Precipitation/Meteorology

NOAA Atlas 14, IDF statistics: https://hdsc.nws.noaa.gov/pfds/

NetStorm Atlas, IDF statistics: http://www.dynsystem.com/netstorm/IDFStats.htm

NOAA, 1-min/5-min archives: https://www.ncei.noaa.gov/products/land-based-station/automated-surface-weather-observing-systems

NOAA, hourly/15-min archives: https://www.ncei.noaa.gov/maps/hourly/

NOAA, all duration archives: https://www.ncei.noaa.gov/maps/alltimes/

NOAA, Climate normals: https://www.ncei.noaa.gov/access/us-climate-normals/

https://www.cocorahs.org/

Hydrometry/Hydrography

USGS, National Water Dashboard: https://dashboard.waterdata.usgs.gov/app/nwd/en/?aoi=default

USGS, National Water Information System: https://waterdata.usgs.gov/nwis/

NOAA, Tide level archives and forecasts: https://tidesandcurrents.noaa.gov/

non-governmental organization, Model my Watershed: https://modelmywatershed.org

Topography

USGS, The National Map: https://apps.nationalmap.gov/downloader/

USGS, LiDAR Explorer: https://apps.nationalmap.gov/lidar-explorer/#/

NOAA, Lidar/Land Cover/Topography: https://coast.noaa.gov/dataviewer/#/  

Buildings:

FEMA, National Buildings layer: https://gis-fema.hub.arcgis.com/pages/usa-structures

OpenStreetMap (gives roads etc.): https://www.openstreetmap.org/

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Parameters:

MRLC, Hydrography/Impervious Surface/Tree Canopy/Land Cover: https://www.mrlc.gov/viewer/

NOAA Improved Impervious Layer: https://www.ecopiatech.com/resources/news/high-resolution-land-cover-data-from-ecopia-ai-now-publicly-available-through-noaa

USDA, WebSoil Survey: https://websoilsurvey.nrcs.usda.gov/app/

USA State Specific GIS

Pennsylvania: https://www.pasda.psu.edu/

Virginia VGIN: https://vgin.vdem.virginia.gov/

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DEMs/LIDAR/Aerial Imagery:

NOAA, Lidar/Land Cover/Topography: https://coast.noaa.gov/dataviewer/#/ 

USGS, The National Map: https://apps.nationalmap.gov/downloader/ 

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Watershed Delineation Boundary Tool:

USGS: https://streamstats.usgs.gov/ss/

Stroud Water Research Center: https://modelmywatershed.org/ 

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Gage Station Data:

USGS, National Water Dashboard: https://dashboard.waterdata.usgs.gov/app/nwd/en/?aoi=default 

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Rainfall Design Storm Depths:

NOAA Atlas 14, IDF statistics: https://hdsc.nws.noaa.gov/pfds/

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Historic Extreme Event Data:

Imagery NOAA: https://storms.ngs.noaa.gov/

Gage Data/Images USGS: https://stn.wim.usgs.gov/FEV/

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New Free Online Mapping Tool!:

https://felt.com/  

Sample Map of My Hometown :-)

EPA Stormwater Calculator:

(Gives SWMM formatted files for multiple year rain gauge data and temperature data for evapotranspiration - also has basic land use parameter estimations of infiltration rates):

https://swcweb.epa.gov/stormwatercalculator/