More wave
Back | HomeBetter late than never 
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Thanks to Diana King, Gary Nuttall and Jim White for the excellent feedback in the comments to Saturday’s forecast.
Saturday itself went as forecast on Friday—-cloud spilling down south eastwards followed by drizzle. What was notable was that although the surface wind was fine, the wind gradient was extremely steep, reaching a good 40 knots at just 1,500’ (I flew a K21 backwards). There was also wave, which I’d mentioned in Thursday’s forecast. Here’s the analysis sounding for Saturday:
The static stability and the speed shear is clear (though so is the frontal cloud, where the ELR meets the dewpoint line).
In the forecasts leading up to Saturday it was notable that the forecast changed from bright and northeasterly on Wednesday to damp and southwesterly on Thursday. It only took a small change in the position of a low to bring about a larger change in the resulting weather, and as far as I can remember all the models were in agreement all along - it was just that what they agreed on, rapidly changed from Wednesday to Thursday.
Sunday looked for a long time earlier in the week that the wind would be close enough to the north for the South Downs to work, but by Thursday doubts were appearing and by Friday it was looking likely that the wind was too far to the west for Parham. Judging by the lack of flights on the ladder (Parham pilots are very good at listing their flights) I suspect this was the case in the end. At least I did realise Friday’s potential the night before on Thursday 
, and indeed Friday was a good day down there.
Elsewhere a main hope for Sunday had been wave in Wales and the Welsh Borders which Phil King first pointed out to me on Thursday. Disappointingly I under-estimated the amount of cloud around on Sunday and that was the undoing at Shobdon, from where Diana King reports that “We were unable to use the primary wave in the lee of Radnor Forest because although there were gaps from time to time they were not large enough or open for long enough. A pity because there was quite strong lift - 4 kts at 3500 - 4000′.” As I mentioned on Saturday all the models showed an area of cloud extending down into England through the Cheshire Gap; it extended just a little too far westard in the event. Eastern and central areas were all much cloudier than any model forecast (though the GFS was closer than most, surprisingly for a global model*). All was not lost at Shobdon though: “Cloud amounts a bit further downwind were not so much. There was very weak lift on what was probably the tertiary wave and Peter Reading contacted at 2800, and eventually reached 10,000′.”
The wave patterns were very apparent on satellite on Sunday, and here’s a lovely picture from Terra:
To orientate yourself the Bristol Channel is visible in the lower left and Cardigan Bay in the upper left. The wave bars coming off the Cambrian mountains are very apparent under the veil of upper level cirrus.
A look at the analysis sounding again shows classic wave indicators of little direction shear but high speed shear, and nice big stable layer:
I had said I thought lift at lower levels would be weaker, when in fact it was quite strong. My error probably was thinking that the less stable air lower down would be less conducive to wave, when thinking about it should actually be better - less resistance to the first rising side of the wave. Adding a virtual parcel shows the stability and the convective inhibition resisting uplift, though there is a little area of instability above the condensation level:
Elsewhere it was a good ridge day. Nympsfield report on their news page that their ridge worked fine, as forecast. Gary and Jim provided excellent feedback from Booker. Again it seems there was strong speed shear even at low levels and I wonder if the sudden smoothness at 1,800’ on tow could have been where turbulence from the ridge was trapped by the high static stability - on Saturday I’d found the atmosphere to be highly stratified. While there was little wave lift the Stokenchurch ridge was working well despite the light winds at Booker - Jim reports “310 / 23Kts which is perfect for the bit from the M40 to Bledlow”. Going on to Wendover is best with a bit more west in the wind, Jim says. (I must say it’s very interesting to read all this - I know the area well, but from the saddle of my mountain bike. I know Whiteleaf particularly well as I broke my wrist there ). Do pilots ever head south down past Watlington Hill from Stokenchurch?
*The GFS also consistently forecasted Monday’s wind to be much lighter than the UK meso did, and was indeed correct.
Early Outlook
Really need to go back through the last few of these and see how reliable they’ve been proving. Anyway… tomorrow (Tuesday) cloudy with a little wind from the west. Wednesday again cloudy and wet in the west as a cold fronts thinks about crossing the country, before doing so on Wednesday night leaving Thursday a brighter breezier day. A southwesterly wind on Wednesday moves a little north on Thursday, probably enough for Nympsfield’s local ridge to work at least.
There’s good agreement that things are going to get rougher going into the weekend. Friday currently looks like having a bright but windy start before heavy rain moves in. Can’t say with worth-while certainty what Saturday and Sunday’s weather will be like but it looks likely that it’s going to be wet and windy at times, but probably fine at others!
In Other News
*One of Sandhill Farm’s K13s is now in very small pieces 
. I wonder if the wind gradient and/or rotor turbulence had a hand in the accident.
*Some folks in Belgium have winch launched a K21 to 5,640’, a new world record. Amazing what you can do with plasma rope and a 10,000’ runway . The winch used was a Mel.
