Snow to 80s in April. How Often Does This Happen?

Snow showers this morning. Yes, it's April 19th

Hinckley

Twinsburg

April snow is not that uncommon. Look at the snow history from Late March through April. These are the individual instances of snow for EACH DAY since April 3rd color coded. 

Yet this week temperatures will be jumping significantly heading into the weekend. (SOI forecast from April 5th seems to be working out). 

SOI Changes in early April connect with these historical weather conditions

All of this means we could go from accumulating snow to 80 in a little over 4 days. When was the last time this happened?  LAST YEAR

I was surprised to find this has happened almost 60 times since 1950! The fastest SNOW to 80s was 19 hours in April 3-4, 2007

How about 80s to snow?  Only 10 times. Most recently in 2018

Big temp swings in March and April occur with high frequency vs other months. These big changes occur in the central US more than in the Great Lakes.

Second Heaviest Snow This Late In The Season. When/How Did We Know?

I'm 5 days late with the late April snow recap.  Here are the snow amounts:  A few locations were close to 8 inches by Thursday morning.

April 21 snowfall

Euclid

Seven Hills

Perry

Snow after March 20 is pretty common in northern Ohio.  Snow after April 20 is a rarity. Only two other years have had more than 2" after April 20.  1974 and 2005. The 2005 event was over a 48 hour period (see the map below).

April 23-25, 2005 snowfall.  Remember this? More than a foot of snow fell across a large part of northern Ohio.  In 3 days it was completely gone!

Signs were pointing to snow or at least a deep storm system with colder air back on April 5--two week prior.

The MJO was heading into favorable cold phases.

A recurving typhoon near the Philippines (which often times portends cooler than normal temperatures eastern US) gave us another good indication.

Eventually the long range models (back on April 8) picked up on this colder idea.

Animation of temperatures: April 10 to 22

Upper level pattern was pretty close to April 2005 snow event. That storm system was deeper.

What's the latest we've had snow? After cross-referencing hourly temperature data, the years are narrowed down to only a few.  Last year was the LATEST MEASURABLE SNOWFALL which broke the record set back in 1907.  Flurries were reported close to Memorial Day in 1907 and 1961 (Akron).

How quickly after late season snow do we see an 80° degree day? Earlier in April it took us around 7 days (6 days listed below) post snow to reach 80°.  Snow on April 1.  83° April 8.

Indications of a late month warmup were surfacing shortly after the colder mid-month pattern was showing up.  SOI composites (analog to similar situations in a La Nina) showed a weak, short-lived ridge of high pressure/warmth around April 26th.

Composite for BIG drop in SOI around April 11-13/La Nina

High temperatures across more than 15 states Monday-Wednesday this week will easily teach 80

Late April Snowfall: How Rare is this?

Yes, it's April 20 and we are talking about accumulating snow!

Sure we've seen late season snow before. It snowed in mid-April in 2019.

...and it snowed in May 2016 (under an inch).

In fact 16 of the last 20 years has featured measurable snowfall after the first day of spring. Over the last 30 years April has featured snowfall almost half of the time. But snow that need shoveled after April 20 is something else entirely.

Below is a calendar of each day starting on March 20 showing the snowfalls for each day. Notice how the frequency of snow drops significantly once we make it through the first 10 days of April. Interestingly we've only had 3 snow events greater than 2" after April 20th. Two of those occurred in 2005. (I remember driving home from work during that big snow on the 24th).  The other one was on May 6, 1974.  We've had snow in May but never more than 2 inches.

The late April snow in 2005 was historic for northern Ohio.  Look at the similarities between this year's pattern and 2005. Position of storm center is close.  2005 was much deeper.

Bigtime cold across the Great Lakes and New England (April 2005)

Much of northern Ohio saw more than a foot of snow that year in late April

This time the overall pattern was showing signs of stronger weather systems by the 3rd week of April (shades of blue).   My tweet from April 5th below.

The cold was building a week ago in central Canada as this temperature animation below shows. This loop is from April 15 to April 29.  Not much warmth here.

Snowfall accumulation is much different this time of year due to warmer ground and road surface temperatures.  These numbers are mainly for grassy surfaces.  Snowfall on roads and sidewalks will end up being under these levels. Sun angle is much higher so any sunshine will accelerate snow melt Wednesday and Thursday. 

Snowfall forecast could mark only the 4th time we've had more than 2" of snow this late in the season.

By the way, this snow doesn't give us any indication of what the rest of spring and summer will be like. More on the summer outlook in the weeks ahead. 

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.