How Do You Know When Spring Has Sprung? How Does This Spring Compare to Normal?

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Spring leaf. April 27, 2021. Credit: USA NPN

This post first appeared on the USA National Phenology Network

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 First Leaf and First Bloom Indices are synthetic measures of these early season events in plants, based on recent temperature conditions. In addition, these models allow us to track the progression of spring onset across the country.

How Does This Spring Compare to “Normal”?

Spring leaf-out (above) has arrived in all but the most northern and highest elevation parts of the country. After arriving early in southern parts of Southwest and Southeast states, cold temperatures halted the progress of spring leaf-out for several days across the northern part of the Southeast, Southern Great Plains, and mid-Atlantic. As a result, spring leaf out arrived days to weeks early across much of the northern Great Plains, Midwest, and Northeast.

Spring bloom. Credit: USA NPN

Spring bloom (above) has arrived in the southern half of the country. Spring bloom is patchy, with much of Texas days to weeks late, while parts of Kansas, Tennessee, Missouri, Illinois, and Indiana are days to weeks early.

How Often Do We See a Spring This Early or Late?

Credit: USA NPN

In places where spring has sprung, how typical is this year’s spring? Darker colors (above) represent springs that are unusually early or late in the long-term record. Gray indicates an average spring.

Credit: USA NPN

For parts of California, Arizona, South Dakota, Minnesota, Wisconsin, and Maine, this spring leaf out was the earliest in the 40-year record.

When Did Spring Arrive at Locations Across the Country?

First leaf index. Credit: USA NPN

The First Leaf Index map above shows locations that have reached the requirements for the Spring Leaf Index model so far this year.

First bloom index. Credit: USA NPN

The First Bloom Index map above shows locations that have reached the requirements for the First Bloom Index model.

What Is Behind These Maps?

The Extended Spring Indices are mathematical models that predict the “start of spring” (timing of leaf out or bloom for species active in early spring) at a particular location (Schwartz 1997, Schwartz et al. 2006, Schwartz et al. 2013). These models were constructed using historical ground-based observations of the timing of first leaf and first bloom in a cloned lilac cultivar (S. x Chinensis ‘Red Rothomagensis’) and two cloned honeysuckle cultivars (Lonicera tatarica ‘Arnold Red’ and L. korolkowii ‘Zabelii’). These species were selected because they are among the first woody plants to leaf out and bloom in the springtime and are common across much of the country.

Primary inputs to the model are temperature and weather events, beginning January 1 of each year (Ault et al., 2015). Maps for the current year are generated using temperature products from NOAA National Centers for Environmental Prediction Real-Time Mesoscale Analysis. More information is provided in our Gridded Product Documentation.

To determine how the current spring compares to “normal,” we difference the day of the year the leaf out, or bloom was reached this year from the long-term average (1981-2010) day of the year it was met. Long-term averages were calculated using PRISM Climate Data daily minimum/maximum temperature data (Oregon State University).

To calculate how often we see spring as early or late as the current spring, we compare the current year’s Spring Index Anomaly value to the anomaly values from the previous decades. Then, we determine how often a spring was at least this early (or late) by taking the number of years in the record divided by the count of years that were earlier (or later) than the current year.

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