NOUS41 KWBC 131700



Service Change Notice 17-100

National Weather Service Headquarters Silver Spring MD

100 PM EDT Wed Sep 13 2017


TO:      Subscribers:

         -NOAA Weather Wire Service

         -Emergency Managers Weather Information Network


         Other NWS Partners and NWS Employees


FROM:    Kathryn Gilbert

         Acting Director, Weather Prediction Center


SUBJECT: Changes to the WPC Excessive Rainfall Outlook,

          Effective October 13, 2017


Effective on or about Wednesday, October 13, 2017, at 1500

Coordinated Universal Time (UTC), the NWS National Centers for

Environmental Prediction (NCEP) Weather Prediction Center (WPC)

will change the Excessive Rainfall Outlook as follows:


- Align the definition with existing practice. The product

definition is proposed to be changed from:

the probability of rainfall exceeding flash flood guidance

at a point


the probability of rainfall exceeding flash flood guidance

within 25 miles (40 kilometers) of a point. 


- Modify the numerical thresholds associated with the risk

categories to conform to the new product definition.


Risk Category         Current Threshold     New Threshold


Marginal              2%-5%                 5%-10%

Slight                5%-10%                10%-20%

Moderate              10%-15%               20%-50%

High                  >15%                  >50%                 


Review of verification over the last 2 years shows that the

existing product is better calibrated toward this new definition

and attendant thresholds. The attendant probability thresholds

are also more intuitive (High Risk is greater than 50%),

enabling more effective risk communication. Further, this

definition is analogous to the Storm Prediction Center

Convective Outlook, thus providing consistency in product

definition across the severe weather and heavy rainfall outlooks.


Products affected by this change include the web graphics at:


as well as the following:


Product        WMO Header     AWIPS ID

-------        ----------     --------

Day 1 Graphic  MENC98 KWNH    RBG94E

Day 2 Graphic  MENS98 KWNH    RBG98E

Day 3 Graphic  MENU98 KWNH    RBG99E


Please send comments or requests for more information to WPC

Development and Training Branch Chief James Nelson at:



National Service Change Notices are online at: