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 what is how hot will it be today or will it 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, or may, not 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. And, 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 spread cloud cover and precipitation accumulation amounts over larger area than what will actually occur. One result is 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.

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 with 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 forecasts 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.

The following forecast parameters are included from September 1 to May 31.

POZ

Chance of Precipitation being freeZing precipitation, if it occurs, at the specified times. This is based on the condition of the occurrence of precipitation. It means that IF precipitation occurs what is the chance that the precipitation will be "freezing " precipitation?

For example, there can be a high POZ but no precipitation is forecast. Meaning that the weather will be dry (the actual forecast) but IF there were to be some precipitation it would likely be "freezing" precipitation.

POS
Chance of Precipitation being Snow if precipitation occurs at the specified times. As with POZ, POS is based on the condition of the occurrence of precipitation. It means that IF precipitation occurs what is the chance that the precipitation will be snow?
TYP
Precipitation TYPe, by category, occurring at the specified times. The categories are:
  • S - Pure snow OR snow grains
  • Z - Freezing rain/drizzle, ice pellets, OR anything mixed with freezing precipitation
  • RS - Rain/drizzle AND snow mixed
  • R - Pure rain and/or drizzle
SNW
24 hr SNoW fall accumulation forecast, by category, ending at that time.
  • 0 - No snow or a trace expected
  • 1 - 0.1" to 1.9" (0.3 cm to 5.0 cm)
  • 2 - 2.0" to 3.9" (5.1 cm to 10.1 cm)
  • 4 - 4.0" to 5.9" (10.2 cm to 15.1 cm)
  • 6 - 6.0" to 7.9" (15.2 cm to 20.2 cm)
  • 8 - ≥8.0" (≥20.3cm)
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.

The following four forecast parameters are included from September 1 to May 31.

PZP
Percent chance of freeZing Precipitation occurring for the previous 12-hr period, ending at that time.
PSN
Percent chance of SNow occurring for the previous 12-hr period, ending at that time.
PRS
Percent chance of Rain/Snow mix occurring for the previous 12-hr period, ending at that time.
TYP
Precipitation TYPe, by category, occurring at the specified times. The categories are:
  • S - Pure snow OR snow grains
  • Z - Freezing rain/drizzle, ice pellets, OR anything mixed with freezing precipitation
  • RS - Rain/drizzle AND snow mixed
  • R - Pure rain and/or drizzle
SNW
24 hr SNoW fall accumulation forecast, by category, ending at that time.
  • 0 - No snow or a trace expected
  • 1 - 0.1" to 1.9" (0.3 cm to 5.0 cm)
  • 2 - 2.0" to 3.9" (5.1 cm to 10.1 cm)
  • 4 - 4.0" to 5.9" (10.2 cm to 15.1 cm)
  • 6 - 6.0" to 7.9" (15.2 cm to 20.2 cm)
  • 8 - ≥8.0" (≥20.3cm)
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 locations in the following states. Note for mobile users: Since there is a large number of locations for which you can obtain MOS guidance, the following pages are NOT responsive to mobile devices. Use the standard pinch-zoom motion to zero in on your desired location.

Alaska Oregon, Washington Idaho, Montana, Wyoming Minnesota, North Dakota, South Dakota California, Nevada Arizona, Colorado, New Mexico, Utah Iowa, Kansas, Missouri, Nebraska Oklahoma, Texas Alabama, Arkansas, Georgia, Louisiana, Mississippi, Tennessee Florida Hawaii Western Pacific Islands Caribbean Islands North Carolina, South Carolina Illinois, Indiana, Kentucky, Ohio Michigan, Wisconsin Delaware, Maryland, New Jersey, Pennsylvania, Virginia, West Virginia Connecticut, Maine, Massachusetts, New Hampshire, New York, Rhode Island, Vermont