NOUS41 KWBC 211441



Technical Implementation Notice 11-15

National Weather Service Headquarters Washington DC

1042 AM EDT Thu Apr 21 2011


To:         Subscribers:

            -Family of Services

            -NOAA Weather Wire Service

            -Emergency Managers Weather Information Network


            Other NWS Partners, Users and Employees


From:       Tim McClung

            Science Plans Branch Chief

            Office of Science and Technology


Subject:    North American Ensemble Forecast System  

            Modification: Effective May 24, 2011


Effective May 24, 2011, beginning with the 1200 Coordinated

Universal Time (UTC) run, the National Centers for Environmental Prediction (NCEP) will make a modification to the datasets used in the North American Ensemble Forecast System (NAEFS) calculations.  The NAEFS output is derived by combining the NCEP Global Ensemble Forecast System (GEFS) and the Canadian Meteorological Center’s (CMC) GEFS.  At this time, NCEP receives

raw GEFS output from CMC and bias-corrects the dataset locally

before use in NAEFS calculations. With this implementation, NCEP

will receive bias-corrected CMC GEFS data directly from CMC, and

will no longer compute the bias corrections at NCEP.


This will result in a slight difference in the CMC data used in

NAEFS. Both modeling centers use the same bias-correction

algorithm but minor differences arise due to two factors: 

First, NCEP and CMC run their bias correction algorithms on

slightly different model grids. Second, NCEP computed the CMC

bias corrections using data up to the previous day, while CMC

calculates the bias correction using data that includes the

current day’s values. Tests have shown the differences in the

CMC bias-corrected dataset and the NCEP version of CMC bias-

corrected data to be minor.


There will be no changes in the content of NAEFS products with

this implementation.


NCEP encourages all users to ensure their decoders are flexible

and are able to adequately handle changes in content order,

changes in the scaling factor component within the product

definition section (PDS) of the GRIB files, 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 implementations.


For questions regarding these changes, please contact:


  Yuejian Zhu

  NCEP/EMC, Global Climate and Weather Modeling Branch

  Camp Springs, Maryland

  301-763-8000 x 7052


For questions regarding the dataflow aspects of these datasets,

please contact:


  Rebecca Cosgrove

  NCEP/NCO Dataflow Team

  Camp Springs, Maryland 20746

  301-763-8000 x 7198


NWS National Technical Implementation Notices are online at: