Public Information Statement

NOUS41 KWBC 151954
PNSWSH

SERVICE CHANGE NOTICE 10-06
NATIONAL WEATHER SERVICE HEADQUARTERS WASHINGTON DC
255 PM EST FRI JAN 15 2010

TO:        SUBSCRIBERS:
           -FAMILY OF SERVICES
           -NOAA WEATHER WIRE SERVICE
           -EMERGENCY MANAGERS WEATHER INFORMATION NETWORK
           -NOAAPORT
           OTHER NWS PARTNERS...AND NWS EMPLOYEES

FROM:      THERESE Z. PIERCE
           CHIEF...MARINE AND COASTAL SERVICES BRANCH

SUBJECT:   ADOPTION AND USE OF /POST-TROPICAL CYCLONE/
           TERMINOLOGY BY NWS NATIONAL CENTERS AND FORECAST
           OFFICES: EFFECTIVE MAY 15 2010

ON MAY 15 2010...THE NWS WILL BEGIN USING THE TERM /POST-
TROPICAL/ TO DESCRIBE WEATHER SYSTEMS THAT ARE NO LONGER 
TROPICAL CYCLONES. THIS CHANGE WILL MAKE NWS TERMINOLOGY 
CONSISTENT WITH THE TERMINOLOGY USED BY OTHER WORLD 
METEOROLOGICAL ORGANIZATION MEMBER STATES. IT WILL ALSO ALLOW 
THE NWS TO MORE ACCURATELY DESCRIBE THE METEOROLOGICAL STRUCTURE 
OF FORMER TROPICAL CYCLONES. 

THE FOLLOWING DEFINITION FOR POST-TROPICAL CYCLONE WILL BE 
INCLUDED IN THIS SEASON/S UPDATE OF NWS INSTRUCTION  
10-604: TROPICAL CYCLONE DEFINITIONS.

POST-TROPICAL CYCLONE...A FORMER TROPICAL CYCLONE. THIS GENERIC 
TERM DESCRIBES A CYCLONE THAT NO LONGER POSSESSES SUFFICIENT 
TROPICAL CHARACTERISTICS TO BE CONSIDERED A TROPICAL CYCLONE.  
POST-TROPICAL CYCLONES CAN CONTINUE CARRYING HEAVY RAINS AND 
HIGH WINDS. FORMER TROPICAL CYCLONES THAT HAVE BECOME FULLY 
EXTRATROPICAL...AS WELL AS REMNANT LOWS...ARE TWO SPECIFIC 
CLASSES OF POST-TROPICAL CYCLONES. 

AS SUGGESTED BY THE ABOVE DEFINITION...THE NWS WILL CONTINUE TO 
USE THE MORE SPECIFIC TERMS OF /REMNANT LOW/ AND /EXTRATROPICAL/    
... WHEN APPLICABLE... TO PROVIDE ADDITIONAL INFORMATION ON THE 
TYPE OF SYSTEM THE TROPICAL CYCLONE HAS OR IS EXPECTED TO 
BECOME. UPDATED DEFINITIONS OF REMNANT LOW AND EXTRATROPICAL 
CYCLONE FOLLOW.

REMNANT LOW...A POST-TROPICAL CYCLONE THAT NO LONGER POSSESSES 
THE CONVECTIVE ORGANIZATION REQUIRED OF A TROPICAL CYCLONE...AND 
HAS MAXIMUM SUSTAINED WINDS OF LESS THAN 34 KNOTS. THE TERM IS 
MOST COMMONLY APPLIED TO THE NEARLY DEEP-CONVECTION-FREE SWIRLS 
OF STRATOCUMULUS IN THE EASTERN NORTH PACIFIC.

EXTRATROPICAL CYCLONE...A CYCLONE OF ANY INTENSITY FOR WHICH THE 
PRIMARY ENERGY SOURCE IS BAROCLINIC /THAT IS...RESULTS FROM THE 
TEMPERATURE CONTRAST BETWEEN WARM AND COLD AIR MASSES/.

THE FOLLOWING EXAMPLE SHOWS HOW THE NEW TERMINOLOGY WOULD BE 
USED IN THE TABLE SECTION OF A TROPICAL CYCLONE DISCUSSION 
PRODUCT FOR THE LAST ADVISORY ON A DECAYING TROPICAL STORM.  

IN THIS EXAMPLE...THE SYSTEM HAS LOST THE DEEP CONVECTION 
REQUIRED OF A TROPICAL CYCLONE BUT IT DOES NOT YET HAVE ANY 
FRONTAL CHARACTERISTICS.  IN ADDITION THE SYSTEM CANNOT BE 
DESIGNATED A REMNANT LOW BECAUSE ITS MAXIMUM WINDS EXCEED  
34 KNOTS.  THEREFORE THE SYSTEM IS MERELY DESCRIBED AS  
POST-TROPICAL AT THE INITIAL TIME. IN 24 HOURS...HOWEVER...THE 
SYSTEM IS FORECAST TO CHANGE STRUCTURE AND BECOME A FRONTAL LOW.  
THE /EXTRATROPICAL/ DESIGNATION IS THEREFORE APPENDED IN THE 
TABLE TO INDICATE THIS FORECAST CHANGE IN STRUCTURE.

FORECAST POSITIONS AND MAX WINDS
 
INITIAL     01/1500Z 46.5N  46.5W    40 KT...POST-TROPICAL
 12HR VT    02/0000Z 48.9N  45.6W    40 KT...POST-TROPICAL
 24HR VT    02/1200Z 52.2N  43.5W    40 KT...POST-TROP/EXTRATROP
 36HR VT    03/0000Z 55.0N  39.8W    40 KT...POST-TROP/EXTRATROP
 48HR VT    03/1200Z 56.0N  33.0W    40 KT...POST-TROP/EXTRATROP
 72HR VT    04/1200Z 56.5N  20.0W    40 KT...POST-TROP/EXTRATROP
 96HR VT    05/1200Z...ABSORBED

THE NHC IS RESPONSIBLE FOR ALL METEOROLOGICAL DECISIONS 
CONCERNING FORECASTING OF TROPICAL AND SUBTROPICAL SYSTEMS FOR 
THE ATLANTIC OCEAN AND THE EASTERN PACIFIC OCEAN...NORTH OF THE 
EQUATOR AND EAST OF 140 DEGREES WEST LONGITUDE.

THE CPHC IS RESPONSIBLE FOR ALL METEOROLOGICAL DECISIONS 
CONCERNING FORECASTING OF TROPICAL AND SUBTROPICAL SYSTEMS FOR 
THE PACIFIC OCEAN NORTH OF THE EQUATOR FROM 140 DEGREES WEST TO 
180 DEGREES WEST LONGITUDE.

IF YOU HAVE QUESTIONS...PLEASE CONTACT:

  TIMOTHY SCHOTT
  NATIONAL WEATHER SERVICE
  MARINE AND COASTAL SERVICES BRANCH
  1325 EAST WEST HWY ROOM 13126
  SILVER SPRING MARYLAND 20910
  301-713-1677 EXT 122

NATIONAL SERVICE CHANGE NOTICES ARE ONLINE AT /USE LOWERCASE/:

  HTTP://WWW.WEATHER.GOV/OS/NOTIF/HTM

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