport-Cleveland). Perhaps we forgot about 2018 with MORE days below normal than 2020.

How about April snow in these years?  Great question.

This year we've had 9 day with at least a trace of snow (through April 22). Two years ago (2018) we had.....13!

So while it has been colder than normal in April, it is not unprecedented.  The recency effect is strong here as we may have placed more weight on this year because it is front of mind.

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A FEW THINGS TO NOTE:  THE COLD APRIL ISN'T A RESULT OF THE RELATIVELY MILD WINTER. SIMPLY BECAUSE THE WINTER WAS "MILD" DOESN'T AUTOMATICALLY MEAN APRIL WAS GOING TO BE COLDER THAN NORMAL. ALSO, THE GLOBAL/CONTINENTAL DRIVERS OF WEATHER PATTERNS IN WINTER CAN PRODUCE DIFFERENT TEMPERATURES, ETC IN SPRING AND SUMMER. THIS IS NOT CONFIRMATION OF "GLOBAL COOLING". IN FACT ONLY A SMALL PERCENTAGE OF THE WORLD HAS HAD TEMPERATURES BELOW NORMAL THIS MONTH.