1. MOS AVN = Dynamical Model –Seven fundamental equations ! AVN MOS = Statistical Model –No seven fundamental equations ! –Equations are statistical, not dynamical !

2. MOS: Equation Development Y1 = mx1 + b1

3. MOS: Temperature Predictors –Model low level temps (i.e. 850mb/2m) –Model relative humidity Accounts for clouds –Model wind direction /speed –Climatology –Previous days min (max) Single site development

4. MOS: Precipitation Predictors –Model mean relative humidity (i.e. 1000- 500mb layer average) –Precipitation output of model –Model vertical velocity (i.e. 700, 500, 850mb) –Model low level wind direction (i.e. 10m) Regional development

5. MOS: Wind Predictors –Low-level wind direction/speed output of model (i.e. 10m, 850mb wind) Single site development

6. MOS Characteristics Requires large sample size –Several years of model output –Increases statistical significance

7. MOS Partially removes systematic model errors (i.e. biases) –If model has a cool bias at 850mb, MOS will account for/remove model bias Works best when models are not tweaked (i.e. no change to physics)

8. MOS: Equation Application

9. GFS MODEL Station: UNV Lat: 40.85 Lon: -77.83 Elev: 378 Closest grid pt: 29.6 km. Initialization Time: 08-02-26 1200 UTC HOUR VALID PMSL THCK 6HRPCN 2m_TMP 850TMP 850REL 700REL 10m_WD 850WND ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ 000 26/12 1006 540 29 -1 91 98 14/003 20/026 006 26/18 997 543 0.38 33 1 99 100 12/005 20/037 012 27/00 993 540 0.13 34 0 97 79 30/005 26/018 018 27/06 997 530 0.02 28 -8 100 92 31/014 34/028 024 27/12 1002 522 0.01 20 -10 89 91 31/014 33/034 030 27/18 1006 517 0.01 23 -13 90 99 30/014 31/027 036 28/00 1010 511 0.01 16 -16 89 75 31/013 31/032 042 28/06 1014 507 0.01 12 -18 91 44 30/011 31/031 048 28/12 1018 504 0.01 11 -19 98 44 29/010 30/032 054 28/18 1022 506 0.01 19 -17 98 17 29/013 30/025 060 29/00 1027 513 0.02 18 -16 98 11 29/008 30/027 066 29/06 1031 519 0.00 12 -14 45 9 26/003 28/019 072 29/12 1030 524 0.00 13 -9 52 90 16/007 24/023

10. GFS MOS KUNV GFS MOS GUIDANCE 2/26/2008 1200 UTC DT /FEB 26/FEB 27 /FEB 28 /FEB 29 HR 18 21 00 03 06 09 12 15 18 21 00 03 06 09 12 15 18 21 00 06 12 N/X 25 27 15 24 16 TMP 36 35 34 33 32 29 26 25 26 25 21 19 18 17 17 19 23 24 21 19 17 DPT 31 31 29 29 26 22 18 16 13 11 9 7 6 6 5 4 4 4 3 8 10 CLD OV OV OV OV OV OV OV OV OV OV OV OV OV SC BK BK BK SC CL SC OV WDR 05 36 30 29 29 29 29 29 29 29 29 29 28 28 28 28 28 28 28 23 15 WSP 03 04 06 11 14 15 13 13 14 14 12 11 11 10 09 13 14 13 06 02 03 P06 100 51 35 24 26 11 10 2 0 0 0 P12 65 41 11 6 1 Q06 3 1 0 0 0 0 0 0 0 0 0 Q12 1 1 0 0 0 T06 1/ 0 2/ 1 0/ 1 0/ 0 0/ 0 0/ 0 0/ 0 0/ 0 0/ 3 0/ 0 T12 3/ 1 0/ 1 0/ 0 0/ 0 0/ 3 POZ 7 0 2 0 1 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 1 POS 41 33 55 65 96100100100100100100100100100 99100100100100100 99 TYP S R S S S S S S S S S S S S S S S S S S S SNW 4 1 0 CIG 3 3 3 4 4 6 6 5 6 6 6 6 6 6 6 6 6 6 8 8 7 VIS 3 3 4 3 5 5 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 OBV BR BR BR BR N N N N N N N N N N N N N N N N N

11. MOS ERRORS: Who’s at fault? Dynamic model (gfs model) –Garbage In = Garbage Out Statistical model (gfs mos) –Imperfect statistical relationships (i.e. lines of best fit are not line of prefect fit!) Forecasting MOS error (utilizing association method)

12. HOW TO BEAT MOS Know how it works MOS tends to do well: –Weather near climatology (equations lean toward modal case) MOS tends to do poor: –Weather departs from climatology ( the “outliers” of the scatter plot) –Bad model data used as input (GI=GO)

13. MOS: Equation Development Y1 = mx1 + b1

14. HOW TO BEAT MOS- temp Tend to go lower than MOS by day if: –It’s precipitating –Overrunning situation –Spatially thin, optically thick cloud (non-climo) –Snow cover (esp. in non climo., treeless area) –Shallow cold air mass –Sea breeze in hot air mass with cold water –YESTERDAY'S OBSERVED MAX/MIN TEMPYESTERDAY'S OBSERVED MAX/MIN TEMP –Expected air mass will be record-breaking –YESTERDAY'S MAXIMUM TEMPERATUREYESTERDAY'S MAXIMUM TEMPERATURE

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

16. MOS ERROR: SPATIALLY THIN/OPTICALLY THICK CLOUD

17. MOS ERROR: Shallow Chill Worse for NGM mos …. not as bad for ETA and GFS MOS

18. MOS ERROR: Shallow Chill

21. 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

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

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

24. HOW TO BEAT MOS Tend to forecast higher than MOS by day: –Mainly sunny –In warm sector Especially if in the cooler season and it’s breezy and prev. night was warm –Expected air mass is record-breaking

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

26. 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

27. HOW TO BEAT MOS Forecast higher than MOS at night if: –Cloudy –Breezy –Higher dew points –Not precipitating

28. MOS ERROR: CYCLONE

29. 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

30. 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

31. 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!

32. HOW TO BEAT MOS Analogous thickness approach!! –Use analogous thickness method to “advect” mos errors to forecast location! –If MOS is busting upstream and same weather regime is heading to forecast site, assume error continues!