How many times during a 365 day period does the temperature read 20 degrees above (or below) the average? How many times does this happen per day, per weekly average, per month?
As someone who grew up in New England it doesn't seem like it would be that rare. There were weeks in July where it was over 95 when the historical average was probably 75 and weeks in February when the average was 10 even if the historical average was 32. What is the standard deviation for temperature in certain areas and how rare is it to see 20 degrees variation if you are looking at multiple windows of time that vary in length?
Sure, global warming did not cause it to be 20 degrees higher than usual? If anything it would be +3 degrees global warming +17 degrees random variation? Why run with 20 in the title if there is no way that global warming would be responsible for the random variation.