NOUS41 KWBC 281600

PNSWSH           

 

Service Change Notice 14-11

National Weather Service Headquarters Washington DC

1040 AM EST Fri Feb 28 2014

 

To:      Subscribers:

         -Family of Services 

         -NOAA Weather Wire Service

         -Emergency Manager Weather Information Network

         -NOAAPORT

         Other NWS partners and NWS employees

 

From:    Mark Tew

         Chief, Marine and Coastal Weather Services Branch 

 

Subject: Changes to text Tropical Cyclone Wind Speed Probability

         Product Effective May 15, 2014  

 

Effective May 15,2014 the following changes will take place with

the text Tropical Cyclone Wind Speed Probability (PWS) product

issued by the National Hurricane Center and Central Pacific

Hurricane Center: 

 

1. Removal of Maximum Wind Speed (Intensity) Probability Table

(Table I) This table gives the likelihood that a tropical

cyclone will be at any of several different intensity

categories (e.g., depression, storm, category 1 hurricane,

etc.) at fixed points in time (e.g., 12 hours, 24 hours, 36

hours, etc.). These probabilities are estimated by creating

a large number of potential track and intensity scenarios

based on the latest official forecast and a climatological

error distribution. NHC has learned that many users consult

this table to estimate the chances that a tropical cyclone

will make landfall at one or more of the various intensity

categories. Unfortunately, the current methodology provides

a highly misleading estimate of landfall intensity. 

 

The problem can be seen by considering a storm that is

forecast to be just offshore at 72 hours. A sizable fraction

of the potential scenarios have already encountered land by

that time, and for these over-land scenarios the expected

intensity at 72 hours will be much lower than what would be

expected for a storm that was still over water and about to

make landfall. Even though the table accurately assesses

what the intensity is likely to be at 72 hours, it grossly

underestimates the likely intensity for when the storm comes

ashore.

 

The table was originally intended to help users assess the

risk of different categories of storms at particular times,

and for storms well away from land the product serves that

function well; however, because of the high likelihood of

misuse for land-threatening storms, and the consequences of

that misuse for the general public, NHC and CPHC are

discontinuing this table until a better procedure to

estimate intensity risk can be developed.

     

2. Wind Speed Probability Table for Specific Locations (Table

II) - The name of one of the parameters in this table will

change from Individual Probability (IP) to Onset Probability (OP). 

This parameter is defined as the probability of the event

beginning during an individual time period (e.g., between

24 and 36 hours).  The name Onset probability better highlights

the difference between this parameter, which provides information

on when the event is likely to begin, and the Cumulative

Probability, which describes the likelihood that the event

will occur at all.

 

The PWS can be found online at:

 

Atlantic and east Pacific basin:

 

http://www.nhc.noaa.gov/

 

Central Pacific basin:

 

http://www.prh.noaa.gov/cphc

 

The WMO headers and AWIPS Identifiers (AI) for the PWS are:

 

Basin                WMO Header              AI

Atlantic             FONT/11-15/ KNHC        MIAPWSAT/1-5/

East Pacific         FOPZ/11-15/ KNHC        MIAPWSEP/1-5/

Central Pacific      FOPA/11-15/ PHFO        HFOPWSCP/1-5/

 

An example of the changes can be found at:

 

www.nhc.noaa.gov/news/20140228_hsu_windProbsTextChanges.php

 

In addition, NWS will no longer provide the stand-alone Intensity

(Maximum Wind Speed) Probability graphic. This graphic contained

the identical information to Table I of the PWS product, which is

being discontinued.

 

If you have comments or questions please contact:

 

    John Kuhn

    National Weather Service

    Marine and Coastal Weather Services Branch

    Silver Spring, Maryland 20910

    Email: John.F.Kuhn@noaa.gov

    Phone: 301-713-1677 extension 121

 

National Public Information Statements are online at:

 

    http://www.nws.noaa.gov/om/notif.htm

 

$$