How We Calculate Freeze Dates

Every date on this site comes from the NOAA/NCEI 1991–2020 U.S. Climate Normals. We do not model or invent dates; we read the published probability values for the nearest reporting station with valid data and present them plainly.

How the freeze dates are calculated

NOAA publishes probability dates for the first fall and last spring occurrence of a minimum temperature at or below a threshold (32°F, 28°F, and 24°F). For each threshold we show three of these values:

• Early (FP10) — the date by which the freeze has occurred in roughly one year in ten. It is the early end of the range.
• Median (FP50) — the typical date; in about half of years the freeze arrives earlier and in half later.
• Late (FP90) — the date by which the freeze has occurred in about nine years in ten.

These are the same percentile normals meteorologists and agricultural extensions use. We report them as published, rounded to the day.

How the prep deadlines are set

A prep deadline is the median first-freeze date shifted earlier by a fixed margin so the work is finished before the freeze in most years. For sprinklers and RVs we use the median first 28°F freeze minus about ten days; for boats, about a week; for snow blowers, three weeks before the estimated first plowable snow. Each module page states the basis it uses.

First-snow estimates

When a station does not publish a first-snowfall date, we estimate the start of snow season as the first month whose snowfall normal reaches one inch, drawn from the monthly normals. Because it is an estimate, we label it as such wherever it appears.

Station selection

Each city is matched to the nearest NOAA station that has valid first-freeze data within about 60 kilometers. Every page names the station, its distance in miles, and its elevation so you can judge how well it represents your exact location. Cities without a qualifying station in range are not published, to avoid presenting thin or missing data.

How the live status is computed

The live badge compares a 10-day forecast from Open-Meteo against simple, published thresholds: a forecast low at or below 28°F flips the sprinkler and RV status to urgent; a low at or below 20°F marks a pipe-risk night; two or more inches of forecast snow says to start the snow blower; and so on. The exact rules are open in the site's rules.js file. If the forecast cannot load, the page falls back to the climate normals and still works.

Limits and accuracy

Normals describe a 30-year average, not a prediction for any single year. A station a few miles away can differ from your yard, especially in hilly terrain or cold-air pockets, and elevation above about 5,000 feet adds local variation. Treat the dates as planning guides and watch the live outlook as the season turns.

How often this updates

The climate normals update on NOAA's schedule (the current set covers 1991–2020). The live outlook refreshes each time you load a city page, cached for up to an hour in your browser.

Last updated: July 11, 2026.