Methodology

The trail-day score

Each trail has a primary activity (hiking, trail running, mountain biking, or dog walking). For each activity, we compute a 0–100 score from a base of 60, then add signed factors. The grade scale:

• Ideal — 85+: drop what you’re doing and go.
• Great — 70–84: a good day.
• Good — 55–69: workable.
• Marginal — 35–54: marginal; you’ll live.
• Skip — under 35: today is not the day.

Inputs that move the score

Weather (live from the National Weather Service)

• Thunderstorms in the short-term forecast subtract heavily from every activity.
• Rain probability above 70% reduces the score — more so for mountain biking, since wet trails rut under tires.
• Wind above 25 mph is hazardous on exposed ridges; above 30 mph is a major penalty. Gust value is preferred over sustained.
• Temperature within an activity’s ideal range adds points; outside its comfort range subtracts.
• Active NWS watches/warnings reduce the score.

Mud score

The mud score maps recent precipitation, trail surface type, and recent temperature onto a five-tier label (prime → swampy).

• Dirt trails: 0–0.3″ in 48 h = prime, 0.3–0.8″ = tacky, 0.8–1.5″ = damp, 1.5–2.5″ = muddy, 2.5″+ = swampy.
• Rocky trails shift one tier toward prime — they shed water faster.
• Paved and boardwalk surfaces are always prime.
• Gravel shifts the thresholds up — it handles more rain before going soft.
• Air temperatures below 45°F push dirt trails one tier toward muddy (slow drying).
• Mountain biking penalizes muddy trails almost twice as hard as hiking does, because wet trails under tires cost trail crews weeks of repair.

Air quality

AirNow AQI penalizes the score progressively once it crosses 100, heavily once it crosses 150. Clean air (AQI under 50) adds a small positive.

Daylight

The score flags trails where daylight remaining is less than the estimated time on trail plus a 30-minute buffer. Bring a headlamp or start earlier.

Park closures

If the park service flags an active closure at the park this trail belongs to, the score is capped at “skip” and the headline says so.

Hard-skip overrides

Regardless of the numeric score, we force a “skip” grade when:

• A park closure is active at the managing agency.
• You’re trying to mountain bike a trail currently rated “swampy” — the ethical override.
• You’re planning a dog walk while thunderstorms are in the forecast.

Limitations

• Mud scoring is a heuristic. A trail’s actual state depends on whether it’s shaded, what soil type it has, and how heavily it’s been used since the rain. We can’t see any of that.
• Parking-fill forecasts are hand-curated hints, not live data. Popular trails really do fill early on weekends.
• State-park alerts are best-effort scrapers; only NPS-managed units have a true API. MD DNR, VA DCR, and DE State Parks are scraped from public alerts pages and may break when those pages redesign.
• Tick and bear advisories are month-based heuristics, not live reports.