354
NOUS41 KWBC 251455
PNSWSH
SERVICE CHANGE NOTICE 26-62
NATIONAL WEATHER SERVICE HEADQUARTERS SILVER SPRING MD
1055 AM EDT THU JUN 25 2026
TO: SUBSCRIBERS:
-NOAA WEATHER WIRE SERVICE
-EMERGENCY MANAGERS WEATHER INFORMATION NETWORK
-NOAAPORT
OTHER NWS PARTNERS, USERS AND EMPLOYEES
FROM: DAVID MICHAUD
DIRECTOR
NWS CENTRAL OPERATIONS
SUBJECT: IMPLEMENTATION OF THE GRAPHICAL TURBULENCE GUIDANCE
NOWCAST (GTGN): EFFECTIVE ON OR ABOUT JULY 28, 2026
EFFECTIVE ON OR ABOUT JULY 28, 2026, BEGINNING AT 1200
COORDINATED UNIVERSAL TIME (UTC), THE NATIONAL CENTERS FOR
ENVIRONMENTAL PREDICTION (NCEP) WILL IMPLEMENT THE GRAPHICAL
TURBULENCE GUIDANCE NOWCAST (GTGN) PRODUCT. THE GTGN PRODUCT
WILL BE DISPLAYED AT THE FOLLOWING LOCATION:
HTTPS://AVIATIONWEATHER.GOV/GFA/#OBS.
ADDITIONALLY, RAW GRIB2 DATA WILL BE AVAILABLE VIA NOMADS, WITH
FURTHER DETAILS BELOW.
IN THE EVENT THAT THE IMPLEMENTATION DATE IS DECLARED A CRITICAL
WEATHER DAY (CWD), AN ENHANCED CAUTION EVENT (ECE), OR OTHER
SIGNIFICANT WEATHER IS OCCURRING OR IS ANTICIPATED TO OCCUR,
IMPLEMENTATION OF THIS CHANGE WILL TAKE PLACE AT 1200 UTC ON THE
NEXT WEEKDAY NOT DECLARED A CWD AND WHEN NO SIGNIFICANT WEATHER
IS OCCURRING.
THE GTGN IS DESIGNED TO PROVIDE A NEAR REAL-TIME ANALYSIS OF IN-
FLIGHT TURBULENCE EVERY 15 MINUTES, SIGNIFICANTLY ENHANCING
SUPPORT FOR THE AVIATION COMMUNITY. GTGN USES A SHORT-TERM (1
OR 2-HOUR) FORECAST FROM THE GRAPHICAL TURBULENCE GUIDANCE (GTG)
AS THE BASIS FOR THE NOWCAST. RECENT OBSERVATIONS OF TURBULENCE
ARE THEN USED TO UPDATE THE GTG FORECAST AND CREATE A BLENDED
ANALYSIS. THE GTGN IS NOT A SUBSTITUTE FOR THE TURBULENCE
INFORMATION CONTAINED IN THE SIGNIFICANT METEOROLOGICAL
INFORMATION (SIGMET) PRODUCT PROVIDED BY NWS FORECASTERS.
OBSERVATIONAL DATASETS CURRENTLY USED BY GTGN INCLUDE AIRCRAFT
OBSERVATIONS FROM PILOT REPORTS (PIREPS), AUTOMATED IN SITU EDDY
DISSIPATION RATE (EDR) REPORTS, EDR ESTIMATED FROM GROUND-BASED
RADAR OBSERVATIONS VIA THE NATIONAL CENTER FOR ATMOSPHERIC
RESEARCH (NCAR) TURBULENCE DETECTION ALGORITHM (NTDA), ESTIMATED
EDR FROM LIGHTNING DATA, AND METAR DATA. THE OUTPUT OF GTGN IS
EDR. EDR IS AN ATMOSPHERIC, AIRCRAFT-INDEPENDENT METRIC OF
TURBULENCE. THE GTG FORECAST IS BASED ON NOAA'S 3-KM HIGH-
RESOLUTION RAPID REFRESH (HRRR) MODEL OVER THE CONUS; THEREFORE,
ITS DOMAIN SERVES AS THE HORIZONTAL DOMAIN FOR GTGN. THE
VERTICAL RESOLUTION OF THE NOWCAST STARTS AT THE SURFACE (100
FEET) AND IS OUTPUT AT EVERY 1,000 FEET UP TO 50,000 FEET.
RAW GRIB2, 3-KM DATA WILL BE AVAILABLE ON THE NCEP NOMADS SERVER
AT THE FOLLOWING URL:
HTTPS://NOMADS.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GTGN/PROD/GTGN.YY
YYMMDD/
WHERE YYYY IS THE YEAR, MM IS THE MONTH, AND DD IS THE DAY OF
ISSUE. THE FILENAME FORMAT IS: GTGN.THHMMZ.3KM.GRIB2.
GRIB2 METADATA SPECIFICATIONS:
- CATEGORY NUMBER: 19
- PARAMETER NUMBER: 30
- PARAMETER NAME: EDDY DISSIPATION PARAMETER (EDPARM)
- PACKING: JPEG2000 COMPRESSION WITH BITMAP
RAW DATA WILL BE AVAILABLE FOR EVALUATION FOR 30 DAYS PRIOR TO
IMPLEMENTATION AT
HTTPS://NOMADS.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GTGN/PARA
PLEASE SUBMIT COMMENTS, QUESTIONS, OR REQUESTS PERTAINING TO
THIS IMPLEMENTATION TO:
ROBERT HEPPER
AVIATION WEATHER CENTER (AWC)
ROBERT.HEPPER@NOAA.GOV
FOR QUESTIONS ABOUT THE DATAFLOW ASPECTS, PLEASE CONTACT:
MARGARET CURTIS
NWS CENTRAL OPERATIONS HPC DATAFLOW TEAM LEAD
NCEP.PMB.DATAFLOW@NOAA.GOV
NATIONAL SERVICE CHANGE NOTICES ARE ONLINE AT:
HTTPS://WWW.WEATHER.GOV/NOTIFICATION/
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