Glidemet

As I forecast on Wednesday, the weekend was a “real mixed bag” with sun, cloud, wind and rain all at various points on both days. Generally the forecasts were good and the weather was just ok to fly, with the wind gusting up to about 25 knots at Bicester on Saturday and a tad lower on Sunday.

What was notable was the widespread wave and good wind for ridge running. Phil King first alerted me to the possibility of wave this week on Monday via a comment to that day’s post. Certainly key indicators were there - a general anti-cyclonic curve to the isobars, and stable layers at suitable heights in forecast soundings, although cloud amounts looked likely to be a problem, and Phil reported on Saturday that Friday had indeed been restricted by cloud.

Helpfully a lot was posted to the BGA National Ladder. First off, here’s a comparison of the wind on Saturday and Sunday:

Saturday’s winds are the more westerly blue barbs and Sunday’s the more northerly orange barbs (well, that was the colours I selected, but exporting an image from the IDV seems to wash out the colours and I’m a bit colourblind, but you should get the idea).

As you can see Saturday had a more westerly wind which was also generally stronger. On Sunday the wind came round much further to the north (something I suggested on both Thursday and Friday, though not so much Saturday), however, down on the south coast the change in direction was less or even opposite.

Judging by the ladder Nympsfield’s runs were working well on Saturday (e.g Trevor Stuart’s flight) but it was particulary good to read Bruce Cooper’s report from Denbigh. Bruce states that the wind was close to the northern limit for the ridge there and this can be seen in the above chart.

Gary Nuttall also left a comment that Booker’s ridges were working well on Saturday, keeping a K13 and a ASW27 up. I didn’t know pure gliders could reach and return from the ridge there - I’d only seen Alan Johnstone’s ASH26 flights on the ladder before.

There was also wave coming out of Wales. Diana King climbed well from Shobdon getting to 13,000’, but very interestingly Alison Mulder reached nearly 12,000’ from Nympsfield. Below is Alison’s track laid over a Modis 250 m image taken around half-way through her flight:

Excellent wave bars are seen standing south east of the southern Cambrian hills extending well into England, and the wave bars Alison used to climb are clearly visible.

Let’s look at a GFS forecast analysis sounding for the area:

Now I’m no expert at analysing soundings for wave but it strikes me that there’s an conditionally unstable layer up to 900 mb followed by a near-isothermic layer up to 700 mb (about 9,000’) combined with a steadily increasing (and quite strong) wind with height coming from the northwest. Diana King commented on Friday’s forecast saying that on Saturday the wave began at about 3,5-4,000’ before reaching 13,500’. My theory is that air from the stable layer was being displaced upwards by the the unstable flow over the hills lower down. However with unstable air again above 700 mb there’s nothing to reflect the energy downwards again, so there wouldn’t be more than one primary wave bar - and that doesn’t tally with the sat pics. Hmmm. If anyone can see what I’m missing, please do tell…

One pointer - I didn’t forecast the wave at all well towards the end of last week and at the weekend. However, the Met Office’s F215 charts (you’ll need to be registered for that link to work, but it’s free) were on the money every day - they run a special model for wave forecasting which they use to write the MTW forecasts on those charts. Although the lead-time is short - the 0200-1100 chart issued at 2130 the night before is the earliest warning you’ll get - they’re very good.

Now let’s turn our attention to Sunday. First off, up in Scotland Kevin Hook put in a cracking 400k reaching FL195. I decided to try overlaying Kevin’s flight on a satpic too and seeing how they lined up. This time the image is an excerpt from the afternoon NOAA-18 image with 1 km/pixel resolution: copyright 2007 RSGB, University of Bern and NOAA.

I asked Kevin how well this satpic matched what he saw from the cockpit. He wrote:

“The wavebar at AMU never moved throughout my flight as you can see from the proximity of each leg to the others in that area. The Satpic shows it just as I remember. As you go west and north, the trace is occasionally on the wrong side of wavebars or over the top of them. I think that this shows that in these areas the wave was much less stable and kept moving up and down wind. Re-forming wave often results in small gaps and corresponds to my impression of clag at TUL and TUW. The blue at the eastern turnpoints and local to Portmoak is also accurate.

“After finishing the task, I watched the wave at TUL set up properly and worked the new line out of interest. The hot spot that I was working at 13:45 is exactly at the point of greatest sink in the Satpic 45 minutes earlier.

“It never ceases to amaze me how different conditions can be on the run out from a turnpoint compared to the run in ten minutes earlier. A change of a few knots in wind speed or of a few degrees in angle can change the picture totally. If anything, I am surprised how closely this single snapshot matches the weather over a six hour flight.”

Very interesting. Again, here’s the GFS analysis sounding for the same time and place:

(Part of the dewpoint line is missing. I don’t know why). If anything it looks like just like a taller and windier version of the same sounding from south Wales on Saturday.

Back down south, judging from the ladder, the wind was not as good for ridge running on Sunday as it had been on Saturday, although Booker again flew a K13 on their ridge until the wind veered and dropped in the afternoon. That said, there appeared to be widespread wave again, and a good climb was made from Talgarth to 10,000’.

So again, thanks all for your comments, emails and ladder flights this weekend. Comparing your flights to the forecast soundings will mean I (and you!) will have a better idea of what to look for in the future, and I’ve refined my knowledge of the winds needed for the ridges too.

Early outlook
Tomorrow (Tuesday) a cloudy and damp day, wind 15 knots from the northwest once again. All back to Nympsfield and Talgarth then . Wednesday looks brighter (though possibly showery in the east) with the wind round to the north now, but quite weak - a bit borderline for Parham.

Thursday sees the high off to the southwest at the moment move a little closer in, giving a brighter day again but quite windless - possible fog risk. Friday not dissimilar (though the Met Office thinks it will be wet - I don’t buy that). The weekend sees too much divergence in the models - the GFS and ECMWF being completely opposed - to make a worthwhile forecast.

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