Weather Underground

Forecasts

All of our forecasts are generated from our proprietary forecasting system that leverages our vast amount of neighborhood weather data that we get from our community - we refer to the system as BestForecast™.

Our ever-expanding network of 250,000+ personal weather stations is the largest of its kind and provides us with a unique ability to provide the most local forecasts based on actual weather data points. BestForecast™ uses the most innovative forecast models available and cross-verifies their output with all of the localized data points. Only our unrivaled amount of local neighborhood weather data can generate forecasts for your front door.

For US locations we give users the option to switch to view the forecasts generated from the National Weather Service’s National Digital Forecast Database (NDFD). In a step to create an unprecedented level of transparency to our forecasting, we publish the recent accuracy of our forecasts for every US location alongside the accuracy of the NDFD forecasts.

What’s the Difference?

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U.S. Current Conditions

U.S. current conditions data comes from 180,000+ weather stations across the country including:

• Almost 2,000 Automated Surface Observation System (ASOS) stations located at airports throughout the country. These are maintained by the Federal Aviation Administration and observations are updated hourly, or more frequently when adverse weather affecting aviation occurs (low visibility, precipitation, etc).
• Over 250,000 Personal Weather Stations (PWS's) that are part of Weather Underground's ever-expanding PWS network. Stations are put through strict quality controls and observations are updated as often as every 2.5 seconds.
• Over 26,000 weather stations that are part of the Meteorological Assimilation Data Ingest System (MADIS) which is managed by the National Oceanic and Atmospheric Administration (NOAA). For further information, see https://madis.ncep.noaa.gov/.

When a visitor requests current conditions from wunderground.com, the geographically closest station is displayed. There is also a Station Select button, which shows a list of the next closest stations.

International Current Conditions

International current conditions are collected directly from more than 29,000 weather stations located in countries around the globe including:

• About 6,000 automated weather stations operating at airports. Typically these stations are owned by government agencies and international airports and data is updated at 1, 3, or 6 hour intervals, depending upon the station.
• Over 8,000 Personal Weather Stations (PWS's) and 16,000 MADIS stations.

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Australian Radar

Some data on this site is sourced from the Bureau of Meteorology. Check here for details.

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COOP Station Snow Data

The snow depth information listed by state is taken by volunteers in the National Weather Service (NWS) Cooperative Observer Program (COOP). More than 11,000 NWS-trained volunteers take observations on farms, in urban and suburban areas, National Parks, seashores, and mountaintops.

The COOP was formally created in 1890 under the Organic Act. Its mission is two-fold:

1. To provide observational meteorological data, usually consisting of daily maximum and minimum temperatures, snowfall, and 24-hour precipitation totals, required to define the climate of the United States and to help measure long-term climate changes.
2. To provide observational meteorological data in near real-time to support forecast, warning and other public service programs of the NWS.

For more detailed information about real-time and historical snowfall measurements from the COOP network, visit the National Climatic Data Center.

Snow Imagery

The snow depth images are taken from a dataset prepared by the United States Air Force (USAF). The data is compiled from a variety of surface and satellite-based measurements for the Northern Hemisphere. The data is updated once per day at about 7pm EST. The data has problems in many areas. In particular, it does not do well in Michigan's Upper Peninsula.

SNOTEL data

Snow data for over 600 locations in the Western U.S. mountains is available from the U.S. Department of Agriculture's SNOTEL network. These stations report snow depth, but not snowfall amount. For more detailed information about real-time and historical snowfall measurements from the SNOTEL network, visit the U.S. Department of Agriculture.

Other Snow Resources:

• Experimental National Snow Analysis
• California Cooperative Snow Surveys
• Rutgers Snow Data Resource Center

