The short answer
A marathon weather 10 day forecast is a rough guide to whether race week will be warm or cool, not an hour-by-hour plan. NOAA says a ten-day forecast is right only about half the time, against about 80% at seven days and 90% at five. Plan from race-day history first, then firm up inside five days.
Key takeaways
- Ten days out, read the forecast as a trend; inside five days, start planning kit and pace.
- Before the forecast is useful, race-day history shows the range you could face.
- Cloud alone can move heat stress by a whole level, so late changes matter.
- Make final pacing and kit decisions from the forecast one to two days out.
On this page
- Can you trust a marathon weather 10 day forecast?
- How much does forecast accuracy drop with each extra day?
- What should you check at 10 days, 5 days and the night before?
- What does race day usually look like before the forecast is useful?
- Why can a small forecast change make a big difference on race day?
- How should your race plan change as the forecast firms up?
- Questions runners ask
- Sources and method
Words this guide uses
- Lead time
- How far ahead of the event a forecast is made; a ten-day lead time means the forecast was issued ten days before.
- Pace-timed forecast
- The weather for each stretch of your route at the time you'll run it, from your start time and pace.
- Reanalysis
- A reconstruction of past weather from observations and a weather model; ERA5 is the best-known one.
Can you trust a marathon weather 10 day forecast?
Trust a marathon weather 10 day forecast for the broad picture only: a warm spell, a cold snap, an unsettled week. NOAA says a forecast ten days or more ahead is right only about half the time.
That doesn't make it useless. It tells you which end of the race's normal range to prepare for. But don't set your pace on it. As our running weather forecast hub explains, the numbers that decide your race are the heat and humidity at the hours you'll be running, and those firm up much closer to the day.
Apple Weather's hourly forecast, which Map your run uses, reaches ten days ahead. So the tool can show your race kilometre by kilometre from ten days out. Treat that first look as a draft.
How much does forecast accuracy drop with each extra day?
It drops steadily. NOAA's National Environmental Satellite, Data, and Information Service gives three figures: about 90% at five days, about 80% at seven, and about half at ten or more.
| Lead time | How reliable, per NOAA | What to use it for |
|---|---|---|
| 10 days or more | Right about half the time | Warm or cool spell; pack for both ends |
| 7 days | About 80% | Likely range of temperatures; draft two plans |
| 5 days | About 90% | Kit choices, travel, rough pacing plan |
| 1–2 days | Most reliable short of race morning | Final pace, drinks, start layers |
NOAA's page doesn't define "accurate", so read these as a general guide rather than a promise for your city. The trend, though, is clear and widely shared.
Forecasts also keep improving. ECMWF, the European forecasting centre, reported in 2010 that its skill had improved by about one day per decade since the early 1980s. Its measure is the large-scale weather pattern, not the temperature on your street. The Met Office says its four-day forecast is now as accurate as its one-day forecast was 30 years ago.
What should you check at 10 days, 5 days and the night before?
Check the trend at ten days, the range at five, and the hour-by-hour detail from two days out. Make your decisions in that order.
- Ten days out: is a warm or cold spell coming? Read the race-day history and plan for both ends.
- Five days out: settle your kit and travel. Write two pacing plans, one for a cool day and one for a warm one.
- Two days out: check the forecast at your start, halfway and last 10 km (6.2 miles). Choose your pacing plan.
- The night before: check again and lay out your kit. Look for rain timing and any change in cloud.
- Race morning: a final look, and the organisers' instructions. Their flags override any forecast.
Our race-day forecast guide shows how to read the numbers at each of those hours.
What does race day usually look like before the forecast is useful?
Race-day history is your best guide until about five days out. It shows the range of weather the race has thrown at runners, and how wide it is.
These ranges come from RunFast's ERA5 race history, 2015–2024. There was no 2020 race at any of the four. Boston 2021 ran on 11 October, London 2021 and 2022 in early October, and Comrades skipped 2020 and 2021 and ran on 28 August in 2022. Reanalysis is not a thermometer at the start line.
| Race | Coldest start air, °C (°F) | Warmest start air, °C (°F) | Peak heat stress at a 4:00 finish (9:00 at Comrades) |
|---|---|---|---|
| Boston Marathon, USA | 3.2 (38), 2018 | 21.1 (70), 2017 | Low to Moderate |
| London Marathon, UK | 5.8 (42), 2016 | 18.9 (66), 2018 | Low to Moderate |
| Berlin Marathon, Germany | 8.2 (47), 2024 | 16.5 (62), 2021 | Low to Moderate |
| Comrades Marathon, South Africa | 4.9 (41), 2023 | 17.3 (63), 2015 | Moderate to High |
An 18°C (32°F) spread at Boston's start means two very different races. Ten days out, the forecast tells you which end you're nearer. Five days out, it starts to tell you the hour. See every year on the Boston race page.
