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

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. MOS short comings: Precipitation BUFFALO, N.Y. KBUF AVN MOS GUIDANCE 9/25/2001 1200 UTC DT /SEPT 25/SEPT 26 /SEPT 27 /SEPT 28 HR 18 21 00 03 06 09 12 15 18 21 00 03 06 09 12 15 18 21 00 06 12 N/X 44 55 45 53 45 TMP 53 54 50 49 47 46 45 49 52 52 49 49 48 47 45 49 50 51 49 49 47 DPT 49 46 45 43 41 40 40 40 40 40 41 44 45 45 44 44 43 44 45 46 44 CLD BK OV OV OV OV OV OV OV OV OV OV OV OV OV OV OV OV OV OV OV OV WDR 24 24 23 23 22 22 22 23 23 24 23 24 24 24 26 26 25 25 27 30 36 WSP 10 10 11 10 11 11 09 12 13 12 09 07 08 07 06 10 14 14 07 05 04 P06 38 45 64 60 54 49 54 42 51 46 29 P12 68 75 70 74 46 Q06 0 1 1 1 1 1 1 1 1 1 0 Q12 1 1 1 1 1 JAMESTOWN, N.Y. KJHW AVN MOS GUIDANCE 9/25/2001 1200 UTC DT /SEPT 25/SEPT 26 /SEPT 27 /SEPT 28 HR 18 21 00 03 06 09 12 15 18 21 00 03 06 09 12 15 18 21 00 06 12 N/X 40 47 41 48 42 TMP 46 45 42 41 41 41 40 42 45 46 44 44 43 43 41 45 46 46 43 45 44 DPT 42 41 41 41 41 40 39 39 39 39 41 42 41 40 39 42 42 42 43 42 41 CLD BK BK OV OV OV OV OV OV OV OV OV OV OV OV OV OV OV OV OV OV OV WDR 25 25 25 23 23 24 24 24 24 25 25 24 25 26 28 29 29 30 30 31 35 WSP 09 09 08 08 10 09 09 10 10 09 07 07 07 07 07 09 10 11 08 06 04 P06 38 50 66 60 50 54 52 32 45 42 24 P12 75 82 74 61 42 Q06 0 1 1 1 1 1 1 0 1 1 0 Q12 1 1 1 1 1

20. 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: 65FNGMMOS: 72F and character of day: optically thick/spat. thin overcast) –If unsure, go CONSENSUS MOS............ wins over long haul! –Analogous thickness methods

21. 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!