When I am planning a cross country flight, friends and relatives often express concern about the weather and offer to share weather forecasts with me. Frequently, they are worried because the TV weather person said something like, “snow tonight, continuing into tomorrow.” I very much appreciate the feelings and motivations behind their concern.
Weather is one of the most vital factors in my flight planning but I am able to take a much more sanguine approach to it because I have more information than is available through the TV or the newspapers or even web sites such as weather.com. I hope that this post will demystify the process a bit for non-aviators.
The first tool that I use is the surface weather forecast map. These are produced once or twice a day, depending on how far into the future you are looking. Here is an example of a map which looks 48 hours into the future:
If I were considering a flight from Syracuse, NY to St. Charles, MO on Monday evening, this map would tell me that conditions look pretty good for most of my route but that there will be an area of snow and snow showers east of Lake Erie, covering northeast Ohio and western Pennsylvania.
Generally speaking, I do not put much credence into forecasts of any type which look more than a day into the future. They give me a “heads up” about things that I might want to keep an eye on, like the snow east of Lake Erie. But if bad weather is forecast for a long time and over a wide area, such forecasts might cause me to start considering alternate plans.
The most precise tool that I use is the TAF or Terminal Aerodrome Forecasts. These are issued several times per day at the larger airports, and look at most 24 hours into the future. The TAF gives, in hour-by-hour detail, what the conditions should be like at the airport. The marvelous thing about the TAF is that instead of “overcast,” it details where the clouds will be. Instead of simply “snow,” it details how much the snow will restrict visibility.
Last week, I planned to fly from Syracuse to St. Charles on Friday, departing Syracuse around noon. On Thursday afternoon, the TV and newspaper weather forecasts were for snow overnight and into Friday, with accumulations of an inch or more. The TAF on early Friday morning, being more precise, offered a much more optimistic picture:
Syracuse NY (Syracuse Hancock Intl) [KSYR] terminal forecast issued on the 24th at 6:21am EST (1121Z), valid from the 24th at 7am EST (12Z) through 7am EST (12Z):
7am EST (12Z): wind 270 degrees at 20 knots gusting to 25 knots, visibility 1 mile, light snow, blowing snow, 1,500 feet broken, 2,500 feet overcast
7am (12Z)-9am EST (14Z): temporarily visibility 1/2 mile, snow, blowing snow, indefinite ceiling 300 feet
10:00am EST (1500Z): wind 300 degrees at 20 knots gusting to 30 knots, visibility greater than 6 miles, 3,500 feet broken
7:00pm EST (0000Z): wind 280 degrees at 8 knots, visibility greater than 6 miles, 15,000 feet scattered
1:00am EST (0600Z): wind 100 degrees at 5 knots, visibility greater than 6 miles, 6,000 feet overcast.
This TAF, issued at 6:21am EST, shows that the weather will be pretty crummy in the morning but by 10:00am, it will improve markedly. Visibility will be greather than 6 miles, which is fine weather for flying, and there will be a broken layer of clouds with bases at 3,500 feet, which is about 3,000 feet above the ground. Since this is winter and high pressure was pushing into the area, I could be pretty sure that these clouds would be fairly thin and the sort of thing that I could fly around or through and end up above. Looking farther into the evening, even those clouds will have blown away, leaving only scattered clouds at 15,000 feet.
Indeed, by 11:00am, the snow had stopped completely and large patches of sunshine called Candy and me to the airport. We took off at noon to picturesque flying over western New York.
If you want to poke into this a little more deeply, I recommend these web sites:
- Wunderground.com is my favorite “general” weather site. You can also get a lot of the aviation weather here, including TAFs. For $5 per year, you can zap the advertisements.
- The National Weather Service’s Aviation Weather Center is free (as in, your tax dollars paid for it) and contains both forecast “prognosis” charts and TAFs, along with much, much more. Look under “Forecasts” in the menu bar.
- A Pilot’s Guide to Aviation Weather Services gives a nice, albiet detailed, overview of the information in the AWC.
- Finally, if you look at a raw TAF for more than a couple of seconds, you will see that it is rife with abbreviations. The secret decoder ring is Aviation Weather Formats: METAR/TAF.
Pilots may also like these resources:
- CSC DUATS. Free but you have to be a pilot or student pilot to get access. I particularly like their free FlightPrep program.
- AOPA has a nice set of weather tools from Meteorlogix. AOPA membership required.
- Trade-A-Plane offers a great set of forecast graphics and access is included with a subscription to the web site/magazine.
I also use information from tools such as the Area Forecasts, Winds Aloft Forecasts, and PIREPs (pilot reports). Long though this article is, it is only an introduction.
