Wx Watch: MOS for the masses

Ever wonder where your TAFs come from? Well, the short answer is that each of the National Weather Service’s Weather Forecast Offices (WFOs) generates them, but this is just the end stage of a complicated analysis of trends and observations. To a large extent, forecasters rely on numerical guidance (computer models) to help them make their 24- or 30-hour forecasts. Everyone’s heard of computer modeling—it’s used in a huge range of applications—and meteorology is certainly no exception.

MOS is just one tool in the forecasting arsenal. Forecasters are sure to check what MOS says, but they maintain a healthy skepticism.

Computer-generated weather models range from a number of graphic depictions to text products called Model Output Statistics, or MOS (pronounced “moss”). There are several MOS products, among them the NAM (North American Mesoscale) and the GFS (Global Forecast System). The NAM and GFS, like so many other weather models, run all day long on high-output supercomputers, and produce their text forecasts four times a day. Each
forecast message gives the computer’s best projection of the weather for various airport locales for three-hour intervals.

Like most everything, MOS is available on the Internet. It’s yet another preflight source of information that can be worth checking if the weather is threatening,or if you’re simply interested in seeing the raw data that forecasters use.

The GFS MOS data published with this article can be accessed online. Click on the state that interests you, and up pops the text for each airport. NAM MOS data can be seen online. Both GFS and MOS data can be pulled up on the University of Wyoming’s website.

• DT = The day of the month, denoted by the standard three- or four-letter abbreviation.
• HR = Hour of the day in UTC. This is the hour at which the forecast is valid, or if the forecast is valid for a period, the end of the forecast period.
• X/N = Nighttime minimum/daytime maximum surface temperatures.
• TMP = Surface temperature valid at that hour.
• DPT = Surface dew point valid at that hour.
• CLD = Forecast categories of total sky cover valid at that hour.
• WDR = Forecasts of the 10-meter wind direction at the hour, given in tens of degrees.
• WSP = Forecasts of the 10-meter wind speed at the hour, given in knots.
• P06 = Probability of precipitation (PoP) during a six-hour period ending at that time.
• P12 = PoP during a 12-hour period ending at that time.
• Q06 = Quantitative precipitation forecast (QPF) category for liquid equivalent precipitation amount during a six-hour period ending at that time.
• Q12 = QPF category for liquid equivalent precipitation amount during a 12-hour period ending at the indicated time.
• SNW = Snowfall categorical forecasts during a 24-hour period ending at the indicated time.
• T06 = Probability of thunderstorms/conditional probability of severe thunderstorms during the six-hour period ending at the indicated time.
• T12 = Probability of thunderstorms/conditional probability of severe thunderstorms during the 12-hour period ending at the indicated time.
• POZ = Conditional probability of freezing precipitation occurring at the hour.
• POS = Conditional probability of snow occurring at the hour.
• TYP = Conditional precipitation type at the hour.
• CIG = Ceiling height categorical forecasts at the hour.
• VIS = Visibility categorical forecasts at the hour.
• OBV = Obstruction to vision categorical forecasts at the hour.

Of course, you have to decode the entries, and to help you do that AOPA has published the definitions and categories for the GFS MOS messages (see page 95). Some are fairly intuitive to interpret, some are not. Once you learn the abbreviations and symbols the interpretation becomes a lot easier and faster. Go online to make a bookmark for decoding GFS and NAM MOS.

Are the models ever wrong? You bet! For example, errors can occur because models do not have fine enough detail to fully incorporate sea breezes, mountain and valley breezes, and other small-scale weather modifiers.

That’s why MOS is just one tool in the forecasting arsenal. Forecasters are sure to check what MOS says, but they maintain a healthy skepticism. Will a frontal passage happen when MOS says it will? A check of satellite imagery and current surface observations may nix MOS predictions. Furthermore, models often disagree with each other. That’s when the forecaster has to rely on his own skills and hunches, most of them based on experience with past models in similar weather conditions, and experience with local conditions.

Want to check a TAF against MOS predictions? It can be an interesting exercise—especially when low ceilings (a “1,” “2,” or “3” in the CIG descriptor at the left edge of the message) and visibilities (a “1” through “4” is the VIS descriptor) seem to be in the offing. Just remember that the TAF rules when it comes to an FAA-official source of forecast information.

Reflectivity, revisited

The August 2011 “ WxWatch: Datalink Lowdown” evoked responses from the two suppliers of Nexrad datalink weather—Baron Services, which provides composite reflectivity images data via WxWorks’ XM WX service, and WSI, which favors base reflectivity images in its InFlight service packages. For an informative look at each company’s rationale for its choice of in-cockpit radar check out AOPA’s website.

Email the author at [email protected].