That may be the case where you are but here the mid-late 20th had its share of deep could. The local long-term co-op (Farmington, POR from 1893) has reached -30 or colder 30 times. 6 came in the 1970s and 8 in the 1990s. The co-op has recorded 32 days with subzero maxima. The 60s lead with 5, then 1910s and 90s each have 4 and 70s, 80s and 2010s with 3. Records at CAR are similar, though they go back only thru 1939. To me it looks like NNE, at least, has good representation thru the decades. Even the nation's biggest UHI has set some all time records in the mid-20th, though Gotham has not gone below -2 since the freak cold plunge in mid-Feb 1943. However, in early 1961 NYC recorded their longest run of maxima 32 or lower, 16 days with the highest temp of 29. (And big snowstorms as bookends, 27.3" total at Central Park, 40-50 in NNJ.)