The Winds Aloft Forecast (FD) is a prediction of wind direction, wind speed, and temperature at altitudes from 3000 MSL to FL390. I’m adding an FD Helicopters Mini-Lesson on this weather product, but it basically focuses on what we might be using the FD for.
Maybe you don’t even look at this report (or the wind streamlines chart) during your pre-flight planning, and only venture to decipher it prior to check rides. For helicopter pilots, the goofy rules that kick in closer to the stratosphere than we’re ever going to be (like wind speeds >99 knots, and the different nomenclature for below-zero temps at altitudes above FL240) make the FD seem more like fodder for trick questions than a practical tool. I’ve always considered it simply as a back-up source for figuring my en-route winds on cross-country flights, but here’s something cool that the FD table can tell you. Check out this FD from over Nantucket (ACK) for today, and specifically look at the 12000 and 18000 columns:
Well, it’s not a brilliant example (but it’s the best I could do today), but between 12000 and 18000, the wind speed is forecast to increase from 12 to 45 knots, or 5.5 knots per 1000 feet. As a rule of thumb, when wind speed increases by >6 knots per 1000 feet, you can expect moderate or greater turbulence.* I’m not going to call it definitive–and keep in mind that the FD is just a forecast–but at the time there were a couple of PIREPs for light to moderate turbulence in the KBOS area.
This came up for me before my commercial cross-country flight. I was looking forward to flying into a mountain airport (KMYL) and the weather was pretty much a go as far as I was concerned. Winds were dead calm at KMYL, and the sky was clear below 12000, as it usually is in the Boise Valley. The one thing bothering me was an AIRMET Tango overlying KMYL. It didn’t go down to the surface, but it did get close enough to the altitude we’d be flying to get into this airport that it had me thinking over whether it was going to be safe to make the flight. I remember being a bit baffled by the calm winds at KMYL and the high winds at the 9000 foot level for the KLWS FD. I talked it over with the CP and, even though he didn’t tell me outright not to make the flight, I didn’t get the feeling that he’d do it. So I bailed on that cross-country, and ended up second-guessed that decision extensively—I’d just cost the school’s owner a 4-hour block on that helicopter, and another student was walking out to do that exact same flight solo (until her instructor called her back after I decided not to go on my flight). It wasn’t until months later that it hit me: that AIRMET Tango was probably there because of the turbulence between the dead calm layer near the surface and an overlying windy layer, and that’s probably about the altitude I’d have been flying at to get over the ridgeline and into KMYL.
So even though the FD might not look especially relevant for a flight at 1000 AGL, you can still use it to guess when and where you might encounter turbulence. In the absence of better info (like a PIREP with wind or turbulence reported), a difference in wind speed at the surface reported on a METAR and forecast winds at the lowest altitude from the FD could be a warning sign. For example, if the winds forecast for KXYZ (elev 18 MSL) on the FD was:
and the METAR was reporting:
KXYZ 132256Z 20006KT 10SM SCT160 15/07 A3005
You might want to consider the possibility of a bumpy ride.
*I’ve seen this in a few places, but the closest that I could get to for a credible source is an old Navy manual, the Aerographer’s Mate 14010. Unfortunately, it’s not in AC 00-45F (Aviation Weather Services), AC 00-6A (Aviation Weather), or the AIM. It was also the topic of a question in AOPA Pilot (Nov 2009).