RobMcKenzie <>
writes:

We are a bit of a microclimate here based on the shape of the
local mountains and our proximity to the Cajon pass. When wind at the surface
thru 15K heights come from the west thru north directions we get a wrapping of
the winds around the tall terrain to our northwest. These are the San Gabriel
Mts.


We almost never actually get a true northwest wind here. It either wraps around
the south side and our winds are SW or it wraps the north side and we get a
north wind (often northeast) When we get wind strong and directly down the
Mountain range (from northwest or southeast) it's often related to thunder storm
gust fronts.


I visualize the San Gabriels being like a rock in a stream. The water can't
quite go over the top so it deflects around both sides and there is a
convergence on the lee side. Sometimes the convergence is north of the Marshall
launch and we get the southwest wind and sometimes it's south of us and we get
the north wind.


So far this is just shape. Another huge factor here is the air temp and
humidity. The ocean brings a moist cool "marine layer" in and often permeates
the entire inland empire splashing like a wave against the ring of our
surrounding mountains held in place like the banks of a pond of water. This
inversion is usually not taken into account by the FSL and blip maps (Dr. Jack
info). Or they know it's there, but not exactly how far inland it has gone. I
have to read between the lines to determine whether what they are saying will
actually happen.


Example: The FSL says it's a good lapse rate up to 9K. This would be great
except when I look outside (and looking at the web cam) it shows a very weak but
noticeable marine layer up to 3K (half way up Marshall) at sunrise. What this
does is make it not heat up to the trigger temp and we could very likely only
get heights to what the marine layer gets up to which could be only 4800' msl
for example. Been stuck in this inversion the days you have so far flown here.
It's a bit of an art (sometimes roll of a dice) deciding whether the temps aloft
will actually play a part in how high our thermals actually go.


Another art is deciding whether a thermal block happens. During weak Santa Anas
if the lapse rate is good enough, the Marshall area will switch upslope and have
fantastic thermals up to maybe 9K or 10K even though it blows over the back at
Crestline. Climbing and drifting out over the city, yet still cycling up at
Marshall. Nice. This is a possibility today. But I'm leaning slightly toward it
staying over the back. Lapse rate not that great and the strength of north is
very weak. Tough call. Perhaps a 40% chance that it does blow in at Marshall but
probably fairly weak soaring if it does.

Rob wrote this before 8 AM on Sunday. By 9 AM he wrote:

Sunday November19th.  I'm thinking we may (75% chance) see a
late morning switch to upslope and flyable for Marshall and Crestline today.
Lift is weak but probably soarable with thermals to 5 or 6K'. Keep an eye on the
windtalker or the Simba report. Yesterday blew down all day.

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