JetStream Max: Model Output Statistics

Upper air forecasts produced by models provide the overall picture of the predicted weather, but they are only pictures. What we want to know is how hot will it be today or if it will rain tomorrow.

This is why we use Model Output Statistics (MOS) models, which produce specific point forecasts of our surface weather. MOS are detailed predictions of surface temperature, precipitation chances and type, thunderstorm chances, cloud cover, cloud height, wind speed, and wind direction at specific points across the country.

Each time a model runs, surface weather forecasts for over 1,500 locations nationwide are provided. These MOS forecast locations are at airports that report weather observations, so chances are, a MOS forecast is available near you.

MOS results are used for guidance in arriving at a forecast. It is called guidance because the model from which MOS is derived may not always have a "good handle" on the weather, so it is used as a first estimate of the expected weather.

For example, MOS temperature forecasts in the vicinity of fronts could be higher or lower by several degrees if the model incorrectly forecasts the speed of fronts. If any precipitation is predicted, then the type of precipitation forecast by MOS can also be wrong.

In addition, while models can forecast areas of unsettled weather reasonably well, individual showers and thunderstorms are not resolved well by models.

Therefore, the models may spread cloud cover and precipitation accumulation amounts over a larger area than where it will actually occur. One result is that MOS forecasts may have a chance of precipitation (even in the 30-40% range) but predict no accumulation.

Short Range MOS

There are two basic types of MOS, short range and long range. The short-range MOS guidance is generated every six hours (0000, 0600, 1200, and 1800 UTC) from the Global Forecast System (GFS) and North American (NAM) models.

Forecast elements are valid from 6 to 72 hours in advance for 3- or 6-hour increments. For example, for the 12 UTC model run, the first forecasted parameters begin at 18 UTC.

The following is how to decode the forecast. Except for chance of precipitation, missing values are indicated by a 9, 99 or 999.

DT: The date of the forecast in Z-time. What is Z-time?

HR: Forecast hour in Z-time.

X/N or N/X: X is the maXimum temperature and N the miNimum temperature for the previous 12-hour period. With the 00z model run, the maximum temperature (X) is listed first. At the 12z model run, the minimum temperature (N) is presented first.

TMP: Z-time-specific TeMPerature forecasts at 2 meters (6 feet) above the ground. Each temperature forecast is presented to the nearest whole degree Fahrenheit. Only three characters are available for the temperature forecasts. Thus, two consecutive forecasts of 100 degrees or more or of -10 degrees or less appear with no spaces between them.

DPT: Z-time-specific Dew PoinT forecasts at 2 meters (6 feet) above the ground. The format follows the TMP format above.

CLD: Forecast categories of total sky cover forecasts valid for the specified Z-times. Total Sky Cover Categories are:

• CL - clear
• FW - Few: >0/8ths to 2/8ths of total sky cover
• SC - Scattered: >2/8ths to 4/8ths of total sky cover
• BK - Broken: >4/8ths to <8/8ths of total sky cover
• OV - Overcast: 8/8ths of total sky cover or totally obscured
• XX - Missing forecast

WDR: Surface Wind DiRection forecasts. This is a 2-minute average forecast of the wind direction at a height of the 10 meters (33 feet). The wind direction is given in tens of degrees and varies from 01 (10 degrees) to 36 (360 degrees). A missing forecast is denoted by the number 99. A calm wind is indicated by the number 00.

WSP: Surface Wind SPeed forecasts. The wind speed is given in knots; the maximum speed allowed in the message is 98 knots (181 km/h). A missing forecast is denoted by the number 99. A calm wind is indicated by the number 00.

P06: Forecast hour in Z-time. The chance of 0.01" or more of liquid equivalent Precipitation (PoP) occurring during a 6-hour period.

P12: The chance of 0.01" or more of liquid equivalent Precipitation (PoP) occurring during a 12-hour period.

Q06: 6-hour precipitation accumulation forecast, by category (0-5), ending at that time. Missing forecasts are denoted by the number '9'. The categories are:

• 0 - No accumulation
• 1 - 0.01" to 0.09" (0.3 mm to 2 mm)
• 2 - 0.10" to 0.24" (3 mm to 6 mm)
• 3 - 0.25" to 0.49" (6 mm to 12 mm)
• 4 - 0.50" to 0.99" (13 mm to 25 mm)
• 5 - ≥1.00" (≥2 5mm)

Q12: 12-hr precipitation accumulation forecast, by category, ending at that time. Missing forecasts are denoted by 9. The categories are:

• 0 - No accumulation
• 1 - 0.01" to 0.09" (0.3 mm to 2 mm)
• 2 - 0.10" to 0.24" (3 mm to 6 mm)
• 3 - 0.25" to 0.49" (6 mm to 12 mm)
• 4 - 0.50" to 0.99" (13 mm to 25 mm)
• 5 - 1.00" to 1.99" (25 mm to 50 mm)
• 6 - ≥2.00" (≥51 mm)

T06: There are two values separated by a "/". The value on the left represents the chance of a Thunderstorm within 15 miles of the station. The value on the right is chance of a severe thunderstorm within 25 miles of the station, with both during the previous 6-hours, ending at that time. What is a severe thunderstorm?

