National Weather Service United States Department of Commerce

For a winter storm to receive a grade, the following criteria must be met:

  • A warning is issued for 5 or more counties (due to snow);
  • Warning criteria is met for 5 or more counties (6+") without a warning issued; or,
  • Warning criteria is met for 2 or more counties (12+") without a warning issued.

Note: a grade is not assigned for events that have warnings issued solely due to impacts or for mixed precipitation events.  This grading system is based largely on objective criteria which can be hard to maintain in complex scenarios.  The goal is to have grades posted within 3 to 5 days of the event.

Elements graded:

  • 6" contour spatial error
  • Magnitude of snow error
  • Onset timing error
  • Peak wind speed error
  • POD/FAR
  • Warning lead time
  • Impact messaging

Other criteria:

  • 6” contour error will only be calculated if there is a large, continuous area of 6+ inches forecasted and observed.  A forecast calling for 4 to 7 inches may include pockets of 6 inch bullseyes which make it difficult to objectively calculate distance between forecast amounts and actual amounts. 
  • Some criteria may not be applicable with some storms, like 6” contour error or wind speed.  Those won’t be included with the grade.
  • If a warning is not issued, the grade will be based on the forecast with 12 hours lead time.
  • Total possible points will be based on criteria being included. 
    • Grade = (actual points/possible points) X 100

Final note: local conditions and individual perceptions from the wide ranging types of our customers may vary from the objective grade calculated over a large area.  

 

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