Hidden in Plain Sight

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It’s not Picasso, but it’s a painting. 

Let me paint an all too familiar picture for you. It is a sweltering summer day, the temperature is hovering in the mid-nineties, and the humidity is paralyzing. A cursory review of the TAF for the nearby Class B airport indicates that it should be a nice day weather-wise for all of your anticipated flights for your shift as a CFI, patrol pilot, or any type of operation for that matter. Just a few hours into your shift, you hear what you believe to be a crack of thunder that causes you to run outside your hangar only to visually verify what your ears told you; a pop-up thunderstorm has occupied the airspace just over your facility. You scurry to get your pre-flighted aircraft back into the hangar to avoid damage you don’t want to explain to the boss. A few choice words are mumbled under your breath about another “blown forecast” by the local Weather Forecast Office (WFO) that issued the TAF for that nearby Class B Airport.

Memory Lane

For those who have forgotten all of the intricacies of a TAF, let’s take a trip down memory lane for a quick review.

Several sources define the TAF or Terminal Aerodrome Forecast. Still, I’ll choose to go with precisely what is published in the FAAs latest Aviation Weather Handbook, just published in December of 2022. The handbook describes the TAF as “a concise statement of the expected meteorological conditions significant to aviation for a specified time period within 5 sm of the center of the airport’s runway complex (terminal)”. The handbook reminds us that “the TAFs use the same weather codes found in METARs.”

The TAFs are prepared by 123 different NWS Weather Forecast Offices (WFOs) for over 700 airports. These forecasts are valid for 24 or 30 hours and amended as required. They provide us with forecast details such as wind direction and speed, prevailing visibility, cloud coverage, ceiling height, and precipitation type. They are essential as they have many implications from a regulatory point of view, such as fuel loading and alternate airport requirements from the Instrument Flight Rules perspective.

Here is the one caveat that many pilots often don’t realize or have forgotten if they ever did know. As outlined above, the TAF forecast coverage area is only 5 statute miles. Having to make a forecast for such a small area is incredibly difficult for meteorologists at the local weather forecast offices. TAFs are often referred to as “point forecasts,” and for a good reason, they are just that; you can think of a TAF as a forecast that covers an area the size of a college campus.

Remember the familiar scenario of the storm that popped up and hadn’t been published in the TAF for the “nearby” Class B? Just how “nearby” can be a crucial factor, even if rain (RA) or Thunderstorms (TSRA) are mentioned in the TAF it is still concentrated on that “point” area of the airport terminal area. If, and it often does, has “Vicinity” (VC) added, such as TSVC, the forecaster has now predicted the weather event to occur within another 5 SM area from that first 5SM area, so now you can think of it as a 10 SM radius from the center of the airport complex. Anyone that has been in aviation for any appreciable time knows that the difference between a MVFR ceiling, or even a low IFR ceiling, and a clear blue sky can be just a matter of 10-15 miles at times (if not less). And this, my friends, is where much of the frustration and mumbling of comments about “blown forecast” come from. So, cut the forecasters some slack! They are not wrong nearly as often as you may think. The forecast maybe doesn’t reach your “nearby” area.

Mind reader?

Have you ever asked yourself, “What were they thinking when they put this TAF out?” Well, good news, you do have a way to understand what the forecaster was considering when they created the TAF.

The best “hidden gem” in weather forecast products is the Area Forecast Discussion (AFD); no, I am not talking about the draconian “Area Forecast” that retired in 2018 and was replaced by the Graphical Forecast for Aviation (GFA). The Area Forecast Discussion, also at times colloquially known as the “Aviation Forecast Discussion,” is written in plain English and allows forecasters to tell you how they quantified the certainty or uncertainty concerning the TAFs they issued.

The exact description of the AFD comes to us from a National Weather Service Directive (10-503), which says: “The AFD is a semi-technical product primarily used as a means to explain the scientific rationale behind a forecast and to summarize watches, warnings and/or advisories in effect. This highly visible product is used to convey forecast and watch/warning/advisory information primarily to federal agencies, weather sensitive officials, and the media”.

All WFOs issue the AFD at least twice daily, and they do not contain a product expiration time. AFDs consist of two primary sections: a narrative description of forecast information and reasoning, and they also include a summary of public, marine, and fire weather outlook/watch/warning/advisory issuances. So, it contains more than just aviation-related weather concerns. The specific “aviation” section is likely the how and why of the product earning the title of “Aviation Forecast Discussion.”

In the official directive put out to the WFO meteorologist, they are advised that the discussion should emphasize significant aspects of the forecast, such as Identification of the most significant hydrometeorological weather affecting the geographical area of responsibility during the 7-day forecast period, Identification of the forecast problem(s)-of-the-day and explanation of their solution(s); An indication of forecast team confidence and probabilistic guidance on weather possibilities not found in other products; Reasoning behind watch/warning/advisory issuance; Differences in model guidance and an indication as to which model appears the most correct and why; reasoning for varying significantly from automated model output guidance products; reasons for significant changes from the previous forecast; Expected timing of events such as beginning or ending of precipitation and degree of uncertainty; and a brief review of the synoptic situation.

That is a lot! But, of most importance is that it is, in fact, a glimpse into the mind of the forecaster and the whys of how they came up with the TAF.

Pot of Gold

The AFD is worth its weight in gold. So many things can be listed and described in it that don’t meet the criteria to be published in the TAF. TAFs require a certain degree of probability before something like thunderstorms can be included. This is where the AFD can forewarn us of the potential for those meteorological events that have uncertain probability both in their timing and location.  

Here is an excellent example of being able to directly read the “why” of how a TAF was generated and gives us a good look at a complicated weather picture:

In this example, the forecaster tells us they went with VCTS and VCSH while keeping VFR ceilings. Considering that the thunderstorm complex described in this AFD may “weaken,” the possibility certainly exists that the VCTS and VCSH in the TAF would appear to be a “bust” if they never arrived. Still, in reality, the AFD would have told you they may, in fact, not arrive.

In another example, a forecaster was brutally honest in this AFD. Smoke from recent Canadian wildfires made the forecast extremely difficult. The Forecaster even mentioned that confidence was “low” due to a lack of model agreement and “forecaster skill/experience with such dense and widespread smoke and haze.”

O’ AFD where art thou?

Hopefully, I have convinced you just how beneficial the AFD can be for your flight operations. The AFD is my #1 go-to at the beginning of every self-weather-brief. But don’t take my word for it; use it yourself to see how helpful it is.

The AFD is “hidden in plain sight”; you just need to know where to go! It is available on www.aviationweather.gov simply by clicking on “Products,” and then down to “Forecast Discussion.”

From here, you simply click on the WFO that covers your area and issues the TAFs for your region.

Additionally, many heavyweight aviation apps, including my favorite, ForeFlight, include the Forecast Discussion – you just have to know where to look! Simply tap on the airport, then weather>TAF, and look for “Forecast Discussion.”

ForeFlight includes a nice feature in their Forecast Discussions, where hazardous weather items such as thunderstorms and fog are highlighted in red to get your attention.

Prettier Picture?

Now that you have a new tool to put in your weather decision-making toolbox, I hope that you will find the AFD beneficial to your flight operations and help you with those mumblings about forecast busts! 

Matt Johnson

Air Medical Pilot

Law Enforcement Officer & Pilot