Reading mountain forecasts
Mountain weather changes more over short distances than almost anywhere else. A few habits help you get more out of any forecast, including the ones on this site.
Check what elevation the forecast is for
A forecast is for a place and an elevation. National Weather Service point forecasts cover a grid cell about 1.5 miles (2.5 km) across and use that cell's elevation, which the point forecast here shows next to the coordinates. If your route climbs well above it, expect colder weather.
As a rule of thumb, temperature drops about 3 to 5.5°F for every 1,000 feet you climb: closer to 5.5°F in dry, clear air, and less in moist or cloudy air. A 3,000-foot climb can mean a summit 10 to 15°F colder than the trailhead, before counting wind chill.
Expect more wind up high
Ridges, passes and summits are often much windier than valleys, and wind makes cold air feel colder. When a forecast calls for gusty winds, assume exposed terrain will get the worst of them.
Know what “chance of rain” means
The National Weather Service's probability of precipitation is the chance that at least 0.01 inch of rain or melted snow falls at a given spot during the forecast period, often 12 hours. The written forecast uses set words for these numbers:
| Chance | Forecast wording |
|---|---|
| 20% | Slight chance |
| 30 to 50% | Chance |
| 60 to 70% | Likely |
| 80% or more | Stated plainly, such as “Rain” or “Snow” |
A 40% chance over 12 hours doesn't say when it will rain. Open the point forecast and look at the hourly charts to see whether the risk is in the morning, the afternoon or overnight. The map's precipitation chance layer summarizes daytime hours, so use it to compare places, then check the hours for your route.
Watch the clock in summer
In many mountain ranges, warm and humid summer days follow a pattern: clear mornings, clouds building over the peaks by late morning, and thunderstorms in the early to mid afternoon. A forecast of “a chance of afternoon thunderstorms” is a reason to start early and plan to be off summits and exposed ridges before the storms usually arrive. See Trail weather safety for what to do if you're caught out.
Rain in the valley can be snow up high
When temperatures near freezing are forecast, precipitation that falls as rain at the trailhead may be snow or ice higher up, especially in spring and fall. Check the forecast for your highest point, and in winter and spring look for a mountain weather and avalanche forecast from your local avalanche center (avalanche.org lists them).
Forecasts get less certain further out
Forecasts for the next day or two are generally much more reliable than those five to seven days out. Use the later days to spot promising windows, then check again the evening before and the morning you leave.
Read the forecast discussion
Each National Weather Service office publishes an Area Forecast Discussion explaining the reasoning behind the forecast and how confident the forecasters are. When the numbers look borderline, the discussion often says whether storms, wind or a cold front could arrive earlier or later than forecast. You can find it through the full forecast page on weather.gov.