GIS Compatible Digital Forecast Information
GIS Compatible Digital Forecast Data Information & Help
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General Information
This digital service provides GIS-compatible weather forecast files which are derived from the National Weather Service gridded forecasts. They are similar to the NWS National Digital Forecast Database (NDFD) grids, except their resolution is 2.5 km instead of 5 km as are the NDFD grids and they are available directly for downloading in shapefile format. The 2.5 km resolution corresponds roughly to a 1 mile by 1 mile square area in size. Forecast grids are available for weather elements at up to 3-hourly resolution and out as far as 7 days, depending on the data set. The elements in the digital service are available for multiple pre-defined sectors including regional, Geographic Area Coordination Center (GACC), and local weather forecast office (WFO) scales.
Zipped Files and Instructions for Downloading
To download a zipped shapefile from the digital forecast database, click one of the predefined domains then click on any of the file links. Most of the data is provided as a polygon shapefile. Wind data are provided as points for wind speed, wind direction and wind gust.
Shapefiles are updated throughout the day. The zipped files are listed on the web page according to the date and time for which the forecast is valid. In the example to the right, the first file in the Max/Min Temp and RH (Temperature and Relative Humidity) column is named "Day 1 (Aug 18)". This file contains the Day 1 forecast of Maximum and Minimum temperature and relative humidity for August 18.
Clicking the name will allow you to download the file. The zipped files are named in a standard convention according to the number of hours from the creation time that the forecast is valid for. Since the Day 1 grid is a 24 hour forecast the zip file in this example is named mxmn_f24hr.zip, with f24hr denoting that it is a 24 hour forecast. These zipped files contain a set of shapefiles. The individual shapefiles are named according to the valid time for the forecast. This will correspond with the date/time listed on the webpage (see above figure). Continuing in this example, the shapefiles, once unzipped, are named "mmWX0804" where "mmWX" denotes the minimum and maximum temperature and relative humidity and "0804" denotes the valid date/time of the forecast, August 18. Note files available at 3 or 6 hourly intervals, such as winds, will have a forecast hour in the file name. The times given for these files on the website are provided in local time.
Windows XP comes standard with a Zip file reader, so all you need to do is double click on the shapefile name to open the compressed file. If you are not running Windows XP, you may need to download a Zip reader software, like pkzip first to unzip the shapefiles.
Explanation of Weather Elements
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Max Temperature
(MaxT) |
The daytime maximum temperature in degrees Fahrenheit out 7 days. |
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Min Temperature
(MinT) |
The overnight minimum temperature in degrees Fahrenheit out 7 days. |
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Relative Humidity
(RH) |
The ratio, expressed as a percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperatures. Displayed every three hours out 72 hours. |
Max Relativity
(Max RH) |
The overnight maximum relative humidity (see above for explanation of Relative Humidity) out 7 days. |
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Min Relativity
(Min RH) |
The daytime minimum relative humidity (see above for explanation of Relative Humidity) out 7 days. |
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Lightning Activity Level
(LAL) |
This is an index of Lightning Activity Level. LAL 1 - No thunderstorms; LAL 2 - Few building cumulus with isolated thunderstorms; LAL 3 - Much building cumulus with scattered thunderstorms. Light to moderate rain; LAL 4 - Thunderstorms common. Moderate to heavy rain reaching the ground; LAL 5 - Numerous thunderstorms. Moderate to heavy rain reaching the ground. LAL 6 - Dry lightning (same as LAL 3 but without the rain). Displayed every six hours out 72 hours. |
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Haines Index
(Haines Ix) |
The Haines index is also called the Lower Atmosphere Stability index. It is composed of a stability term and moisture term. The index has been shown to be correlated with large fire growth on initiating and existing fires when surface winds do not domination fire behavior. Haines Indices range from 2 to 6 for indicating potential for large fire growth. Displayed every three hours out 72 hours. |
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Wind Speed
(Spd) |
The expected sustained 10-meter wind speed (in knots) for the beginning hour of the period. Displayed every three hours out 72 hours. |
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Wind Direction
(Dir) |
The expected sustained 10-meter wind direction from which the wind is blowing for the beginning hour of the period, using 36 points of a compass. Displayed every three hours out 72 hours. |
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Wind Gust
(Gst) |
The maximum 3-second wind speed (in knots) forecast to occur within a 2-minute interval at a height of 10 meters. Wind gust forecasts are valid at the top of the indicated hour. Displayed every three hours out 72 hours. |
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Quantitative Precipitation Forecast
(QPF) |
QPF stands for the Quantitative Precipitation Forecast. This forecast is for the total amount of precipitation expected for every 6 hour window. In the event of snow, this value represents the liquid water equivalent. Displayed in six hour periods out 60 hours. |
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Probability of Precipition
(POP) |
The likelihood of occurrence (expressed as a percent) of a measureable amount of liquid precipitation (or the water equivalent of frozen precipitation) during a 6 hour period of time. Displayed every six hours out 60 hours. |
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Temperature
(Temp) |
Is the expected temperature in degrees Fahrenheit valid for the indicated hour. Displayed every three hours out 72 hours. |
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Sky Cover
(Sky) |
Is the expected amount of opaque clouds (in percent) covering the sky, valid for the indicated time period. Displayed every three hours out 72 hours. |
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Heat Index
(Heat Ix) |
The "Apparent Temperature" is an accurate measure of how hot it really feels when the Relative Humidity (RH) is added to the actual air temperature. Displayed every three hours out 72 hours. |
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