How to Read an Hourly Weather Forecast for an Outdoor Project

A daily forecast gives you a high, a low and one rain chance. For a project that has to go on dry and stay dry for a set number of hours, that isn’t enough. The hourly forecast shows when things happen, and when is usually what decides the job.

Where to find it

At weather.gov, search for your city or ZIP code. The forecast page for your location has a link to an hourly weather graph, which plots the next few days of temperature, dew point, wind, sky cover, rain chance and relative humidity on one screen. There is also a table version. Many weather apps show hourly data too, but the NWS graph puts all the lines that matter for a project in one place. It is the same National Weather Service data the forecasts on this site use.

The lines that matter

Temperature

Look at more than the high. Note when the temperature climbs above your product’s minimum in the morning, how long it stays there, and how quickly it falls in the evening. For products that need the temperature to hold, such as driveway sealer, deck stain and concrete, follow the line through the night.

Dew point

Plotted on the same chart as temperature. When the two lines are close, the air is near saturation. Where they converge in the evening, dew is likely, and a coating that hasn’t set can be damaged. Many coating labels want a gap of at least 5°F. See dew point for painting and staining.

Chance of precipitation

The hourly probability of measurable rain at your location. Look at the whole span from when you start to the end of your product’s rain-safe time, not just the working hours. See what a chance of rain means.

Rain amount

The quantitative precipitation forecast gives the expected amount, often for 6-hour blocks. A high chance of a few hundredths of an inch is very different from a high chance of an inch. For fertilizer and pre-emergent, some rain is welcome, but a downpour isn’t.

Wind and gusts

Sustained wind and gusts are listed separately. Gusts matter for spraying, spray painting and working on ladders. Very light wind at night and early morning, together with a clear sky, points to dew and, for weed killer, the still air in which spray drift is most likely.

Sky cover

Clear skies mean hot surfaces in the day and fast cooling at night, with more dew. Cloudy nights stay warmer and wetter-looking but produce less dew. Full sun on dark surfaces in summer is a reason to work early or in the shade.

Relative humidity

High humidity slows drying; very low humidity with wind and heat dries concrete too fast. For concrete, the combination of temperature, humidity and wind decides the evaporation rate, which is what the concrete forecast checks hour by hour.

Reading it for three projects

• Pouring a slab: find a morning with temperatures in the 50s to 80s, moderate wind and no rain for the first several hours after finishing. Then check that the next two nights stay well above freezing. On hot, dry, windy days, the early morning is the window.
• Pressure washing before staining: the wash itself is easy: just avoid freezing nights. The stain is the hard part, so count the dry days after washing. You need the wood to dry out, and then a window for the stain.
• Spray painting a small project outdoors: look for a mild hour with wind under about 10 mph, humidity not too high and the air well above the dew point. Late morning to early afternoon is usually best.

How much to trust it

• Days 1 to 3: generally reliable for planning, though shower timing can still shift by a few hours.
• Days 4 to 7: good for spotting likely days; less reliable for hourly timing. Confirm closer to the day.
• Scattered summer storms: the chance is an honest statement that some places will get rain and others won’t. Keep a tarp handy.
• Your yard is not the grid cell. Forecast values describe an area about 2.5 km across. Valleys, lakeshores, hills and tree cover change the local picture.

Read the forecaster’s notes

Each National Weather Service office publishes an Area Forecast Discussion, linked from the forecast page. It is written by the forecasters and explains their reasoning: how confident they are about rain timing, whether a front might stall, whether fog or frost is possible. It uses some jargon, but for a big project, a quick read can tell you whether a “20% chance” is a confident low number or a guess.

Let the forecast do the counting

Reading an hourly graph for every project and every day of the week takes time. The project forecasts on this site read the same National Weather Service hourly data and apply each project’s limits for you, then show which hour caused a problem. Use them to narrow the week down, and the hourly graph to check the details of the day you choose.