How the county maple atlas is built

1. Production: the Census of Agriculture

Farms, taps and gallons of syrup per county come from Table 37, “Maple Syrup: 2022 and 2017”, in the 2022 Census of Agriculture county volume for each of 17 states. Figures are copied as published. Where USDA withholds a county's taps or gallons to avoid disclosing an individual operation (shown as “withheld”), we leave it blank rather than estimate it. Counties with at least 5 maple farms in 2022 get an atlas page (385counties); the rest appear only in their state table. The parser checks that county farm counts add up to each state's published total for both years before anything is written.

2. Weather: 36 seasons of daily temperatures

For each county we take the daily maximum and minimum temperature at its county seat (or largest town; the page says which) from the PRISM 4-km daily grid, July 1990 through June 2026, served by NOAA's Regional Climate Centers (ACIS). PRISM is a model that interpolates station observations to a grid, so it is not a thermometer in anyone's sugarbush, and one cell cannot represent a whole county, least of all a mountainous one. A PRISM “day” ends at 7 a.m. Eastern; we checked against five airport stations (about 1,320 days each) and pair each calendar day's overnight low with that afternoon's high the way the live forecast does.

3. The sap-day rule

A sap day is overnight low under 30.2 °F and afternoon high over 37.4 °F, the freeze-thaw definition from field research in Quebec sugarbushes (de Lima Santos et al., 2025), and the same rule our live forecast uses. A classic day is low 20–29 °F and high 40–49 °F, the textbook pattern extension services describe. Weekly charts count sap days per 7-day bin from December 1; monthly shares use calendar months.

4. Season signals, and how we chose them

USDA NASS published each state's approximate average season opening and closing dates from 2011 to 2022. That gives 117 state-seasons of producer-reported truth across 13 states. We scored dozens of candidate rules against it, weighting each county by its 2022 farm count. Rules built only on freeze-thaw clusters cannot time the opening, because in the southern half of the maple belt freeze-thaw days happen all winter: they land three to six weeks before producers actually tap. Heat accumulation does the work.

The fit is the same on odd and even years (closing 5.5 vs 5.5 days; opening 5.8 vs 7.4), so the constants are not an artefact of the years used to choose them. Three quarters of closing estimates land within a week of the reported date; 91–93% within two weeks. The rule is tightest in the northern tier (Maine, New Hampshire, New York, Vermont, Wisconsin, Minnesota: 2–5 days) and runs early at the warm edge (Ohio, Indiana, West Virginia), where producers keep collecting after the heat threshold.

5. Anchoring to USDA

Because the rule carries a state-specific bias, the typical dates shown on a county page are anchored: the state's USDA median date plus the difference between that county's signal and the farm-weighted state signal. That keeps only the within-state differences (south before north, valley before hill) and removes the rule's bias. Illinois, Iowa, Kentucky and Missouri have no USDA dates, so their pages show the raw weather signal with a caveat. Year-to-year spread is each county's own interquartile range of signal dates over 36 seasons.

6. Limits worth knowing

• One grid cell per county; your woods may run days earlier or later with slope, aspect and elevation.
• Producer-reported dates reflect decisions (when people chose to tap) as well as weather.
• USDA records are short for Indiana, Minnesota and West Virginia (three years each).
• PRISM's authors advise against using it for very long-term trends; trend statements on this site use long-record station observations instead.
• The Census counts farms; backyard sugarmakers who don't sell are not in it.

7. Reuse

The dataset is yours to use with attribution (“LuckySap County Maple Atlas, getluckysap.com”), alongside the credits below: the Census figures are public domain; PRISM data require naming the PRISM Group, Oregon State University, prism.oregonstate.edu, and the access date (2026-10-05); town coordinates are from GeoNames (CC BY 4.0). county-maple-atlas.csv holds one row per county with the Census figures, typical dates, and per-season sap-day counts. Generated 2026-10-06.