Why Spring Is Getting Earlier in Many US Cities

The 1981-2010 normals and the 1991-2020 normals — exactly a decade apart — show a consistent shift toward earlier springs in most of the continental US.

For Chicago, the new average last spring frost is three days earlier than the previous normals period. For Boston, about four days earlier. Atlanta shifted by about a week.

This is not a climate argument — it's arithmetic. The 30-year window shifted forward by a decade, and the newer decade warmed on average. Whether the trend will continue at the same rate is a forecasting question the normals don't answer.

What the normals do tell gardeners: the old "plant after Memorial Day" rule for Chicago tomatoes is now roughly a week conservative.