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Terminal Aerodrome Forecast
(TAF) amendments 
Most of the official weather forecasts you will
get on a standard briefing of via DUATs are issued by the Aviation
Weather Center (AWC) in Kansas City, Mo. This includes the
area forecast (FA), AIRMETs (WA), SIGMETs (WS) and convective SIGMETs
(WST). Terminal Aerodrome Forecasts or TAFs, however, are not
issued by the AWC nor are they issued by Flight Service; they are
issued by your local NWS Weather Forecast
Office (WFO). The meteorologists
at the local WFO are very familiar with any local weather effects and
have
the best opportunity to produce a quality forecast. Pictured
to the right is the WFO in Greer, South Carolina (KGSP). Click
here to view a list of the local Weather Forecast
Offices.
Your local WFO typically has the
responsibility for issuing the TAFs for 6 or 7 terminal areas. At the
Greenville-Spartanburg WFO, for example, they issue the terminal
forecasts for six airports to include KAND, KGSP, KGMU, KCLT, KHKY and
KAVL as shown in the image below.
Scheduled
TAFs are issued four times a day at 0000,
0600, 1200 and 1800 UTC. They are typically transmitted 20 to
40 minutes prior to these times. Once the TAFs hit the wire,
the forecaster must continue to compare the forecast to the actual
observations for the airport to be sure it accurately depicts the
ceiling, visibility, wind and weather occurring at the airport.
When there is a discrepancy or the forecaster feels that the
TAF isn't representative of the weather that may occur in the terminal
area within the TAF's valid period, they will issue what is called an
unscheduled TAF, better known as an amendment.
Amendments are the absolute best way to provide
the highest quality forecast. Just like instrument students
are taught by their instructors not to chase the needles, forecasters
are similiarly encouraged not to chase the observations. For
example, an unexpected, but brief rain shower may quickly develop and
pass by the terminal area temporarily lowering visibility below the
visibility in the TAF. The forecaster may be tempted to issue
a quick amendment, but if the condition is expected to be brief,
there's no value to issuing an amendment - especially if it doesn't
change the flight category (i.e., VFR, MVFR, IFR, LIFR, VLIFR).
The forecaster doesn't have to literally watch the
observations minute by minute per se.
Instead, they have a software program appropriately called
"weather watch" that monitors the observations. Based on
programmed criteria, the software compares the terminal forecast to the
latest observations for each TAF site issued by that forecaster and
flags the forecast element as green when they match. When the
program highlights a forecast element as yellow or red, this means the
difference is near or has exceeded the amendment criteria. As
can be seen on the right (click here for a larger image),
the forecast for the most part matches the current observations for
those airports in the the Greenville-Spartanburg WFO (most elements are
green). However, there are three terminal areas (KAVL, KHKY
and KAND) that show yellow for wind implying that the forecast is not
quite in line with the current observations.
This allows the forecaster to
quickly scan the display to determine if there is an immediate need for
an amendment to one or more of the TAFs. The forecaster that
issues the TAFs is assigned to the "short term desk" and has other
duties to include the area
forecast discussions and the issuance of severe thunderstorm
or tornado warnings for the WFO. The forecaster shown on the
left is located at the Greenville-Spartanburg WFO and has recently
transmitted the 1800 UTC scheduled TAFs and is now working on the
gridded forecasts for the GSP region. Click here to see the national
gridded forecasts. The weather watch software compares the
current observations to the gridded forecasts looking for differences
as well. |
