NOUS41 KWBC 031925 AAA



Service Change Notice 17-41 Updated

National Weather Service Headquarters Silver Spring, MD

325 PM EDT Wed May 3 2017


To:       Subscribers:

          -NOAA Weather Wire Service

          -Emergency Managers Weather Information Network


          Other NWS Partners, Users and Employees


From:     Dave Myrick

          NWS Office of Science and Technology Integration


Subject:  Updated: Upgrade of National Water Model

          Effective: May 8, 2017


Updated to change implementation date from May 4 to May 8 due to

Critical Weather Day


Effective on or about Thursday May 4, 2017, beginning with the

1200 Coordinated Universal Time (UTC) run, the National Centers

for Environmental Prediction (NCEP) will begin operationally

running Version 1.1 of the National Water Model (NWM).


The NWM is an hourly cycling uncoupled analysis and forecast

system that provides streamflow for 2.7 million river reaches

and other hydrologic information on 1km and 250m grids.  It

provides complementary hydrologic guidance at current NWS River

Forecast Center river forecast locations and significantly

expanded guidance coverage and type in underserved locations.


The NWM ingests forcing from a variety of sources including

Multi-Radar Multi-Sensor (MRMS) radar-gauge observed

precipitation data and High-Resolution Rapid Refresh (HRRR),

Rapid Refresh (RAP), Global Forecast System (GFS) and Climate

Forecast System (CFS) Numerical Weather Prediction (NWP)

forecast data.  USGS real-time streamflow observations are

assimilated, and all NWM configurations benefit from the

inclusion of over 1,260 reservoirs.  The core of the NWM system

is the NCAR-supported community WRF-Hydro hydrologic model. 

WRF-Hydro is configured to use the NoahMP Land Surface Model

(LSM) to simulate land surface processes.  Separate water

routing modules perform diffusive wave surface routing and

saturated subsurface flow routing on a 250m grid, and muskingum-

cunge channel routing down NHDPlusV2 stream reaches.  River

analyses and forecasts are provided across a domain encompassing

the CONUS and hydrologically-contributing areas, while land

surface output is available on a larger domain that extends

beyond the CONUS into Canada and Mexico (roughly from latitude

19N to 58N). In addition, NWM forcing datasets are provided on

this domain at a resolution of 1km.


List of Enhancements in Version 1.1:


 1. Forecast cycling

    - Extension of short range forecast from 15 to 18 hours

    - Increased frequency of medium range forecast from 1 to 4

times per day

 2. Parameter Updates

    - Parameter calibration to reduce hydrologic biases

    - Improvement of the Multi-Radar/Multi-Sensor (MRMS), High

Resolution Rapid Refresh (HRRR) and Rapid Refresh (RAP)

precipitation blending in NWM Analysis

    - Improvement of HRRR/RAP precipitation blend in NWM Short

Range forecast

    - Incorporated a data-driven approach to channel parameter

estimates utilizing a regression model based on USGS cross-


 3. Stream Connectivity Refinements

    - Corrections to NHDPlusV2 streamflow connectivity errors

noted by RFCs

    - Addition of new oCONUS hydrologically contributing areas

 4. Additional Upgrades

    - Fix for overly large infiltration rate in sandy areas

during intense precipitation

    - Fixes to underlying topography leading to isolated extreme

OCONUS ponded water values

    - Fix for high altitude excessive snow melt

    - Fix for isolated negative underground runoff

    - Enhancements to USGS data decoder and preprocessor

    - Corrected locations and stream reach attributions to

several previously mismatched USGS gauge locations.

    - Shift from NetCDF3 to NetCDF4 output file format.  End

user software may need to be adjusted to adapt to this change. 

NetCDF version 4.2 or later libraries are recommended.

    - NetCDF metadata and data structure updates to improve

compatibility with NetCDF file readers--as above, decoders may

need to be adjusted to adapt to these changes.


NetCDF format changes: 


  1. In order to reduce the size of NWM output files and align

metadata with existing standards and conventions, a few changes

have been made to the files’ internal data structure. In

particular, many variables are now stored as integers with

associated scaling factors as opposed to floats. Most 3rd party

commercial and open-source applications, like ESRI ArcGIS, QGIS,

IDV, Panoply, and the Weather and Climate Toolkit should handle

the packed data automatically.  Further information on data

packing is available at:


  2. In addition, some dimension, variable, and attribute

changes have been made. It is highly recommended that users

review the available metadata before attempting to use the NWM

1.1 data files. A listing of file metadata changes is presented


All Files:

   - "time" dimension is now an UNLIMITED dimension

   - Units of "time" variable have changed to "minutes since

     1970-01-01 00:00:00 UTC" or epoch time

   - "reference_time" dimension and variable have been added

   - "units" attributes have been modified to align with CF


   - "scale_factor" and "add_offset" attributes have been added

     to variables containing packed data values

   - The "_FillValue" attribute has been added to all data



 "channel_rt" files:

