Analysis of Model Forecasts of Significant Cold Fronts Using MOS Output Steve Amburn, SOO WFO 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, SOO WFO Tulsa, Oklahoma