Spring Comes Early Across the U.S.
Over the past several decades, with the exception of the Southeast, spring weather has been arriving earlier in most parts of the United States. This shift affects all sorts of biological processes that are triggered by warmer temperatures — not just flowering, but animal migration and giving birth and the shedding of winter coats and the emergence from cocoons. The calendar above uses an index for the onset of spring developed by Mark D. Schwartz (University of Wisconsin-Milwaukee) and USA National Phenology Network colleagues. This index, based on temperature variables measured at individual weather stations estimates the first day that leaves appear on plants in a given state. To come up with a U.S. estimate, we took the average change across 716 weather stations spread across the lower 48 states.
For the U.S., the average shift of “first leaf” was approximately 3 days earlier moving from March 20 (1951-1980 average) to March 17 now (1981-2010 average).