   - "station" dimension and "station_id" variable have been

    renamed to "feature_id" and now include 2,716,897 reach



 "land" files:

   - "south_north" and "west_east" dimensions have been removed

   - "x" and "y" coordinate dimensions and variables have been


   - "soil_layers_stag" and "snow_layers" dimensions have been

    removed from all "land" files except for those associated

    with medium_range forecasts

   - "ProjectionCoordinateSystem" variable has been added


 "terrain_rt" files:

   - "x" and "y" coordinate dimensions and variables have been


   - "ProjectionCoordinateSystem" variable has been added


 "reservoir" files:

   - "station" dimension and "lake_id" variable have been 

    renamed to "feature_id"

   - "latitude" and "longitude" variables have been added


 "forcing" files:

   - "ncXX" dimensions have been removed

   - "x" and "y" coordinate dimensions and variables have been


   - "nv" dimension has been added to work with new

    "time_bounds" variable

   - "ProjectionCoordinateSystem" variable has been added


General Framework:


Version 1.1 of the NWM will be run in four configurations that

are largely similar to Version 1.0--Analysis and Assimilation,

Short-Range, Medium-Range and Long-Range.  Notable changes


1. The extension of short-range forecasts from 15 hours to 18


2. The increase in medium-range forecast cycling frequency from

one time per day (06Z) to four times per day (00Z, 06Z, 12Z and


For a complete description of the overall configuration, input

data sources and other elements, users are referred to the NWM

Version 1.0 Technical Implementation Notice.


End users are able to view the output via the interactive map

and image viewer on the Office of Water Prediction (OWP)

website,  Additionally, the

full set of NWM output and a subset of forcing files are

available on NCEP web services at:


Output Changes on the NCEP web services:


  1. The directory structure naming convention will be changing

for the forcing files, where:

  fe_analysis_assim/ -> forcing_analysis_assim/

  fe_short_range/ -> forcing_short_range/

  fe_medium_range/ -> forcing_medium_range/


  2. In version 1.0 all “nc” files were gzipped and labeled with

an extension “gz”. With version 1.1 the files use internal

compression, will no longer be gzipped and no longer contain

that extension. An example:

  Changes to

  Where CC is cycle  


  3. The file naming structure will be changing for the forcing

files, where:


    Changes to

    Where CC is cycle and ## is 00-02 (where tm is time-minus,

meaning the number of hours prior to valid cycle time)


    Changes to

    Where CC is cycle and ### is 001-018


    Changes to


    Where CC is cycle and ### is 001-240


  4. New file output will be available


    Where CC is cycle and ## is 00-02 (previously only ##=00 was



    Where CC is cycle and ## is 00-02 (previously only ##=00 was



    Where CC is cycle and ### is 001-018

    Where CC is cycle and ### is 001-018

    Where CC is cycle and ### is 001-018

    Where CC is cycle and ### is 001-018 (previously only

##=001-015 was available)


    Where CC is cycle and ### is 001-018 (previously only

##=001-015 was available)



    Where CC is cycle (00,06,12,18) and ### is 003-240

(previously only CC=06 was available)


    Where CC is cycle (00,06,12,18) and ### is 001-240

(previously only CC=06 was available)



  5. Users will find that long range products have enough of a

lag time in creation that they may appear in the previous day’s

output directory.  For example, long range mem 1 products for

the 18z cycle will not show up until the day after their

initialization time.  For this reason, users are encouraged to

look back in the previous dated directory for long range product



The 15 minute USGS observation timeslice files in the

corresponding usgs_timeslices directory will only contain files

valid on that date.  Previously, users could find data for the

previous day in the current directory, but with the new version

they will need to look back in the corresponding dated

directories to find older data. 


   6. One other item to note with this upgrade is that the

following files will be available 6-10 minutes sooner than with

version 1.0.


 Where CC is cycle and ## is 00-02


Most NWM NetCDF output files are directly viewable using

standard NetCDF visualization utilities.  The exceptions are the

point-type NWM channel output files containing streamflow and

other variables.  In particular, due to storage space

limitations, the latitude and longitude of each point are stored

outside of the file, but are available at:


Scripts are also available at this location which will append

this geospatial data to a user selected output file, enabling

viewing of channel output files within NetCDF visualization



A consistent parallel feed of NWM data is

available on the NCEP server via the following URLs:


NCEP encourages all users to ensure their decoders are flexible

and are able to adequately handle changes in content order and

also any volume changes which may be forthcoming. These elements

may change with future NCEP model implementations. NCEP will

make every attempt to alert users to these changes prior to any



For more general information about the NWM, please see:


Any questions regarding this implementation should be directed

below. We will evaluate any feedback and decide whether to



For questions on the science aspects, please contact:

  Brian Cosgrove

  OWP/Analysis and Prediction Division

  Silver Spring, MD



For questions regarding the data flow aspects of these datasets,

please contact:

  Carissa Klemmer

  NCEP/NCO Dataflow Team

  College Park, Maryland



NWS National Service Change Notices are online at: