Understanding the Role of Deterministic versus Probabilistic NWP Information

EFS Tools Used to Assess Uncertainty and Probability ยป Spaghetti Plots

This image shows a spaghetti plot with the 5220, 5460, and 5760-m contours from each EFS member of the NCEP Short Range Ensemble Forecast (SREF) assigned a different color. The grey shading shows the spread about the ensemble mean and the thick black contour shows the ensemble mean position of each of the 3 contours. The darker shading depicts larger spread in the figure.

500 hPa spaghetti plot from Short-Range Ensemble Forecast system (SREF, contours are 522, 546, 576dm) with the spread shaded in meters, showing the forecast over the US from initial time 15 UTC 6 December 2007 valid 03 UTC 8 December 2007. Significant troughs are indicated over the Western U.S. and Great Lakes region. Maximum uncertainty is up- and downstream of the western trough, but on the axis of the Great Lakes trough.

The 500 hPa data clearly show areas of both high and low uncertainty based on the spread of the contours. Adding the spread helps with this activity. Clearly, there is good agreement at 500 hPa over Florida where there is no shading and large disagreement over the Great Lakes where the contours diverge. Also, note the darker shading in the confluent flow along the East Coast.