FAQ - Sources
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U.S. Forecasts
All U.S. forecasts and advisories on the wunderground.com web site are generated by the National Weather Service (NWS). The NWS transmits the data to a satellite data feed called NOAAPORT that insures rapid delivery of NWS information within seconds of when it is sent out. The Weather Underground's maintains a NOAAPORT satellite dish at its co-location facility in Santa Clara, California that continually collects NWS information. The NWS issues forecasts specific for every county in the U.S. two to four times per day, or more often when conditions warrant. If you feel that there is a problem with the forecasts for your city, the best thing to do is go to: http://www.nws.noaa.gov to complain to your local NWS office.
We have a National Weather Service forecast for every county in the U.S. However, there is not an airport in each county that reports weather conditions, so for those counties, we report the weather forecast for that county, and the current conditions from the nearest airport that sends weather reports to the National Weather Service.
Current Conditions
Current condition data also comes from the NWS NOAAPORT data feed. Most U.S. current condition data comes from the Automated Surface Observation System (ASOS) stations located at about 1600 airports throughout the country. The Federal Aviation Administration maintains these sites and provides location and contact information. The current condition information is updated once per hour at about 10 minutes before the hour, or more frequently when adverse weather affecting aviation occurs(low visibilities, precipitation, etc). Some stations using human weather observers (typically military bases) do not report conditions at night When a user requests a U.S. forecast from wunderground.com, the geographically closest current condition station is displayed, along with a pull-down menu of the three next closest stations.
Over 95% of our Current Conditions data comes from automated weather stations installed at airports. There are always a few malfunctioning automatic weather stations; our current software does not screen the data to look for these malfunctioning stations, so you will see occasional locations with wacky weather readings. Feel free to report these to us.
International current condition data updates at 1, 3, or 6 hour intervals, depending upon the station. Some countries, such as Iraq, Afghanistan, and many countries in Africa, send no data at all. Transmission errors and equipment failures are common in the international data stream; when a station goes more than 24 hours without sending a report, we will not display the current condition (or forecast) information for that site. The Weather Underground receives data for about 6000 international cities.
Official current conditions for locations within Canada now come from Environment Canada.
International Forecasts
The International forecasts are generated twice per day using output from the AVN computer forecast model run at the U.S. National Weather Service's National Center for Environmental Prediction. Custom software designed at The Weather Underground takes output from the AVN model and comes up with forecasts for 6000 international cities.
The forecasts do reasonably well for a large number of locations, but do poorly in some locations, particularly mountainous areas. We are developing new software enable us to improve these forecasts in the near future.
Weather Advisories
The advisories we post are indicated by the National Weather Service. If you should see an advisory that looks wrong, it is because they entered the wrong ending date, which means the advisory remains for longer than necessary. We must remove the advisory, manually, which we may not always catch. If you see an incorrect advisory, please feel free to contact us and let us know.
