[shadow=pink,left]For the last few years I've had an inkling that RASP's surface temperature forecasts (and hence updraft velocity, cloudbase, etc) under-predict the effect of high cloud. Of course, it is only a layman's observation and might well be wrong, but a number of days in the past were forecast to be pretty epic from an XC point of view, but ended up being mediocre with high cloud shutting off most of the sun.
According to RASP, tomorrow looks quite a good day for us, (good updraft velocities, little shear and 5000ft bases by mid afternoon) but there is some high cloud forecast, so I'm watching to see if this reduces these values and plays a significant part on the day.
GFS suggests that there'll be a fair bit of altostratus over the UK
RASP shows it on their predicted sounding for 12pm
...and more significant amounts by 2pm and 4pm
Anyway, I just thought I'd post my ponderings, try out the image function on the new website (and, yes I'm sorry - play with font size, drop shadows etc!) but hopefully provoke some thought and discussion about using RASP to predict good flying days!
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Well, there were a few thin streaks of high cloud, but not as extensive or as deep as forecast, and not enough to make a difference to the day. In fact the RASP predictions for cloudbase, temperature and updraught velocities were pretty close to what transpired, so it's difficult to draw any conclusions on whether they undervalue the effects of extensive top cover.
