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HOW TO BEAT MOS

HOW TO BEAT MOS. Need to understand how it works MOS does well when: Weather is close to climatology (equations lean toward the modal case) MOS does poor when: Weather departs from climatology ( the “outliers” of the scatter plot) Bad model data used as input (GI=GO).

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HOW TO BEAT MOS

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  1. HOW TO BEAT MOS • Need to understand how it works • MOS does well when: • Weather is close to climatology (equations lean toward the modal case) • MOS does poor when: • Weather departs from climatology ( the “outliers” of the scatter plot) • Bad model data used as input (GI=GO)

  2. MOS: Equation Development Y1 = mx1 + b1

  3. HOW TO BEAT MOS • Forecast lower than MOS by day if: • It’s precipitating • Overrunning situations • Spatially thin, optically thick cloud (non-climo) • Snow cover • Shallow cold air mass • Sea breeze in hot air mass with cold water • Expected air mass will be record-breaking

  4. MOS ERROR: OVERUNNING 850mb Predictor gives a very poor forecast!

  5. MOS ERROR: SPATIALLY THIN/OPTICALLY THICK CLOUD

  6. MOS ERROR: Shallow Chill

  7. MOS ERROR: Shallow Chill

  8. MOS ERROR: Shallow Chill

  9. MOS ERROR: Shallow Chill

  10. Beating MOS • How to account for shallow chill problem: • Recognize pattern • Look at 2m temps from model (ETA/AVN) • If much colder than MOS, then lower MOS

  11. MOS ERROR: FRONTS Relaxed gradient aloft gets translated to the surface

  12. MOS ERROR: FRONTS Relaxed gradient aloft gets translated to the surface

  13. HOW TO BEAT MOS • Forecast higher than MOS by day if: • Mainly sunny • In warm sector • Especially if in the cooler season and it’s breezy • Expected air mass is record-breaking

  14. HOW TO BEAT MOS • Forecast lower than MOS at night if: • Clear • Calm • Low dew points • Snow cover • (unless its ‘climatological’!)

  15. HOW TO BEAT MOS • Which city is more likely to have the bigger bust in the following situation? • Clear skies, light winds, snow cover • ST. LOUIS vs. INTERNATIONAL FALLS

  16. HOW TO BEAT MOS • Forecast higher than MOS at night if: • Cloudy • Breezy • Not precipitating

  17. MOS ERROR: CYCLONE

  18. HOW TO BEAT MOS • PRECIPITATION • Will tend to miss mesoscale events tied to topography • Lake-effect • Under predicts upslope areas, Over predicts in downslope areas • WIND • A little inflation of sustained winds

  19. HOW TO BEAT MOS • Other considerations: • NGM beyond 48-hours …. Watch out! • Beware if MOS exceeds 850mb ‘rules’ • Lean toward MOS product that makes the most sense: • (i.e. AVNMOS: 65F NGMMOS: 72F and character of day: optically thick/spat. thin overcast) • If unsure, go CONSENSUS MOS ............ wins over long haul! • Analogous thickness methods

  20. HOW TO BEAT MOS • Analogous thickness approach • Use analogous thickness method to “advect” mos errors to forecast location! • If MOS is busting at upstream site and same weather regime is heading to forecast site, then assume error will continue!

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