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)

MOS: Equation Development

Y1 = mx1 + b1

850mb Model Temp VS. Observed Surface Temperature

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Observed Surface temperature (C)"THE PREDICTAND"

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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

MOS ERROR: OVERUNNING

850mb Predictor gives a very poor forecast!

MOS ERROR: SPATIALLY THIN/OPTICALLY THICK CLOUD

MOS ERROR: Shallow Chill

MOS ERROR: Shallow Chill

MOS ERROR: Shallow Chill

MOS ERROR: Shallow Chill

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

MOS ERROR: FRONTS

Relaxed gradient aloft gets translated to the surface

MOS ERROR: FRONTS

Relaxed gradient aloft gets translated to the surface

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

HOW TO BEAT MOS

• Forecast lower than MOS at night if:– Clear– Calm– Low dew points– Snow cover

• (unless its ‘climatological’!)

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

HOW TO BEAT MOS

• Forecast higher than MOS at night if:– Cloudy– Breezy– Not precipitating

MOS ERROR: CYCLONE

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

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

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!