National Snow Analyses 3D Pro Interface

Instructions

Below are several links to ".kmz" files suitable for viewing with KML interpreters (such as Google Earth). Software and plugin information can be found here. More information on these products can be found below.

Information for Internet Explorer users

Snow Model and Observation Deltas

• The "delta" file displays the most recent differences between observed and modeled snow at observing stations, based on data gathered over a 24-hour period. Differences are separated into three categories: snow water equivalent, snow water equivalent derived from snow depth, and snow density. Note that all three categories are displayed by default. Display of any category may be toggled using the radio buttons in the "Places" section of a KML viewer.

  Clicking on the symbol at any station will trigger the display of a balloon showing more detailed information for that station and a web link to a time series at that station from the "Interactive" section of our website.

  Be advised that data provided by automated stations sometimes suffer from quality control problems. Furthermore, data provided by stations do not necessarily represent conditions in the surrounding area. However, nearly all the most recent available SWE, snow depth, and snow density observations for a 24-hour period over the coterminous U.S. are included in the "delta" file, even though many will not be used in snow data assimilation. The snow data assimilation process includes a critical manual element, and the "delta" KMZ file illustrates the raw material from which NOHRSC staff initiate that process.

  1. Snow Water Equivalent

     The difference between observed and modeled SWE is the most direct comparison available, and is the most valuable resource contributing to snow data assimilation. Values in red represent stations where the observed SWE is larger than the modeled SWE, while values in blue represent the opposite scenario. A legend is included in the KMZ file and is displayed by default.

  2. SWE derived from observed snow depth

     The second category of data in the "delta" file is the difference between SWE derived from observed snow depth (i.e., observed snow depth multiplied by modeled snow density) and modeled SWE. The legend of symbols and colors for these observations is identical to that used for direct SWE observations. The utility of the differences obtained in this manner is limited by the accuracy of modeled snow pack density.

     Differences in SWE based on snow depth are shown only for those stations which reported snow depth during the observing period, but did not report SWE.

  3. Snow pack density

     The third category of data in the "delta" file is the difference between observed and modeled snow pack density. A separate legend is included for density differences. Reds and oranges represent cases where modeled density is lower than observed density, while greens and blues indicate the opposite.

     Density observations are not explicitly provided by observing stations; however, many stations, particularly in the Western U.S., routinely observe both SWE and snow depth. For any given "delta" file, if a station has observed SWE and snow depth within six hours of each other, those values are used to compute a pseudo-observation of snowpack density. It is actually rare for snow depth and SWE observations to occur at different times at a station (for a given day the number of stations where this occurs can usually be counted on one hand), but because of the scarcity of density observations, particularly in the Eastern U.S., and the importance of modeling density accurately, the looser restriction is sometimes valuable. Occasionally during the snow season, as seasonal snow packs evolve, snow density assimilation is performed in order that observations of snow depth can more effectively contribute to the SNODAS.

Snow Water Equivalent Nudging Fields

• The "nudge" file displays the total nudging applied to modeled snow water equivalent (SWE) for an assimilation cycle of the NOHRSC Snow model (NSM). Blue areas indicate the addition of SWE, while orange areas indicate SWE removal. A color scale is included, and appears across the top of the display.

  When snow data assimilation is performed, observations of snow depth and snow water equivalent (SWE) judged representative of their surrounding area are compared with corresponding NOHRSC Snow Model (NSM) values to produce a field of differences. These differences are used to produce a raster of "nudging" values, which are applied to a repeat six-hour run, or assimilation cycle, of the NSM. During the snow season, assimilation is performed routinely, but not more than once a day, and then only when differences between representative observed snow and NSM values are seen to be spatially correlated. If assimilation is performed, it typically occurs over the 06-12 UTC model cycle.

Snow Analyses Overlays

• The "Snow Analyses Overlays" are small .kmz files which drape overlays of snow model input and output rasters over terrain. These files do not contain the imagery itself, rather they point to data available online. Clicking on the link should open and load the imagery into the client application.
• By default, the daily image of NSA snow water equivalent will be displayed. To view different rasters, deselect the checkbox corresponding to the currently-displayed raster, and select the checkbox corresponding to the raster that you wish to view.

Snow Reports and Stations

• The "Snow Reports and Stations" section contains links to larger .kmz files which provide placemarks for stations that have recently reported snow.* Click on the link to the desired file, and your browser should open it in the client.
• A large collection of placemarks (normally around 3000) will be displayed. More than likely, you will need to zoom in before you can distinguish among them. Each displayed placemark consists of an icon and a station identifier.
• Stations denoted with a red snowflake have reported snow during the last 24 hours. Stations denoted with a blue snowflake have reported snow during the last week, but not during the last 24 hours. Stations denoted with a transparent blue snowflake (these are rare) have reported snow during the last two weeks, but not during the last week. Stations denoted with the NRCS logo are automated SNOTEL stations.
• Clicking on the placemark for a station creates a pop-up label with a more detailed description of that station, including its name, type, and elevation. At least two other links will also appear in the label: The "Station Info" link will open your browser to a page showing terrain and satellite imagery centered on the reported location of the station. The "NSA Time Series" link will open your browser to a page showing several graphs depicting time series of NSA data and station observations.

  *Note that a station may report zero values, and that many do this routinely. The presence of a placemark for a station does not necessarily mean that there is snow at the station.

Notes for Internet Explorer users

Due to peculiarities with some versions of Internet Explorer, opening the ".kmz" files can require additional instructions. There are two methods of dealing with Internet Explorer's handling of the files; either of them should work.

• Click on a link to a .kmz file.
• Internet Explorer will prompt whether you want to open or save this file. If the name of the file shown in the dialog has a .kmz suffix, you should be able to open it directly with the KML intepreter. If it has a different suffix (for example: delta_2006192396.zip), another step has to be taken.

• Click the Save button.
• In the next Save As dialog, change the saved filename so that it ends with .kmz (as it originally did), and click Save.
• Double-click the saved file, and the KML interpreter should open it.

• Click the Open button.
• Internet Explorer will either let you choose an unzipping program, or will open it with one that was already specified.
• This will unzip the file, showing the .kml file inside.
• Double-click that file, and the KML interpreter should open it.

National Weather Service National Operational Hydrologic Remote Sensing Center Office of Water Prediction 1735 Lake Drive W. Chanhassen, MN 55317
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