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Map Information

Temperature	A color-filled contour map showing current temperature. The key below the image shows the corresponding temperature value for each color.
Radar Map	A radar map showing the current precipitation data for the United States.
Visibility	A color-filled contour map showing current visibility. Visibility is the maximum horizontal distance that can be seen. The key below the image shows the corresponding visibility for each color. Note that most observing stations in the U.S. use are automated, and use a visibility sensor that has a maximum range of 20 miles. Thus, the maximum visibility reported in many areas is 20 miles, even though it is actually much greater.
Wind	A color-filled contour map showing current wind speed. Wind vector arrows are also displayed to show the wind direction. The key below the image shows the corresponding wind speed for each color.
Heat Index	A color-filled contour map showing current heat index. Heat index is the apparent temperature considering both the temperature and relative humidity. The key below the image shows the corresponding heat index for each color.
Wind Chill	A color-filled contour map showing the current wind chill. Wind chill is the apparent temperature considering both the temperature and the wind speed. The key below the image shows the corresponding wind chill for each color.
Humidity	A color-filled contour map showing the current relative humidity. Relative humidity is the ratio of water vapor contained in the air to the maximum amount of water vapor that can be contained in the air at the current temperature. The key below the image shows the corresponding relative humidity for each color.
Dew Point	A color-filled contour map showing the current dew point. Dew point is the temperature to which the air needs to be cooled in order for the relative humidity to reach 100 % (when a cloud would form). The key below the image shows the corresponding dew points for each color.
UV Forecast	A color-filled contour map showing the forecasted Ultra Violet index (UV index) at noon local time. The UV index indicates the strength of the sun's ultraviolet rays that cause sunburn. The higher the number, the stronger the UV rays coming in contact with your body. The key below the image shows the corresponding UV index and minutes to skin damage at noon for each color. For more information on UV index, see The Climate Prediction Center.
Satellite Map	The current infrared satellite image for the United States. Infrared satellite images indicate the temperature of cloud tops. The cooler the clouds, the brighter the color on the map. Since air temperature generally decreases with increasing altitude, the cooler (brighter) the clouds, the higher the clouds. Unlike the visible satellite image, infrared satellites work at night.
Fronts	This map shows the current radar, fronts positions, and areas of high and low pressure for the United States. Blue fronts are cold fronts, red fronts are warm fronts, alternating red and blue fronts are stationary fronts, and purple fronts are occluded fronts. Front positions are updated every 3 hours.
Snow	A color-filled contour map of the current snow depths across the United States. The key below the map shows the corresponding snow depths for each color. The data is taken from a U.S. Air Force data set that is updated once per day at about 8pm EST. The data appears to have underestimation errors in Michigan's Upper Peninsula.
Flight Rules	A map depicting the the flight conditions at each airport that reported in the past 2 hours across the United States. Visual Flight Rules (VFR) - the pilot has the responsibility for seeing and avoiding other aircraft, towers, mountains, etc. Marginal Visual Flight Rules (MVFR) - conditions between VFR and IFR. Generally defined as visibility between 3 and 5 miles, and ceilings between 1,000 and 3,000 feet. Instrument Flight Rules (IFR) - the pilot may be in clouds, flying solely by instruments, and therefore unable to see other aircraft. The controller takes the responsibility for aircraft separation. Low Instrument Flight Rules (LIFR) - ceiling below 500 feet AGL and/or visibility less than one mile.
Jet Stream	A color-filled contour map of the wind speed where the atmospheric pressure is 300 hPa. Arrows showing the direction the wind is blowing at that level are overlaid. A pressure of 300 hPa corresponds to an altitude of approximately 8000 - 10000 meters above ground--the altitude where the jet stream is typically found. The data shown is the 6-hour forecast from the AVN computer model, which is run twice daily at 00 and 12 GMT by the National Center for Environmental Prediction (NCEP). The jet stream images are updated twice per day, at approximately noon and midnight EST.
Ozone Pollution	The data plotted is sent to us approximately one hour after it is collected, between 8am and 8pm, from the The Environmental Protection Agency (EPA). Data is currently missing for eight states, but data for some of these states will be added over the next year. The EPA uses its Air Quality Index to provide general information to the public about air quality and associated health effects. An Air Quality Index (AQI) of 100 for any pollutant corresponds to the level needed to violate the federal health standard for that pollutant. For ozone, an AQI of 100 corresponds to 0.08 parts per million (ppm) over an 8-hour period -- the current federal standard. Over half of the U.S. population lives in areas where the AQI exceeds 100 and violates the federal health standard at least once per year.
NDFD Graphical Forecasts	Data for these maps are gathered from the National Digital Forecast Database (NDFD). Currently we offer Prevalent Weather, Temperature, Maximum Temperature, Minimum Temperature, Dew Point, Relative Humidity, Sky Cover, Precipitation Amount, 12 hour Probability of Precipitation, Snowfall, and Convective Hazard Outlook maps for the the lower 48 US states. All of these are considered to have "Operational" status. We will provide additional NDFD-derived imagery as more products / regions become operational. For more information regarding the NDFD, please visit the NDFD Home Page.