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Public Information Statement

NOUS41 KWBC 011848
Public Information Statement
National Weather Service Headquarters Washington DC
148 PM EST Thu Mar 1 2012
To:       Subscribers:
         -Family of Services
         -NOAA Weather Wire Service
         -Emergency Managers Weather Information Network
         Other NWS partners and NWS employees
From:    Mark Tew
         Chief, Marine and Coastal Services Branch
Subject: Minor Modification of Saffir-Simpson Hurricane Wind
         Scale Thresholds Effective May 15, 2012
Effective May 15, 2012, the Saffir-Simpson Hurricane Wind Scale
(SSHWS) will be adjusted slightly to resolve issues associated
with the conversion of units used for wind speed. This change
follows a public comment period conducted in 2011. 
The National Hurricane Center (NHC) and Central Pacific Hurricane
(CPHC) assign intensities of hurricanes in 5-knot (kt)
increments. For advisory products, these intensities are
converted to miles per hour (mph) and kilometers per hour (km/h),
and then rounded to the nearest 5-mph or 5-km/h increments. 
Challenges occur when the current Category 4 intensity is 115 kt
(132.3 mph). Although 115 kt is within the Category 4 range, the
equivalent rounded wind speed in mph is 130 mph, which is in the
Category 3 range. To classify the hurricane as a Category 4 in
both sets of units (kt and mph), NHC and CPHC must incorrectly
convert 115 kt to 135 mph in the advisory products. A similar
issue occurs when the current Category 4 intensity of 135 kt is
converted to km/h.
Effective May 15, 2012, to resolve these rounding issues,
Category 4 on the SSHWS will be broadened by one mph at each end
of the range, yielding a new range of 130-156 mph.  This will
also result in a minor modification of the Category 3 and 5 wind
speed thresholds. The SSHWS will change as follows:
Category 3: 111-130 mph (96-113 kt, 178-209 km/h)
Category 4: 131-155 mph (114-135 kt, 210-249 km/h)
Category 5: 156 mph or higher (136 kt or higher, 250 km/h or
Category 3: 111-129 mph (96-112 kt, 178-208 km/h)
Category 4: 130-156 mph (113-136 kt, 209-251 km/h)
Category 5: 157 mph or higher (137 kt or higher, 252 km/h or
There will be no change to the wind speeds currently assigned to
Categories 1 and 2.
With this change, a 115-kt Category 4 hurricane will have its
intensity properly converted to mph and rounded to the nearest 5
mph (130 mph) and remain within the Category 4 mph range.
Important note:  Since intensities are assigned using 5-kt
increments, neither storms in the historical record nor any
future storms will have their SSHWS category changed as a result
of this modification to the scale.
The NWS wishes to remind media, partners, and the public the
Saffir-Simpson Hurricane Wind Scale provides information on wind
impacts only.  The scale does not provide commentary or
information on other impacts or characteristics of tropical
Additional information on this change can be found at:
If you have any questions, please contact
   John Kuhn
   National Weather Service
   Silver Spring, Maryland 20910
   301 713 1677 ext. 121
National Public Information Notices are online at:

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Page last modified: Thursday, 01-Mar-2012 19:59:04 UTC