WHAT IS THE DIFFERENCE BETWEEN FIRST LEAF AND FIRST BLOOM?
The First Leaf Index is based on the leaf out of lilacs and honeysuckles, which are among the first plants to show their leaves in the spring. This Index is associated with the first leafing of early-spring shrubs and other plants.
The First Bloom Index is based on the flowering of lilacs and honeysuckles. This Index is associated with blooming of early-spring shrubs and leaf out of deciduous trees.
HOW DOES THIS SPRING COMPARE TO “NORMAL”?
Spring leaf out continues to spread north across 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. Spring leaf out is now arriving days to weeks early across the Midwest and Northern Great Plains.
Spring bloom shown to the top, right has arrived in parts of Southwest and Southeast states. Spring bloom is early in California and Arizona and patchy in Southeast states.
Taking a closer look at Central PA, some areas saw spring leaf out occur more than a week early. For example, northern counties like Cameron, Elk and Jefferson Counties saw spring leaf out 9 to 10 days early. Meanwhile, the majority of Central PA followed average trends.
HOW OFTEN DO WE SEE A SPRING THIS EARLY OR LATE?
In places where spring has sprung, how typical is this year’s spring? Darker colors represent springs that are unusually early or late in the long-term record. Gray indicates an average spring. For parts of California and Arizona, this spring leaf out was the earliest in the 40-year record.
How typical is it for spring to arrive early in Central PA compared to recent decades? An early spring, or early spring leaf out, can occur every 1 to 4 years. As we saw earlier, 2021 would be included in that statistic since northern areas of Central PA saw some early green!
WHEN DID SPRING ARRIVE AT LOCATIONS ACROSS THE COUNTRY?
The First Leaf Index map to the bottom, left shows locations that have reached the requirements for the Spring Leaf Index model so far this year.
The First Bloom Index map to the top, right shows locations that have reached the requirements for the First Bloom Index model.
On average, counties south of I-80 in Central PA are the first to see signs of spring. This typically happens during the first week of April. Then, counties to the north slowly follow. While southern Central PA followed this trend, northern Central PA was eager to great spring!
A few areas along the southern boarder of the state, like Bedford and Somerset, saw some green as early as the end of March. However, the majority of Central PA saw spring leaf out during the first week of April. This lines up with average timing. Then, as revealed earlier, northern Central PA experienced spring leaf out around a week and a half early.
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-baesd 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 year the leaf out or bloom was reached this year from the long-term average (1981-2010) day of 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 a 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. 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.