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Sailplane reports on RUC convergence forecasts

Posted By: Jack Glendening
Date: 26-June-03 0538Z

I earlier posted documentation of a remarkable case of a moving
non-precipitating convergence line, initiated over the Sierras, which
was predicted by RUC and verified by satellite cloud photos and by
reports from sailplane pilots who flew in it. Since that time I've
received numerous reports from sailplane pilots in another region of
California, the northwest region of the Central Valley, where they are
using daily RUC predictions of convergence-produced upward motion as a
basis for their flights, and finding it accurate enough that they have
come to rely on it. I would like to briefly report on these because
they come from a more lengthy set of experiences, from a different
region, and illustrate a smaller-scale phenomenon seldom reported on.
One caution here is that I will only be referring to convergences
generated by relatively large-scale mountain ranges, which are
reasonably resolved by RUC's 20 km resolution.

These convergences are usually generated by flow over the Mendocino
Mts - their occurrence is not rare, but is not a daily phenomenon and
the location at which it occurs, whether it moves, etc. all vary.
Pilots find the RUC predictions most useful for determining whether a
convergence will be occurring that day and where it might be expected
to occur, so they can know if and where to look for it. Vertical
velocities are often largest toward the end of the day, both in
predictions and reality, and some pilots have gained enough confidence
in the forecasts that if a convergence is predicted they are willing
to venture very far from the airport and depend on the convergence to
get them back to the field late in the day, when no thermal lift exists.

One quote the pilot most experienced in utilizing these lines (over 40
flights in convergences):

"Almost every forecasting expert I've talked to is very skeptical
about this parameter - I used to share their opinion. After many many
flights and actual experience I don't give a damn what they say - they
are WRONG." <http://www.drjack.net/cgi-bin/forum.cgi?read=105#105>

[BTW I am one of the people he is referring to there]

That same pilot believes enough in the predictions that on a day when
RUC was predicting a very atypical convergence line, one in the
_northeast_ corner of the valley, he took a tow in the direction
opposite to the normal one and glided expecting to find the
convergence - and found it, to the great surprise of other soaring
pilots:

"Tow to Chico?? or get there on a flat glide with only 2,700 ft???
Nobody does things like that!!"
<http://www.williamssoaring.com/_live1/00000261.htm>

So these pilots are finding that many relatively small-scale
predictions, of which I have tended to be skeptical due to model noise
etc, are verifying remarkably well (though I emphasize that the
predictions are being used to answer qualitative questions, i.e. will
there be a strong convergence today, how will it change with time,
etc).

What triggers this report is the occurrence a few days ago of an
unusual prediction of very strong conditions at a time of year when
they are extremely rare, which had the locals shaking their heads:

"Okay, I don't believe it either, but BLIPMAP (RUC) is showing a VERY
weird day, with a small area right around Williams going to 13,000.
There is also a mighty Godzilla convergence (150+) going north to T-15."
<http://www.soaringnet.com/_williams_disc/0000006d.htm>

but that turned out to be another correct forecast:

"... June 23, 2003 was one of the great days that WSC has had, it was
not a typical Maxwell shear (convergence) day but a low pressure
induced variety of convergences, one for the books and BLIPMAP (RUC)
called it!" <http://www.soaringnet.com/_williams_disc/00000079.htm>

In summary, the response of many pilots to these relatively
small-scale RUC forecasts has been very favorable, and these pilots
are very concious of forecast deficiencies since for them not getting
back to the gliderport means significnat inconvenience and expense.
I'll end with a quote from a long-time weather-observing pilot there:

"We are learning more things about soaring, more about weather,
especially convergence, and BLIPmaps, than we ever thought was even
possible." <http://www.williamssoaring.com/_live1/00000261.htm>

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