No pilot today should act like it's still 1927 and the airplane waiting on the ramp is a new Boeing 40A loaded with 1,000 pounds of mail waiting to head west from Chicago. Back then the weather briefing consisted of calls to a farmer 100 miles to the west and another 200 miles down the line to ask what their weather is like, combined with a glance at the sky.
One of the reasons that today's pilots can tap into an elaborate system of weather forecasts and predictions is that all too often airmail pilots ran into unforeseen weather. Tapping into the weather data system should commence at least an hour before the planned takeoff. It should begin with a look at what the weather should be doing along your route at the time you want to fly. To help you make sense of all of this, let's get some meteorological definitions out of the way.
An analysis is a map, or maybe text, describing what the weather is actually doing at a particular time.
A prog (for prognosis) is a forecast of what the weather will be doing at a particular time in the future.
Valid time, often abbreviated VT, is the time for which a particular forecast is made and is noted on the map or in the text.
Day 1 on weather products is the day the forecast was made. A forecast for the next day is Day 2, and so forth.
All of the times used on weather products are in Zulu time. This amounts to the standard time in London, England. (It used to be called Greenwich Mean Time or GMT, but that is no longer the name.)
We'll see how all of this works as we look at some products a pilot could consult while deciding if weather is likely to be safe for the flight.
The first product, Figure 1, is the simplest. It's the convective outlook produced by the National Weather Service's Storm Prediction Center in Norman, Oklahoma. You can find these maps - which provide a general look at where thunderstorms are likely - on the Web www.spc.noaa.gov/products/index.html. The label at the bottom shows that the map was issued on the fourth day of the month, "04" at 12:56 a.m. Zulu time. (Weather products use the 24-hour clock with 00 as 12 a.m.) It's valid between 1 a.m. and 12 noon.
To convert Zulu to local time, if the United States is on standard time, subtract five hours from Zulu to get Eastern time, six hours for Central, seven hours for Mountain, and eight hours for Pacific time. If the country's on daylight savings time, subtract one hour less for each time zone.
To figure out where these maps are showing that thunderstorms will be, imagine you are walking along a line in the direction indicated by the arrow. The storms will be on your right. Figure 1 shows that ordinary thunderstorms are expected over much of Texas and Louisiana, and probably also northern Mexico, but the NWS doesn't forecast for Mexico. The green arrow with slgt indicates a slight risk of severe thunderstorms with winds faster than 50 knots, or tornadoes, or large hail. If you're thinking of flying in the area where thunderstorms are possible, think some more. Thunderstorms won't be going on everywhere in the area for the 12 hours the forecast covers, but you need to ensure that you stay away from them.
Related Articles
