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OVERVIEW: The TAF Shell Generator constructs rudimentary TAFs based on Model Output Statistics (MOS) from www.nws.noaa.gov, extracted model data from www.wxcaster.com (Earl's Numericals), and initializes from observations. WSCC tables from AFCCC can be used for the first 6 hours of the TAF. MOS bulletins have a time resolution of 3 hours, and Earl's Numericals have a time resolution of 6 hours, so data from these bulletins are interpolated to 1 hour data points. The TAF Shell Generator is capable of making TAFs from the NAM (ETA), GFS, or an ensemble that averages out both models. These TAFs are useful for a first look at the weather, and will help with forecasting for WWA locations which do not have TAFs regularly created for them." INSTRUCTIONS:
#NOAA MOS bulletins and Earl's Numericals have a bad habit of hanging up in the cache here at the 25OWS. Before running the TAF Shell Generator it is advisable to open the MOS and Earl's Numerical bulletins in a browser and refresh the browser if the bulletins are not the most recent. #Click the ""Make TAF"" button for the desired model. #When prompted, enter the 4 letter ICAO of the station you want the forecast for. #If prompted, enter the date and time for the TAF in ddhh format. Example: enter ""0206"" (without the quotes) for a TAF that begins on the 2nd day of the month at 0600Z. You will only be prompted if the check box labeled ""Use Current Time"" is unchecked. Notes: #The most recent observation available is used for initialization. If something other is desired, place the desired observation in the Manual Observation Input box before generating the TAF. Data in this box will automatically override the current observation. #The program has been known to crash if the observation used for initialization contains errors. In this case, copy the observation into the Manual Observation Input box, correct the error, and click the ""Make TAF"" button again." "Limitations: #A maximum of only 3 cloud layers are forecasted: one low, one mid, and one high layer. #Icing intensity is not forecasted... it defaults to light for all icing except for clear icing with freezing precipitation. #Precipitation intensity is not forecasted.. it defaults to light except for severe thunderstorms, when it defaults to moderate." "Forecasting Procedures and Data Thresholds:"
"Wind Stratified Conditional Climatology: WSCC data from AFCCC can be used for the first 6 hours of the TAF. Ensure the ""Use WSCC"" box is checked to use the data." "Wind: Winds for the first line of the TAF are derived from the initial observation. Winds on subsequent lines are derived directly from the MOS for the direction and average speed. When the average speed exceeds 15 knots, a gust speed of 8 knots above the average are added on (the 8 knot difference is based on wind studies conducted for the Pacific Northwest over the past few years). Wind speed changes of 10 knots or more and direction changes of more than 30 degrees when the speed is greater than 15 knots will cause a new line to be specified." "Visibility: Visibility for the first line of the TAF comes from the initial observation. Additional lines derive visibility from the VIS line on the MOS.
New lines are specified for visibility when it changes through the following thresholds: 0800 or the base's field minimum, 3200, 4800, and unrestricted." "Weather: Weather on the first line of the TAF is based on the initial observation. Weather groups on subsequent lines are derived as follows: #FG, BR, and HZ come from the OBV line on the MOS. Checks are made to ensure ""FG"" when the visibility is less than 1000m, ""BR"" when the visibility is 1000m to less than 9999m, ""HZ"" when the temperature-dew point spread is greater than or equal to 5°C, and no obstruction if the visibility is 9999m. #""FZFG"" is forecast if the MOS forecasts ""FG"" and the temperature is less than 0°C. #Precipitation is forecast when the P06 line on the MOS is greater than or equal to 20%. Precipitation type is determined as follows:
#""FZRA"" is forecast when ""RA"" is forecast and the temperature is less than or equal to 0°C. #When the Lifted Index from Earl's Numericals is less than 0, instability is indicated and ""SH"" is prefixed to the precipitaion. #If the T06 line on the MOS is greater than or equal to 20%, ""TS"" is prefixed to the precipitation." "Clouds: Ceiling on the first line of the TAF is from the initial observation. Ceilings on subsequent lines are determined from the value on the CIG line on the MOS.
Remaining cloud layers are derived from the maximum relative humidity in Earl's Numericals throughout 3 layers: low clouds (Sfc - 6500 ft), mid clouds (6500 - <18,000 ft), and high clouds (18,000 - about 30,000 ft). Coverage of that layer is determined by the maximum relative humidity: FEW = 65 - 72.5%, SCT = 72.5 - 80%, BKN = 80 - 90%, OVC = > 90%. Coverages are adjusted so that the summation principle applies." "Icing: To find icing, the program first looks at Earl's Numericals to find layers where the temperature is less than 0°C, relative humidity is greater than 80%, upward vertical velocity is present, and no thunderstorms are forecast. It then averages out the icing potential over the time period that the line is valid for, and adjusts it so that icing is not forecast below the ceiling. The layer of icing potential must be at least 2000 feet thick to be reported. If freezing rain is forecast, severe clear icing from surface to 1000 feet will automatically be included. Likewise, moderate clear icing will automatically be included for freezing drizzle." "Turbulence/LLWS: Turbulence is found by looking at Earl's Numericals for wind direction and speed for each layer and then calculating the shear vector. Turbulence intensity is then calculated in accordance with AFWA TN-98/002 table 2-10. 3-5.9 knots per 1000 feet = light, 6-9.9 knots per 1000 feet = moderate, 10-14.9 knots per 1000 feet = severe, and greater than or equal to 15 knots per 1000 feet = extreme. Turbulence intensity is then averaged out over the time period that the line is valid for. If surface winds are 20 knots or greater, turbulence in low levels is added in accordance with AFWA TN-98/002 figure 2-51. 20-24.9 knots = light from surface to 3000 feet, 25-34.9 knots = light-moderate from surface to 3000 feet, 35-49 knots = moderate from surface to 5000 feet, and greater than 49 knots = severe from surface to 5000 feet. If moderate or greater shear is forecast at 2000 feet or less, ""WSCONDS"" is added." "Altimeter Settings: Altimeter settings are derived from the Mean SLP on Earl's Numericals and initialized from the initial observation. For every 1 millibar of change on Earl's Numericals, the altimeter setting will change approximately .03 inches. The minimum altimeter setting for the duration of the TAF line is selected." "Temperatures: Temperatures come directly from the MOS. The N/X line is used when the temperatures fall within the time frame of the forecast. The TMP line is used when there is no max or min temperature on the N/X line during the time frame of the forecast. Times are determined by interpolating from the TMP line." "WWA Criteria: Watch/Warning/Advisory criteria is automatically highlighted for selected locations. Other locations can be manually checked for generic criteria by clicking the ""WWA Check"" button. Highlighting can be removed by clicking the ""Clear Highlighting"" button. Red indicates elements that meet WWA criteria. Green indicates elements that might meet WWA criteria, but further investigation is required." "Remarks: The TAF Shell is indicated as an automated TAF by the remark ""AUTO"" on the last line followed by the model that was used to create it… NAM, GFS, or NAM-GFS (ensemble). A ""W"" may follow the model indcating that WSCC was used, and a ""T"" will be appended if user-defined thresholds were used." |
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