Labor Day (September 2, 2002) was a remarkable day in the Sierras with many long flights. I was particularly interested to hear of reports that a convergence line developed which moved eastward with time, since the BLIPMAPs predicted the occurrence and eastward movement of just such a convergence! I have created a special viewer sequence to look at the hourly development and movement of that convergence line forecast, with viewing instructions and a brief description below.
The BLIPMAP convergence predictions can be viewed by going to either a
viewer that starts looping automatically - no clicking is
required!
OR
the usual BLIPMAP viewer
where I have already pre-set the correct values for
"Date", "Time", and "Parameter" but you are required to do the hard
work of first clicking on "INSERT MAP" to load the hourly maps and
then using the FRAME "FORWARD" AND "BACKWARD" buttons to move through
the images.
The plots readily show movement of the convergence line. Evaluating magnitudes is trickier since the coloring scales automatically with the maximum plot value: the vertical velocity reaches the day's largest values between 22-0Z and is relatively constant over that period, but for other times comparison to the color bar is required since the maximum vertical velocity is then weaker. If you are not sure what to look for, the convergence line is very prominent over the Sierras at 21Z with light blue and yellow-red coloring - so you can start from 21Z and go forward/backward in time to see the movement of the line.
What I see is an initial early morning development of a convergence line at the southern end of the Sierras, at 17Z extending roughly from around China Lake northward along the west side of the Sierras. As the day progresses the line extends itself further to the north and moves eastward, the eastward movement being somewhat more rapid to the north since in the south it remains over Mt. Whitney until 1Z. By 20-21Z the line has extended to north of Lake Tahoe and it subsequently moves east of Lake Tahoe in the north and east of the Sierras to+beyond the White Mts in the south. By 2Z it has largely dissipated, though isolated pockets remain at the north end.
I would be interested in hearing any flight reports with specifics
which con confirm or disagree with these predictions. I note
that they appear to match almost exactly the description in Brian
Choate's "900k out and return from Truckee" report posted to the HGC
Community Board
http://groups.yahoo.com/group/hgcgroup
which I quote below:
Gavin Wills and myself launched out of Truckee about 12:30 monday
09/02/02 into a convergence line that ran down the west of Lake Tahoe
on the Sierras.
We flew the duo on a recreation flight down the Sierras to Mt Whitney,
got there at 3:30 where we decided to turn around and head home, we
crossed east over Lone Pine to the White Mts and headed north, where we
experienced the strongest lift I have ever seen, we were cruzing at
17,000 ft 100kts and the averager was 16.6 kts! We left Boundry Peak,
and continued up the same convergence (which had now drifted east) to
the Pine Nut Mts and continued to follow the convergence line all the way
to Airsailing, we turned Airsailing at 16,000ft with an easy final
glide to Truckee. With a straight line distance of 900k in 6 hrs.
Sept 9 Addition
Satellite cloud images and a second convergence line
You can view visible satellite images showing the convergence
line movement using one of the following javascript viewers
These viewers start looping automatically - no clicking is
required!
At slow bandwidths, loading can be very slow due to the large image sizes
Hourly intervals (1.2MB, ~4 min loading at 50K/s bandwidth!)
OR
30 minute intervals (2.4MB, for high bandwidths only)
Comparison of these images to the forecast is astounding, because the satellite clouds not only show good agreement with the predicted Sierra-Tahoe convergence line and its movement (including such features as the movement of cloud growth eastward from the Sierras to the White Mts with time) but because there is also agreement between a different convergence line predicted to extend at approx. right-angles to the Sierra-Tahoe convergence line, cutting across the NW corner of Nevada, and observed growth of such a line in the satellite cloud image! This latter line is particularly remarkable since unlike the Sierra-Tahoe line there is no topographic ridgeline in that direction! Instead, it appears to be a convergence that extends far downstream of Mt Lassen. Has anyone ever flown such a convergence line in that direction?
Convergence can be a sensitive parameter so not every convergence
feature predicted by the model will occur in reality, but the
agreement of
two different convergence line forecasts with observed clouds
suggests that forecast convergences are very apt to occur in the
actual atmosphere and can often be flown.