Category: Weather

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:

FT	3000	6000	9000	12000	18000	24000	30000	34000	39000
ACK	3310	3414-05	3212-11	2812-17	2845-28	2863-39	286552	295352	293752

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:

FT	3000	6000	9000	12000	18000	24000	30000	34000	39000
XYZ	3129	3133-01	3138-04	3044-08	3045-17	2845-28	286042	275952	275460

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).

The North Pacific is a source for some pretty impressive low pressure systems. Stick them in the GOM, and they’d look like hurricanes. The one that blew in this week had a pressure below 1000 mb and winds in the 40-60 knot range, which rivals a category 1 hurricane. The difference, I suppose, is that the warm waters of the GOM produce convective activity that strengthens the low, but this source of energy is missing in the higher latitudes of the Pacific. This storm caused some problems on the coast, but the winds dissipated pretty quickly.

What I wanted to point out about this particular system was the correlation between the pressure gradients and the winds. In this overlay of the SFC Prognostic Chart and the Wind Streamlines, you can see the low in the upper left corner, off the Washington coastline. Just southeast of the low the isobars are stacked pretty close together, but spread out as you move south along the coastline. The wind streamlines reflect the effect this has on wind speeds: southeast of the low, right where the isobars are stacked, the winds peak at hurricane force. North and south of the low, the winds meander around and peter out as the pressure gradient–the distance between the isobars–dissipates.

Do you know what the hatched area that stretches from central Mexico, through Utah and all the way up the west coast on the Wind Streamlines graphic is? Here’s a hint: the wind streamlines image I used was for the 3000 MSL level.

Turns out it wasn’t a fog blanketing the area…we were getting dust dropped on us from a dust storm to the north. The MODIS satellite imagery from yesterday shows the dust getting kicked up and blown to the southwest. And walking through the yard today I could see a puff of dust kick up with every step. Yes, I often ask myself why I moved here from Seattle.

I went back and scanned the METAR data for yesterday. An hour after I posted, KPSC started reporting haze (HZ), which pretty accurately describes what the skies looked like by mid-afternoon…

METAR KPSC 041653Z 01019G23KT 7SM HZ FEW100 14/01 A2977 RMK AO2 SLP081 T01390011

And later in the day there were a few METARS that coded it for what it was…

METAR KPSC 050253Z 01016KT 5SM BLDU CLR 14/M01 A2981 RMK AO2 SLP096 T01391006 53018

One clue should have been the temp-dew point spread, which never got close enough for fog formation.

The storm got awful enough that they shut down sections of I90 to the north of us. Oddly enough, from the METAR data you’d have never seen it…there wasn’t a weather station within the storm, and the AIRMETs for that morning only showed a Tango over eastern Washington.

I couldn’t see across the river this morning. Typical winter weather in the Columbia River Basin…cold air, moisture, and the bowl shape that we live in is conducive to fog and clouds getting in here and staying for days. From home I can look at the web cam at KRLD, which is 11 o’clock and a couple of miles from home. They don’t have a ASOS, but KPSC (8 miles to the east and a bit further from the river) is reporting:

KPSC 041453Z 01017G22KT 10SM BKN080 12/01 A2974

Clearly the KPSC picture is a little different, and the picture on the far right shows the WA DOT traffic cam that is a little closer to that airport. It does show the winds are about what I thought, 15-20 knots. Dewpoint is off–maybe the river is adding the additional cooling and/or moisture that’s keeping the fog on the ground at KRLD. What I still find interesting is the combination of heavy fog and wind–this isn’t what the textbooks tell you is supposed to happen. The windsock at KRLD is standing straight out, and a 15-20 knot wind is supposed to pick that fog up and make it a low stratus layer. But I can’t see 1500 meters across the river, and the visibility at KRLD sucks.

If you’re still reading expecting me to give you an answer, sorry to disappoint. But I can throw in some other confounding observations. On the visible satellite (below, left) you can see the clouds filling the Basin as the sun comes up. From the east. Usually moisture comes into this region from the Pacific Ocean to the west. Low pressure to the southeast is the reason for the easterly winds, and I can only assume that the moisture was brought in here with the front. The other unusual thing is that the Cascade Mountains also form a barrier to low clouds, and if it’s a clear day here and IMC in Seattle, you can often see a north-south line of clouds formed by the terrain holding them to the west. That’s not what’s going on today though. On the IR satellite (right side), the clouds overhead are relatively bright. If I remember correctly, bright clouds on IR indicate colder, high clouds. Fog is low and warm, and doesn’t always stand out on IR. There must be a higher cloud layer obscuring the low fog layer around us.

The take-home from this, for the low-time pilot with little knowledge of real-world weather and a limited understanding of the local weather patterns, is that I might not be a happy camper if I’d planned on scooting over to KRLD this a.m. I’d have launched that way expecting gusty winds based on the KPSC ASOS, but with the winds and 8000 CIG/10 sm VIS reported, I wouldn’t have expected to find IMC (or, at best, MVFR) at KRLD. PIREPS wouldn’t have helped me (none are reporting sky conditions), and the few airports in this region are reporting VFR. If I was coming from the west, I’d have been faced with the decision to press on to KPSC and try to land there, or turn around and head back into the desert. In this case, having that WDOT weather cam at KRLD would have made a huge difference in deciding whether to make the flight or not. I wouldn’t have thought to include a weather cam in my pre-flight planning.