T12: There are two values separated by a "/". The value on the left represents the chance of a Thunderstorm within 15 miles of the station during the previous 12-hours, ending at that time. The value on the right is chance of a severe thunderstorm within 25 miles of the station during the previous 12-hours, ending at that time.

 CIG: Forecasts of eight categories of CeilInG heights (the elevation of the base of clouds) for the specified times. The categories are:

1. ceiling height of < 200 feet (<61 meters)
2. ceiling height of 200 - 400 feet (61 - 122 meters)
3. ceiling height of 500 - 900 feet (152 - 274 meters)
4. ceiling height of 1,000 - 1,900 feet (305 - 579 meters)
5. ceiling height of 2,000 - 3,000 feet (610 - 914 meters)
6. ceiling height of 3,100 - 6,500 feet (945 - 1,981 meters)
7. ceiling height of 6.600 - 12,000 feet (2,012 - 3,658 meters)
8. ceiling height of > 12,000 feet (>3,658 meters) or unlimited ceiling.

VIS: Forecasts of seven categories of VISibility for the specified times. The categories are:

1. visibility of <1/2 mi (<800 meters)
2. visibility of 1/2 to <1 mi (800 to <1,600 meters)
3. visibility of 1 to <2 mi (1,600 to <3,200 meters)
4. visibility of 2 to <3 (3,200 to <4,800 meters)
5. visibility of 3 to 5 mi (4,800 to <8,000 meters)
6. visibility of 6 mi (9,600 meters)
7. visibility of >6 mi (>9,600 meters)

OBV: Forecasts of five categories of OBstruction to Vision for the specified times. The categories are:

• N - None of the following...
• HZ - haze, smoke, dust
• BR - mist (fog with visibility ≥5/8 mi (≥1,000 meters))
• FG - fog or ground fog (visibility <5/8 mi (<1,000 meters));
• BL - Blowing dust, sand, snow

Long Range MOS

Unlike the short range MOS, the extended-range GFS MOS guidance is generated only twice daily from the 00z and 12z cycles the model run.

Forecast elements are valid from 24 to 192 hours in advance in 12-hour increments. Except for chance of precipitation, missing values are indicated by the numbers '9', '99' or '999'. Following is how to decode the forecast.

​FHR: Forecast HouR, i.e. how many hours from the model run time. The next line provides the "local time" day of the week and date of the month. For example, FHR = 24 means that forecast is for 24 hours AFTER September 2 at 12z, meaning September 3 at 12z (morning in mainland U.S.).

X/N: X is the calendar day maXimum temperature. N is the calendar day miNimum temperature for the day. With the 00z model run the maximum temperature (X) is listed first. At the 12z model run, the minimum temperature (N) is presented first.

TMP: Z-time-specific (00z or 12z) TeMPerature forecasts at 2 meters (6 feet) above the ground. Each temperature forecast is presented to the nearest whole degree Fahrenheit. Only three characters are available for the temperature forecasts. Thus, two consecutive forecasts of 100 degrees or more or of -10 degrees or less appear with no spaces between them.

DPT: Z-time-specific (00z or 12z) Dew PoinT forecasts at 2 meters (6 feet) above the ground. The format follows the TMP format above.

CLD: Forecast categories of total sky cover forecasts valid for the specified Z-times. Total Sky Cover Categories are:

• CL - clear
• FW - Few: >0/8th to 2/8ths of total sky cover
• SC - Scattered: >2/8ths to 4/8ths of total sky cover
• BK - Broken: >4/8ths to <8/8ths of total sky cover
• OV - Overcast: 8/8ths of total sky cover or totally obscured
• XX - Missing forecast

WSP: Surface Wind SPeed forecasts. The wind speed is given in knots; the maximum speed allowed in the message is 98 knots (181 km/h). A missing forecast is denoted by the number 99. A calm wind is indicated by the number 00. Wind direction is not provided.

P12: The chance of 0.01" or more of liquid equivalent Precipitation (PoP) occurring during the previous 12-hour period.

P24: Forecast hour in Z-time. The chance of 0.01" or more of liquid equivalent Precipitation (PoP) occurring during the previous 24-hour period.

Q12: 12-hr precipitation accumulation forecast, by category, ending at that time. Missing forecasts are denoted by 9. The categories are:

• 0 - No accumulation
• 1 - 0.01" to 0.09" (0.3 mm to 2 mm)
• 2 - 0.10" to 0.24" (3 mm to 6 mm)
• 3 - 0.25" to 0.49" (6 mm to 12 mm)
• 4 - 0.50" to 0.99" (13 mm to 25 mm)
• 5 - 1.00" to 1.99" (25 mm to 50 mm)
• 6 - ≥2.00" (≥51 mm)

T12: The chance of Thunderstorms during the previous 12-hours, ending at that time. T24: The chance of Thunderstorms during 24-hour period from 1200-1200 UTC time period ending at that time.

CLIMO: This is the CLIMatOlogical normal maximum/minimum temperatures and chance of precipitation during the 96 - 120 hr period, which is approximately the mid-point of the forecast. Not all stations will contain climatic information, in which case missing values are represented by 999.

Retrieving MOS

​​​Retrieve MOS guidance through the MOS Text Bulletins page.