discrepancy between XC Map and Route cross section

[Josh 1234]

I noticed a discrepancy between the XC Map and Route cross section
today for the Dunlap to Owens route for tomorrow. The route is
roughly between Squaw Valley, CA (just E of Fresno) and Independence,
CA. Looking at the Map, the Top of Usable Lift Above Ground has a
minimum of 2-3k ft. But looking at the Route cross section, the
middle part shows lift not getting above the ground. Not nearly as
encouraging.

[Chris Galli (XC Skies)]

Hi Josh. OK. Hold on to your hat and glasses...:)

With the GFS, there are actually 3 different versions to determining
the top of lift. The first two most people understand because it's
described in a mouse-over note on the main Map application. The layer
called Top of Usable Lift (Alt) is an experimental version of the more
basic approach to finding the top of usable lift (as noted in our
parameters descriptions).

http://www.xcskies.com/science/products/xcmap/docs#thermal

The other version of lift determination is in the route and new 3 day
point forecast code. This code uses a modified nested grid approach that
is likely even more accurate than the two other approaches (not always,
of course). All 3 approaches will be very close to each other except
along dramatic terrain features (such as the Owens Valley, the central
Alps, etc.) or when the air mass is highly sensitive to small changes in
surface temps. When terrain plays the determining factor on how unstable
the area will be, these three approaches will start to show different
results due to their sensitivity.

Looking specifically at your route (or at least a cross section from
Dunlap to Independence), I can see that the sensitivity to small changes
in surface temperature is high along the western slopes of the Sierras.
If you look at a cross section 20 miles north across the Sierras, you'll
see that the top of lift is a good 4000K+ over the ground. To the south,
another cross section shows a stable western slope over the Sierras like
the Dunlap line.

When this sort of thing happens, I start looking at the NAM and RUC for
clues on the forecast. NAM is run out 3 days so looking at that I can
see that the map overlays for GFS vs NAM are nearly identical for
mid-day. This would lead me to think that the lift potential will likely
be better than what the route forecast is showing.

Interesting to note that the Dunlap to Indp. route for today and
yesterday showed a similar feature of stability on the western slopes,
but taking a quick look at the 1km satellite loop for today, I can see
that it was plenty unstable with mature cumulus over the entire Sierras.
Another reason to think that tomorrow will be good given the conditions
are not changing much for that region over the next 24 hrs.

-Chris