On Feb 27, 9:47*pm, Fred wrote:
Good article, John. *It deserves wider distribution. Thanks for the
analysis.

Anyone who flies out west knows about "rivers of sink". However, many
can't decouple the MacCready of the flight director from the computer
(SN-10 users, for example). I have typically used a height reserve
above my chosen MacCready setting (something like 1,000 feet for 25
miles). Flying dry in weaker winter conditions, I usually use a
MacCready of 3 to 4 and up that to 5 to 6 in summer conditions flying
ballasted.

I have also been experimenting with forecasting these "rivers of
sink". Even in the blue, there tends to be some minor wave activity.
If you have a local RASP, you can see these on the Boundary Layer Up/
Down Motion plot or on the HRRR plots of average vertical velocity.
We have seen quite good agreement between these forecast wave/
convergence lines and both lift and sink. For example, last Sunday we
had a line of cumulus form exactly where we had a forecast wave line,
running from southwest to northeast about 20 miles south of our field.

In the blue you can't see these lines, but it is useful to know which
way they are aligned. If you end up flying in a "river of sink",
chances are you are running down one of these wave lines and you need
to turn at right angles to it to get back in lift or zero sink. The
forecasts might not predict them in the right place, but usually get
the alignment right. Check the plots before you fly. Better yet,
print a copy and carry it with you.

Mike