❝ How do you know when spring has begun? Is it the appearance of the first tiny leaves on the trees, or the first crocus plants peeping through the snow? The Spring Leaf Index is a measure of these early season events in plants, based on recent temperature conditions. This model allows us to track the progression of spring onset across the country. The map shows locations that have reached the requirements for the Spring Leaf Index model (based on NOAA National Centers for Environmental Prediction Real-Time Mesoscale Analysis temperature products).
Click through to the article and a dynamic model of this map.