On Saturday January 28th 2017, I was fortunate to enjoy almost 6 hours of soaring at a maximum altitude of 13,600 ft. I started the day with low expectations as the conditions were not in line with what we’d typically expect from a Pilchuck ridge soaring day. The wind was coming from the west, which – as I already learned from previous experiments – does not align well with Pilchuck’s south-facing ridge line. While ridge soaring was unlikely to work, I was encouraged by the favorable wave conditions so I bet $100 on another science experiment. The bet paid off!
About a week prior to my lucky day, I noticed a strong jet stream at 300 hPa moving towards our area. The jetstream shown on windytv.com was shaped like a horseshoe – a shape favorable for wave formation in our area as it provides strong airflow along the south/north axis. This jetstream is caused by the polar vortex, an area of low pressure around the North Pole, and rotates around the pole, passing through our region every once in awhile to produce good wave soaring conditions – assuming the timing is right (daylight hours, clear sky, etc).
Over the next couple of days, conditions remained consistent across model runs. Every morning I’d open up BUFKIT to download the latest GFS forecast for Everett and check the key conditions for a wave day: 1) wind coming from the south; 2) at least 15 knots at 5,000 ft; 3) wind increasing with altitude; 4) no wind shear, and; 5) no clouds within the first few thousand feet above Pilchuck. When I get closer to the day of interest, I run a script I wrote to assess the day’s vertical profile against the HRRR weather model, a high resolution weather model, and check the sixth ingredient required for wave formation: the vertical temperature profile.
The Scorer Parameter is used to predict the formation of gravity waves and takes into account the vertical profile of the horizontal wind AND the stability of the air. Trapped mountain waves typically occur when the value decreases with altitude. [There are other nuances like wave length, which I am ignoring for now.] The horizontal blue line in the chart below shows the maximum altitude reached (13,600 ft). The curving blue line shows the Scorer Parameter (Y2) with altitude. Almost exactly at my achieved maximum altitude, the value of Y2 curves to the right.
While, in theory, everything looked great; there was one spoiler in the mix. The wind forecast was more westerly than what we’d typically expect to support ridge soaring on Mt Pilchuck. On past expeditions, I was unable to sustain flight against Pilchuck’s ridge without the wind blowing from the south. Textbooks will tell you that the wind needs to hit the ridge within a +/- 15° angle perpendicular to the ridge line. Saturday’s forecast predicted a wind direction between 220 and 240, far outside the usual range. To my surprise, this did not turn out to be an issue for catching wave straight off tow.
The figure below shows the areas of rising air as measured by my logger and the areas where the wind is expected to hit the mountain at its highest velocity. The wind vectors draw below are created with Windninja, free software produced by the USDA. I’ve plotted Saturday’s flight in orange and added two other wave flights in blue to provide an idea of the wave’s location with a south wind. All of the data below was stitched together with QGIS, another free piece of software used to create maps. Reviewing the diagram, it appears that a north-facing crest may have triggered the wave.
It was fairly tricky to stay in the wave as the area of lift was small and the wind at altitude strong (up to ~40 knots). XCSoar proved to be of great help in identifying the area of best lift and positioning my glider in the wave’s core. The screenshot below shows how how my glider’s trail is colored green in rising air, with varying thickness based on lift strength. After locating the best part of the lift, I flew back and forth, always turning into the wind to avoid getting blown out of the wave. XCSoar will detect the straight line of rising air and automatically draws a thick blue line to indicate the path to follow. Additionally, with a strong cross-wind, XCSoar will draw a thin gray line to indicate my actual course. The wind direction and strength is shown as a dark gray arrow. Getting an initial wind reading is important as it helps position the wind relative to the terrain below. Getting off tow, I’d make three circles to get an initial reading. As you can see below, I had to crab into the wind to stay within the wave. You may also note that my flight path is not exactly perpendicular to the wind. This is because I am maximizing a limited area of lift triggered by a small mountain section below. If the wind was perpendicular to a longer straight ridge line, then the wave’s core should be expected parallel to the ridge line.
With a good grasp on the wave’s location and an initial climb to 13,600 ft, I was ready to have some fun and explore the area. I first headed towards Big Four mountain, located downwind to the east. I got to the eastern edge of my detour in 10 minutes, losing 1,100 ft of altitude. I found a small area of weak zero-lift wave behind Big Four, but it wasn’t strong enough to comfortably hang out with a strong headwind waiting for me on my way back west.
Getting back to the wave took 23 minutes, and cost 4,300 ft of altitude. All of a sudden my glider’s 1:70 L/D, heading east, turned into a mere 1:20 L/D (!), heading back west. One moment you feel king of the world, 10 minutes later you’re sweating to make it back home! [XCSoar did a good job setting expectations, telling me that Green valley remained in glide range despite a strong headwind. Also, wind speed drops with altitude so my performance would have improved as I got lower.]
Back at Pilchuck, I worked my way back up to 13,000 ft and headed out south, crossing Spada Lake. From there I flew back to my elevator in the sky and took another ride up to then head out north towards Three Fingers. Coming back to Pilchuck, I noticed a thin layer of condensation was getting thicker. Until then I was comfortably warm even at high altitude. With the sky closing in above me and the sun setting on the horizon, the greenhouse effect of my canopy stopped producing heat and I quickly turned cold. I took one last long glide towards Port Susan and touched down at Arlington 15 minutes before sunset.
Do you want to forecast wave on your own? Read my next blog post: How to Catch a Wave: From BUFKIT to Scorer