NOUS41 KWBC 111945



Service Change Notice 18-39

National Weather Service Headquarters Silver Spring MD

345 PM EDT Wed Apr 11 2018


To:       Subscribers:

          -NOAA Weather Wire Service

          -Emergency Managers Weather Information Network


          Other NWS Partners and NWS Employees


From:     Allison Allen

          Chief, Marine, Tropical and Tsunami Services Branch


Subject:  Arrival of Tropical-Storm-Force Winds Graphics become

          operational on or around May 15, 2018


Effective on or around May 15, 2018, the National Hurricane

Center (NHC) and the Central Pacific Hurricane Center (CPHC)

will begin operational issuance of the Arrival of Tropical-

Storm-Force Winds Graphics.


The anticipated arrival of sustained tropical-storm-force winds

from a tropical cyclone is a critical threshold for coastal and

inland communities. For example, emergency managers use this

information to help determine when to begin and complete coastal

evacuations, while members of the public need to know when to

prepare their homes or businesses. Once sustained tropical-

storm-force winds begin, such preparations usually become too

dangerous or difficult.


Historically, many decision makers have inferred the arrival of

sustained tropical-storm-force winds from NHC and CPHC products

deterministically, without accounting for tropical cyclone track

or size uncertainty. The risk in not factoring in these elements

of uncertainty is that communities may have less time to prepare

if a tropical cyclone speeds up or increases in size beyond the

initial forecasts.


To better meet users' needs, a set of graphics was developed

that depict when sustained tropical-storm-force winds from an

approaching tropical cyclone could arrive at individual

locations. The maps were developed and tested using social

science techniques, including one-on-one telephone interviews,

focus groups, and surveys with emergency managers, broadcast

meteorologists, and NWS meteorologists to gather opinions on the

idea, content and design of the products.


The timing graphics are created using the same Monte Carlo wind

speed probability model currently used to determine the risk of

tropical-storm- and hurricane-force winds at individual

locations. This model constructs 1,000 plausible scenarios using

the official NWS tropical cyclone forecast and its historical

errors. Additional information on this product and the

underlying technique are online at:


There will be two thresholds for producing the Arrival of

Tropical-Storm-Force Winds Graphic and posting them on the NHC

and CPHC websites for all tropical cyclones, post-tropical

cyclones, and potential tropical cyclones for which advisories

are written:


1. Earliest Reasonable Arrival Time: This graphic identifies

   the time window that users at individual locations can

   safely assume will be free from tropical-storm-force winds.

   Specifically, this is the time before which there is no

   more than a 1-in-10 (10 percent) chance of seeing the onset

   of sustained tropical-storm-force winds. This time frame is

   when preparations should ideally be completed for those with

   a low tolerance for risk.


2. Most Likely Arrival Time: This graphic identifies the time

   before or after which the onset of tropical-storm-force

   winds is equally likely. This graphic would be more

   appropriate for users who are willing to risk not having

   completed all their preparations before the storm arrives.


Timing information will only be available for locations that

have at least a 5 percent chance of experiencing sustained

tropical-storm-force winds during the next 5 days.


Each of these thresholds will also be available overlaid on top

of the cumulative 5-day probability of tropical-storm-force

winds, providing a single combined depiction of the likelihood

of tropical-storm-force winds at individual locations, along

with their possible or likely arrival times.


The graphics will be updated with each new full tropical cyclone

advisory package from NHC and CPHC. Arrival times will be

depicted with higher temporal resolution (i.e., in 6-hour

intervals) during the first day of the 5-day forecast,

increasing to lower temporal resolution (i.e., in 12-hour

intervals) after the first day of the 5-day forecast

period.  Arrival times will be referenced to 8 AM and 8 PM local

time, using a constant time zone that corresponds to where the

cyclone is located at the time of the advisory.  For example, if

a cyclone is located in the Eastern Time Zone at the time of an

advisory but is forecast to move into the Central Time Zone

during the 5-day forecast period, all times on the graphic will

be referenced to the Eastern Time Zone.


When advisories are issued, the default graphic will be

available as a clickable thumbnail within the storm window box

for active cyclones on the NHC main webpage

and the CPHC main webpage:


After clicking on the thumbnail, users can select any of the

four options.


Additional information and map examples are online at:


For questions regarding this notice, please contact:


Jessica Schauer

NWS Tropical Program Leader

NWS Marine, Tropical and Tsunami Services Branch

Miami, FL



National Service Change Notices are online at: