Interpretation of Global Forecast Model "Flipflops"

Two (00 UTC 8 and 9 April 2002) ensemble forecasts separated by 24 hours, valid 12 UTC 16 April 2002, for 5700-m(?) 500-hPa height contour. The operational MRF T170 forecast is in bold yellow. Graphic courtesy of Stephen Jascourt, UCAR/COMET

Abstract

All forecasters are familiar with the radical changes from run to run that seem to occur with medium-range (and sometimes even short-range) forecasts in the Global Forecast Model. Do such 'flipflops' mean that there has been a fundamental change in the model forecasts?

In these cases, we will describe two recent model "flipflops" in a pair of time-adjacent MRF runs, and show how ensemble forecasts shed light on what is actually occurring in the MRF.

Learning objectives

In this case study, the main lessons are:

  1. Model flipflops usually result from forecast uncertainty, rather than radical changes in the initial data, because the data assimilation system gives high weight to the first guess (i.e. the short-term forecast at analysis time).
  2. As a result, individual operational model forecasts often will look like one of the ensemble members from an older forecast valid at the same time, and almost always fall within the range of forecasts given by the prior ensemble run.
  3. The ensemble mean and spread is a better measure of fundamental run- to-run model forecast change than consecutive operational forecasts.

Credits

by Dr. Bill Bua

Thanks to: