Qualitative Verification of the Operational GFS and Ensemble Mean Qualitative Verification of the Operational GFS and Ensemble Mean for 1200 UTC 13 March 2003 By: Duc Nguyen

Outlines   GFS Initialization for 1200 UTC 13 March 2003   144-Hour Forecast Control Run & Ensemble Mean Plots Spaghetti Plot   240-Hour Forecast Control Run & Ensemble Mean Plots Spaghetti Plot   372-Hour Forecast Control Run & Ensemble Mean Plots   Summary   Conclusions: What does it all means in term of FORECASTING?

GFS initialization of 500 mb heights & absolute vorticity for 1200 UTC 13 March 2003 Area of focus is Eastern Pacific/Western U.S Area of focus is Eastern Pacific/Western U.S   Major trough axis extending from Gulf of Alaska to southern coast of CA   Southwesterly flow along the West Coast   Zonal flow centered at 30 N, west of trough axis

Ensemble Mean and Normalized Spread Major trough along West Coast & in good agreement with ensemble members Southwesterly flow along west coast Zonal flow west of trough axis 144-Hour Forecast Valid 1200 UTC 13 March 2003 Control Run and Ensemble Spread Major trough along West Coast with spread of ~100 meters Southwesterly flow along west coast Zonal flow west of trough axis

Spaghetti Plot for 144-Hour Forecast Valid 1200 UTC 13 March 2003 Ensemble members (all colors), ensemble mean (black), and operational GFS (gray) are in good agreement (which are reflected in spreads & normalized chart) Major trough axis along the west coast Excellent agreement of the zonal flow along our latitude, west of the major trough axis

240-Hour Forecast Valid 1200 UTC 13 March 2003 Control Run and Ensemble Spread Ensemble Mean and Normalized Spread A ridge along the coast with large spreads indicated by orange & red Northwesterly flow along the West Coast Trough along 150 W, location where verification shows zonal flow Short-wave ridge along the coast West/northwesterly flow along the West Coast Small short-wave trough along 150 W, where verification shows zonal flow

Spaghetti Plot for 240-Hour Forecast Valid 1200 UTC 13 March dm for control run & ensemble mean agree with few of the ensemble members 522-dm for control run & ensemble mean are out of sink with ensemble members Weak trough axis along West Coast A suggestion for zonal flow, west of trough axis along the West Coast

372-Hour Forecast Valid 1200 UTC 13 March 2003 Control Run and Ensemble Spread Ensemble Mean and Normalized Spread An intensive looking trough over the West Coast with huge spreads (gen meters) Southwesterly flow along the West Coast Northwesterly flow west of major trough and NO zonal flow An weak looking trough over the West Coast with moderate spreads Southwesterly flow along the West Coast Zonal flow clearly evidence along our latitude

Summary  144-hour forecast  144-hour forecast : Both operational GFS & ensemble mean are verified with southwesterly flow which is controlled by the BIG trough axis over the West Coast, and a zonal jet west of trough axis.  240-hour forecast  240-hour forecast : Both operational GFS & ensemble mean are not verified. Operational GFS shows a stronger ridge with a large spread. Ensemble mean is more conservative, showing a weak ridge along the west coast with moderate spread.  372-hour forecast  372-hour forecast : Both operational GFS & ensemble mean shows different solutions. Operational GFS was verified with BIG trough over the West Coast. Though did not capture the zonal flow west of it. It captured the northwesterly flow, west of the major trough axis. Ensemble mean was not verified with the BIG trough; however, it captured the zonal flow along our latitude.

Conclusions: What does it all means in term of FORECASTING?  Ensemble mean is more conservative. It can capture the general pattern better than the operational GFS.  GFS is more extreme. It represents only a single & “controlled” solution to the forecast. It can either be VERY ACCURATE OR TOTALLY WRONG.  In our case, the zonal jet stream with a weak trough axis could produce more precipitation compared to a major trough and a northwesterly flow.  In general, if a forecast were made for this particular pattern, it’s more ACCURATE to believe the ENSEMBLE MEAN OVER THE OPERATIONAL GFS.

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