How accurate is a modelled soil temperature?

Every soil reading on this site is the 0–10 cm (0–4 inch) soil layer of NOAA's Global Forecast System (GFS), the U.S. National Weather Service's global weather model, on its 0.25° grid (about 25 km between grid points). Each city uses the nearest land grid cell; the page gives the distance. Cities whose nearest land cell is more than 30 km away (small islands, long sandbars) get no page.

• Past days come from the model's analyses at 00, 06, 12 and 18 UTC. The next 7 days come from the newest run (every 6 hours of it). A day is the average of the four values at 06, 12 and 18 UTC and 00 UTC the next day (about 1 am to 7 pm Eastern); the low and high are the lowest and highest of those four.
• Thresholds and dates use the 5-day running mean of those daily averages, so one warm afternoon does not open a window. "Held 55°F from 3 April" means: the first day of the spring when the 5-day mean stayed at or above 55°F for at least 10 days running, after being below it.
• Measured 2 in and 4 in readings come from NOAA's U.S. Climate Reference Network (USCRN): buried probes at 5 cm and 10 cm, reported every hour. They are shown only for the nearest station within 150 km and are never mixed into the model numbers.
• Growing degree days (base 50°F, simple average method: (high + low) / 2 − 50, never below zero) and chill hours come from NOAA NCEI GHCN-Daily highs and lows at the city's nearest station that reports at least 96% of days. Gaps of up to 5 days are filled by straight-line interpolation. Chill hours are hours at or below 45°F from 1 November, estimated hour by hour from each day's high and low (a cosine curve from the low at 6 am to the high at 3 pm and back); the hours between 32 and 45°F are shown as well.

A model is not your lawn. Sun, shade, slope, mulch, irrigation and soil type move a real lawn's temperature by several degrees. The measured error against the reference stations is on this page; for a single yard, a $15 soil thermometer pushed in 2 to 4 inches is the best check.

The garden thresholds (50 to 55°F for crabgrass pre-emergent, 50 to 65°F for cool-season grass seed, 65°F for warm-season grass, 60°F for tomatoes, 40°F for peas and lettuce) are common rules of thumb from lawn and garden guidance, not product instructions. Follow the label of any product you apply. This site is not affiliated with or endorsed by NOAA or the National Weather Service.