MOS data is available from the NGM, the GFS (also known as AVN and MRF on the TAMU weather interface), and the NAM (also known as ETA on the TAMU weather interface). The most complete bulletin is the 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.

NGM MOS is a tradeoff. The model itself is the least accurate, but it has been run in its current configuration for the longest time, so the equations relating the model forecasts to what actually happens is the most accurate. 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