I spent a good bit of last week checking on the NOAA GOES visible satellite loops to see if I could spot the Station Fire. I wasn’t able to, but I know it’s possible. The video below is from the University Corporation for Atmospheric Research, and it shows the 2007 fires in Baja very nicely. That image is supposed to be from GOES-West, which is the satellite that provides the visible, infrared, and water vapor imagery for the Western half of the US. The resolution looks a little better than what I’m used to seeing from the GOES satellites though, so I’m suspicious. Although it’s very obvious here that we’re looking a smoke and not clouds, the giveaway would be the point source for the smoke. Clouds won’t behave quite like that.

ChopperChick posted some more great photos from the Station Fire, but there were 2 particularly cool ones. On the left is the image from the Terra Satellite’s MODIS. Although the Terra Satellite is pretty awesome in it’s own right, it’s not very helpful from an aviation weather perspective. (It’s a useful tool for studying climactic change, and you can view the fire response imagery here.) Check out the 2 intense white splotches just left of the center of the image. Unlike the smoke trails cast off by most of the fires, there’s something different going on there. If you zoom in, you can see the thick, brown smoke at the base, and the white parts…those would be cumulus clouds. Specifically, pyrocumulus, and that’s what’s shown on the right.

I remember seeing these for the first time on the way out to do a stage check when I was finishing my private pilot ticket. This was right about the same time that the Salmon River fires were burning in the central mountains of Idaho (another MODIS image, below). Clear skies everywhere, except that over the mountains to the north, there were 2 massive cumulus clouds piling up. We even saw some lightning up in the tops of one of the clouds.

Like all cumulus clouds, pyrocumulus form because a moist air mass is lifted aloft and cools. In this case, the fire provides the lifting mechanism and the moisture comes (at least in part) from the burning vegetable matter. The weather around pyrocumulus clouds is also what you’d expect from cumulonimbus or towering cumulus: turbulence, updrafts, downdrafts, and IMC and icing in the cloud.

One of the problems that I have with learning (as opposed to just memorizing) weather is that it can sometimes be difficult to pick out the principles when looking at the real-world complexity of it all. Take today for example. On the SFC Prog Chart, I see a pretty strong region of lows over the Great Lakes, one off the East Coast, and a couple of weaker systems over the Midwest and West Coast. What’s the first thing we learn about lows? That they draw surrounding air into them, and that the inflow is deflected to the right because of Coriolis effect, causing counter-clockwise circulation around the system. Now take a look at the Visible Satellite loop. The system over the Great Lakes is a decent illustration of what happens around an area of low pressure. But from the perspective of a student just learning, how strong an impression would that low off of the East Coast make?

That’s where I got to thinking about how I’d teach this. Why not pick out an extreme example rather than trying to pick out something lame from the day-to-day weather? Take the weather picture from 4 years ago today–right about the time Hurricane Katrina was making landfall near New Orleans. A hurricane (aka, cyclone) is basically just a low pressure system taken to an extreme. They dominate the weather around them, and it’s easy to see what all a low does by looking at a hurricane. The pressure at the center of a hurricane (usually in the 800-1000 mB range) is well below any of the examples on today’s weather map. The first thing you’ll notice from the Visible Satellite image is the circulation around Katrina: air moves in along the isobars and breaks to the right to create the counter-clockwise swirl. This wind flow pattern is driven by warm, humid air rising from the surface at the center of the low.

From what I hear, stuck-wing drivers like departing out of a low since the rising air improves their climb performance…whether this is relevant for those of us who never climb above 1500 AGL, I couldn’t say. This rising air around a low also drives cloud formation as the moist air is pulled aloft and cools. Once again, Katrina illustrates this to the extreme: the spiraling bands moving toward the eye are lines of cumulus and cumulonimbus clouds. This is where the rule of thumb that bad weather is usually associated with low pressure systems comes from. (Although I’d call bullshit that the opposite is true–that highs mean good weather. I’ll post more on that this winter when there’s a high sitting over the Columbia River Basin, locking us into low IFR and icing conditions.)

So what happens to all the air that is pulled into the low? The rising air erupts out of the top of the low and spreads out from there. Apparently, with a hurricane, how the rising air is drawn away is an important factor in how powerful the storm will be–if the rising air isn’t being drawn off, it acts as a cap on the column of air at the center of the low, weakening the storm. In the upper atmosphere above the low, there is actually a reversal in the circulation. If you look on the northwest corner of the Katrina image, you can actually see this: there are high clouds that are spiralling clockwise. These are cirrus clouds that are formed from from moist air lofted into the upper atmosphere.

Any other thoughts or learning points for teaching about low pressure systems? Add them to the comments–I’ll be setting this up as my first community ground lesson soon.