Analysis of Model Forecasts of Significant Cold Fronts

Using MOS Output

Steve Amburn, SOOWFO Tulsa, Oklahoma

The Question

• Does a model bias exist when significant cold fronts move through eastern Oklahoma?

• Forecasters say yes.– Too cool ahead of the front– Too warm behind the front

Method

• Define significant front (~15°F delta)• Select the frontal cases (over 30)

• Use proxies for model data– MAV and MET MOS for periods 1-5– MEX MOS and ECMWF output for periods 6-13

• Compute applicable statistics

Data• Time period: Oct 2007 through Feb 2009• Periods 1-5 = 38 Significant Fronts• Periods 6-13 = 35 Significant Fronts• Significant = high temp change ≥ 15°F – Day before to day after frontal passage

• MOS data examined • GFS MOS (MAV and MEX)• NAM MOS (MET)• ECMWF (3-hourly max/min output)

# Mean Absolute Errors (Pds 1-5)

Count of Pos/Neg Errors (Pds 1-5)

# Mean Absolute Errors (pds 6-13)

Count of Pos/Neg Errors (pds 6-13)

Statistics for Periods 1-5

• MAV/GFS– Pre-frontal

• Avg bias = -5.31°F• MAE = 6.12°F• # errors to warm = 20• # errors too cool = 164

– Post-frontal• Avg bias = 4.62°F• MAE

= 5.07°F• # errors to warm = 160• # errors too cool = 19

• MET/NAM– Pre-frontal

• Avg bias = -4.42°F• MAE = 5.15°F• # errors to warm = 35• # errors too cool = 150

– Post-frontal• Avg bias = 2.97°F• MAE

= 3.95°F• # errors to warm = 137• # errors too cool = 34

Statistics for Periods 6-13

• MEX/GFS– Pre-frontal

• Avg bias = -9.32°F• MAE = 10.05°F• # errors to warm = 21• # errors too cool = 256

– Post-frontal• Avg bias = 7.29°F• MAE =

8.03°F• # errors to warm = 240• # errors too cool = 27

• ECMWF– Pre-frontal

• Avg bias = -8.66°F• MAE = 9.70°F• # errors to warm = 24• # errors too cool = 248

– Post-frontal• Avg bias = 0.89°F• MAE

= 6.04°F• # errors to warm = 128• # errors too cool = 131

Summary for Significant Fronts

• GFS and MET for Periods 1-5– Both have significant cool bias ahead of fronts– Both have significant warm bias behind fronts

• GFS and ECMWF for Periods 6-13– Both have significant cool bias ahead of fronts– GFS has significant warm bias behind fronts– ECMWF showed almost no bias behind fronts

Analysis of Model Forecasts of Significant Cold Fronts

Using MOS Output

Steve Amburn, SOOWFO Tulsa, Oklahoma

918-832-4115