Updated: Sep 5, 2021
Say you are flying to Effingham Count Airport (1H2) in southern Illinois within the next eight hours. However, there isn't a Terminal Aerodrome Forecast (TAF) issued for that airport. What can you reasonably do to understand the weather you might face during your arrival at 2200 UTC? Use the nearest TAF? How about the Graphical Forecast for Aviation (GFA)?
Effingham County Airport sits in a hole (show to the right) with terminal forecasts located on all sides for the most part. Decatur, Illinois (KDEC) is the closest airport that has a scheduled TAF issued. It is about 50 nautical miles to the north of Effingham County. Fifty miles isn't all that far, so can a pilot expect the weather at Effingham County to be the same as Decatur? It is certainly easy for a pilot to leap to that conclusion.
Don't count on the nearest TAF to be accurate in all cases, especially one that is 50 nm away. A TAF is a point forecast that is valid only five (5) statute miles from the center of the airport's runway complex. This leaves the pilot with the unfortunate job of extracting site-specific weather data from the GFA (see below). That is not an easy task considering that the GFA tool doesn't present all of this information in one view, so you have to hunt for it.
Don't despair. Model Output Statistics (MOS) may provide some much needed help. MOS (pronounced "moss") isn’t new and has been around since the 1970s. In recent years it has surfaced and now is being used by some pilots on a regular basis. Model Output Statistics, as the name implies, is derived from the output of weather prediction models. Numerical weather prediction models are run on a scheduled basis on the computers at the National Centers for Environmental Prediction (NCEP). Models such as the North American Mesoscale (NAM) and Global Forecast System (GFS) are executed every six hours.
These models do not automatically produce a point forecast for a specific town or airport, however. The job of MOS is to take the “raw” model forecast and attempt to improve on it using a statistical method to produce an objective and more useful site-specific forecast. Therefore, MOS is simply a computer program that is executed after the underlying models have completed and uses historical information called geoclimatic data that gives it an advantage over just the basic model output from the GFA.
The NAM and GFS both have their own version of MOS called the MET and MAV, respectively. While MOS may not fulfill the regulatory role, it does provide the pilot with much needed site-specific detail that isn’t found from using the GFA alone and provides an extended forecast well beyond the TAF’s 24 or 30 hour period. It is not just for pilots either; it is also used by the professional forecasters at the local weather forecast offices (WFOs) to help them construct and amend TAFs.
These MOS forecast predicts the sensible weather elements. These predicted elements fall into one of three categories: probabilistic, categorical and continuous. For example, continuous data includes, but is not limited to, temperature, dewpoint temperature and wind (direction and speed). For some predicted elements categories are used. Instead of providing a cloud height forecast of 500 feet AGL, a category of “3” is assigned that represents a range of cloud heights from 500 feet AGL to 900 feet AGL as shown in the table on the right. Precipitation and thunderstorm occurrence, on the other hand, are always presented as a probability. The output of MOS is found formally in a tabular bulletin format called FOUSnn where nn is the specific bulletin number.
If we go back to our original example, Decatur's TAF was calling for TEMPO 2921/3001 5SM -RA OVC015 and the wind to be 32013KT. The GFA (not shown) valid at this time was forecasting BKN030 with SCT -SHRA. Cloud bases in the GFA are shown in MSL unless otherwise noted. Given a field elevation of 600 feet, that places the ceilings at roughly 2,500 feet for an arrival at 22Z.
You can already see that the nearest TAF and GFA are close, but don't necessarily agree. The Decatur TAF suggests the need for an instrument approach and an alternate whereas a pilot using the GFA would not need an alternate and should expect a visual approach.
The good news is that a MOS site exists for Effingham County that can help provide a clue as to the weather upon arrival. According to the GFS LAMP MOS, at 2200 UTC the sky is expected to be overcast (OV) with a 59 percent chance of rain. The categorical ceiling forecast shows a 4 which equates to a ceiling between 1,000 feet and 1,900 feet according to the table above. Visibility is being forecast at a category of 7 which means better than 6 statute miles. Finally, the winds are expected to shift around from the southwest to the northwest with winds of 260 degrees at 8 knots at 22Z.
While MOS isn't always perfect and never should be used as a replacement over the official TAF guidance, it can help provide a site-specific forecast typically better than the GFA. In case you were wondering, the 2225 UTC surface observation (below) for Effingham County airport turned out to be very close to the MOS forecast above valid at 2200 UTC.
K1H2 292225Z 25008KT 10SM OVC016 A2981 RMK AO2
Currently, EZWxBrief provides access to the Localized Aviation MOS Program (LAMP) bulletin on the EZAirport page as shown above. LAMP is a derivative of the GFS MOS product. The nice thing about LAMP is that it provides hourly forecast time step for a 24 hour period and is updated every hour. It generally performs better than the GFS MOS product alone especially in the first 6 hours of the forecast. See the EZWxBrief Pilots Guide for more information on this forecast product.
Lastly, EZWxBrief also considers the MOS forecast as one of several inputs when constructing the EZForecast you see on the EZMap display. Like some apps that simply show the MOS forecast, EZWxBrief integrates MOS along with several other forecasts to determine the forecast you see on the EZMap.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist