Embedded Files
MOS data is available from the the GFS and the NAM. The most complete bulletin is the (now extinct) NGM MOS bulletin. If you can decode the NGM MOS bulletin (below), you should have little trouble with the others.
MOS is based on the historical record of model forecasts compared to observations. Using linear regression, equations have been developed which relate the actual weather to the model forecast. The output is a forecast of what will happen, based on what did happen with similar forecasts in the past.
For the NAM MOS in particular, beware of screwy-looking forecasts: they may be due to the MOS equations being developed over a short time window with not enough weather to produce reliable statistics.
MOS forecasts are frequently quite good. However, they should not be used as guidance when the weather conditions are changing rapidly or when there is reason to suspect the model forecast. Because MOS forecasts are statistical interpolations from past observations, they also sometimes fail to predict record warm and cold temperatures. If you have high confidence in your own forecast, don't be swayed too much by computer forecasts which disagree.
Below is a random MOS report, from Detroit, Michigan on Wednesday, October 16, 1996. Each line is a link; click on a link to obtain a description of the information on the line. The top half of the report contains information critical to the forecasts we make; the bottom half (QPF and beyond) is sometimes useful. You are responsible for being able to read the top half of a MOS bulletin.
DTW EC NGM MOS GUIDANCE 10/16/96 1200 UTC DAY /OCT 16 /OCT 17 /OCT 18 / HOUR 18 21 00 03 06 09 12 15 18 21 00 03 06 09 12 15 18 21 00 MN/MX 57 72 54 60 TEMP 70 70 66 62 61 59 59 65 69 69 64 61 59 57 55 56 57 55 51 DEWPT 56 56 57 57 56 55 55 56 56 55 55 54 52 51 50 49 46 44 42 CLDS SC SC SC SC BK OV OV OV OV OV OV OV OV OV OV OV OV OV OV WDIR 23 23 17 18 19 18 17 17 18 17 15 15 16 19 20 21 23 23 23 WSPD 11 10 05 06 06 07 08 12 16 14 11 14 15 15 16 19 23 25 18 POP06 8 36 27 20 22 55 66 62 50 POP12 45 39 80 75 QPF 0/ 1/ 0/1 0/ 0/0 2/ 3/3 2/ 1/2 TSV06 9/ 5 23/ 3 11/ 5 7/ 4 12/10 24/ 6 20/ 7 13/ 3 12/16 TSV12 27/ 7 15/ 8 30/13 26/ 9 PTYPE R R R R R R R R R R R R R R R POZP 0 1 0 1 0 0 0 0 0 0 0 0 0 0 0 POSN 0 0 0 0 0 0 0 0 0 0 0 2 6 10 21 SNOW 0/ 0/ 0/0 0/ 0/0 0/ 0/0 0/ 0/0 CIG 7 7 7 7 6 6 6 6 6 7 7 6 4 VIS 5 5 5 5 4 4 4 4 5 5 5 5 4 OBVIS H H H H F F H H N N N N F
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