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Who constructs that TAF?
It
is not common knowledge for a pilot to know the specific
organization that generates the various aviation weather forecasts they
use every day. This is certainly a point that is clearly not
emphasized in any training curriculum used by flight schools and
typically is not discussed in many aviation texts. Pilots are
simply taught the basic characteristics of those forecasts and how to
utilize them in the context of their preflight planning
analysis. But it can be useful to know the source of the
official forecasts and understand the challenges faced by these
underappreciated meteorologists.
There are several official weather
products that pilots use that are generated by human forecasters.
These include area forecasts (FA), AIRMETs (WA), SIGMETs
(WS), convective SIGMETs (WST) and terminal aerodrome forecasts.
Terminal aerodrome forecasts, better known to pilots as TAFs
are provided to pilots as part of a standard briefing and will be
included on a DUATS briefing. Despite what some pilots are
taught, they are not automatically generated by a computer and are not
issued by the staff at the Lockheed Martin Flight Services.
Lockheed Martin Flight Services only reports the
weather and forecasts and never issues any specific forecasts including
TAFs.
With the exception of airports controlled by the military, TAFs are
constructed by highly skilled meteorologists located at the local
Weather Forecast Offices (WFOs). Each weather forecast office shown
here (upper right) has the responsibility of issuing the TAFs for one
or more terminal areas the fall within their county warning area (CWA)
with an average of roughly five or six per forecast office. Click here to view a larger
image. For example, the Greenville-Spartanburg weather
forecast office located in Greer, SC is responsible for issuing the
TAFs for six airports that fall in their CWA. These include
Anderson County Airport, Asheville Regional Airport, Charlotte/Douglas
International Airport, Greenville Downtown Airport,
Greenville-Spartanburg International Airport and Hickory Regional
Airport as shown in the diagram on the left. Click here to view a larger image.
Only a few forecasters at the
local WFOs around the country are also pilots. However, they
do understand how their forecast and subsequent amendments may impact a
pilot’s preflight planning and decisions while en route. As a
result, these meteorologists are highly motivated to provide as
accurate of a forecast as possible within the constraints of the
directives provided by the NWS.
A TAF is considered a point
forecast and is not meant to be used as a zone or area
forecast. Given this challenge, the local
forecasters have the best chance to identify the wind, ceiling,
visibility and weather that might occur at a specific airport over the
next 24 or 30 hours (depending on the TAF site). To meet that
challenge, each WFO identifies local forecast rules that will capture
the effects of terrain (mountains, bodies of water, urban setting,
etc.) as well as climatology. For example, a terminal area
located
within the city may have a much different forecast than one located 10
miles
outside of the city. Also a terminal area next to a river may
hold onto the fog a
lot longer than a terminal area 10 miles away from the river.
Thunderstorms and fog represent some of
the most difficult weather to forecast. The difference
between a forecast of OVC001 and CLR is significant to a pilot, but in
some circumstances the meteorologist may have conflicting guidance and
often has to rely on their local knowledge to decide if that radiation
fog will form in the early morning hours.
A widespread convective outbreak is normally very
well forecast, but when the thunderstorms are expected to be more
isolated or scattered, a forecaster will tend to leave out a chance of
thunderstorms, knowing they may have to amend the forecast after the
thunderstorms begin to develop. In some circumstances, it
simply comes down to meteorological risk; they are hoping for the best
without always forecasting the worst.
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