Why can a small forecast change make a big difference on race day?
Because heat stress depends on the sun as much as the air. A forecast that swaps sunshine for cloud can move WBGT by a whole level without changing the temperature.
Take 24°C (75°F), 70% humidity and a light wind. In full sun at midday, the RunFast engine puts WBGT at 26.0°C (79°F), High. Under thick cloud it's 21.0°C (70°F), Moderate. That 5°C (9°F) gap comes from cloud alone.
Rain works the same way on a cool day. Boston's 2018 start was 3.2°C (38°F) in cold rain, and the feels like when wet fell below freezing. A dry start at the same temperature is a different race.
How should your race plan change as the forecast firms up?
Start with two plans and drop one. Write a cool-day pace and a warm-day pace a week out, then choose two days out.
If the final forecast puts you in Moderate heat stress for the second half, the warm-day plan starts slower and spends less early. If it's High or worse, slow down further and take the organisers' advice seriously. If it's cold and wet, plan layers you can shed and keep your hands warm.
Whatever the forecast says, the race's medical team and flags have the final word. For what heat does to your pace in numbers, see our hub on running in heat and humidity.
Questions runners ask
Why are long-range forecasts less reliable?
Weather models start from today's observations, which are never perfect, and the atmosphere magnifies small errors as time goes on. NOAA puts it simply: models have to use estimates and assumptions, and those estimates become less reliable the further ahead you look. That's why a ten-day forecast is a trend, not a timetable.
Is a 14-day forecast worth checking before a marathon?
Only as a loose hint. NOAA says forecasts of ten days or longer are right only about half the time. Two weeks out, you learn more from the race's history: the range of start temperatures and heat stress in past years. Use that range to pack for both ends, then let the forecast narrow it.
Has weather forecasting got better over time?
Yes, steadily. ECMWF, the European forecasting centre, reported in 2010 that its forecast skill had improved by about one day per decade since the early 1980s. The Met Office says its four-day forecast is now as accurate as its one-day forecast was 30 years ago. Today's five-day forecast is a sound basis for planning.
Should I change my goal time because of a forecast a week out?
Not yet. A week out, NOAA's figure is about 80% right, so the details can still change. Write two plans instead: your goal pace for a cool day and a slower one for a warm day. Choose between them two days out, when the hour-by-hour forecast for your course is far more reliable.
What if the forecast changes on race morning?
Go with the latest forecast and the organisers' instructions. Check the hour-by-hour forecast for your finish time when you wake, and look for any heat or weather alerts from the race. If heat stress has moved up a level, start slower. If rain has arrived, keep a throwaway layer on for longer and protect your hands.
See the heat at your pace.
Drop in your route (GPX, FIT or TCX), pick the day and your pace, and see the feels like, dew point and heat stress for every kilometre, timed to when you’ll be there. Free, in your browser, no sign-up.
Sources and method
- NOAA National Environmental Satellite, Data, and Information Service (2026). How reliable are weather forecasts?
- European Centre for Medium-Range Weather Forecasts (ECMWF) (2010). Landmark in forecast performance
- Met Office (2026). How accurate are our public forecasts?
- Journal of Occupational and Environmental Hygiene (2008). Modeling the wet bulb globe temperature using standard meteorological measurements (Liljegren et al.)
- Copernicus Climate Change Service (ECMWF) (2026). ERA5 hourly data on single levels from 1940 to present
Forecast reliability figures are quoted from NOAA NESDIS, ECMWF (2010) and the Met Office as written; NOAA does not define "accurate" on its page, so treat its percentages as a general guide. ECMWF's measure is the forecast day at which the anomaly correlation of 500 hPa height (the large-scale pattern) drops to 60%, not surface temperature at a place. Race-day ranges come from RunFast's race history: hourly ERA5 reanalysis at grid points along each course, at the official start and an even-paced 4-hour finish, 2015–2024, with the gaps noted in the text; reanalysis is not a thermometer at the start line. The cloud example uses the RunFast engine's WBGT model (Liljegren et al., 2008) at 24°C (75°F), 70% humidity and a 10 km/h (6 mph) wind, with the sun 60° high: 900 W/m² of sunshine against none. Imperial values are conversions.
Wes Thomson
Wes Thomson builds RunFast, which times the heat, humidity and dew point to a runner's own pace along the route, and its sister Windline, live wind forecasts for cyclists. Both run on the same forecast engine. He writes these guides with one focus: what the weather will do to your run, kilometre by kilometre, and how to plan around it.