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Southwest California area forecast discussion 
National Weather Service Los Angeles/Oxnard California
420 am PDT Tuesday Oct 13 2015

..updated aviation discussion...


A low to our southwest will bring possible shower and thunderstorm
activity this afternoon through Thursday evening. A slow cooling
trend this week with near normal temperatures by Sunday. A second low
to our north will arrive Saturday and bring possible showers from
late Saturday into early next week.


Short term (today-thu)... overnight lows look a lot more like highs
than lows this morning. Temperatures in many locations are still in the
upper 70s and 80s. The highest minimum temperature record for downtown l.A.
And Burbank are 71 and 73 degrees respectively. Its seems quite
likely that these records will be smashed this morning. These
impressively warm overnight lows are the result of a surge of high
clouds that arrive too late yesterday afternoon to moderate temperatures
but have prevented radiational increase in humidities
which lessens the ability for the air to cool. Maximum temperatures will again
rise to well above normal values due to the huge running start they
will have this morning. The only variable will be how much middle level
cloudiness there will be. Right now it looks like there is a
substantial amount of clear skies rotating into the area so skies
should be at worst partly cloudy. There is quite a bit of moisture
today and it should be rather unstable across the mountains and Antelope
Valley and this will result in a slight chance of thunderstorms this afternoon and

All models are consistent with the handling of the upper low to our
SW. It will hardly move today through Wednesday morning and then it
will slowly move to the NE reaching Point Conception by Thursday
afternoon. What the models have absolutely no agreement on is the
moisture pattern and hence the precipitation pattern. This is very
typical of systems coming up from the SW where there is little or no
observational data. Broad brushed a slight chance or chance probability of precipitation across
the entire area from Wednesday afternoon to Thursday evening as the
low approaches with some upper level divergance and weak vorticity
advection. The best chance of showers on Wednesday will be over the
mountains and then on Thursday it will be over the sba/vta region which
will have the greatest positive vorticity advection. Precipitable waters are very high and as the low
approaches Thursday there could be some heavy showers or thunderstorms. This
tropical system will be quite unstable and thunderstorms will be
possible along with the showers.

Finally as this is a tropical system humidities will once again rise
and make things feel more uncomfortable than the temperature would

Long term (fri-mon)... the upper low moves to the east Friday and
Sat and this should end the shower threat (maybe a afternoon thunderstorm in
the mountains friday) heights will fall to 576 dm so maximum temperatures will fall to
near normal values and humidities will lower as well so it will
finally feel pretty comfortable.

Another trough/upper low moves into the area Saturday night. The ec is
quite aggressive with this feature while the GFS shows a weaker
system. Trended into the forecast with mostly cloudy skies and a chance of
shower. Will monitor the system and if the GFS wants to trend to the
wetter more aggressive or vice versa. Whatever the case with the rain
the clouds and lower heights will conspire to bring below normal temperatures
to the area.



At 080z...there was a surface-based inversion. The top of the
inversion was 1400 feet with a temperature of 29 degrees celsius.

Overall...moderate to high confidence in 12z taf package. Middle/high
level clouds will drift across all sites through the period with VFR
conds expected. There is a slight chance of thunderstorms this
afternoon over the Ventura/Los Angeles County mountains as well as
the Antelope Valley. For tonight/Wednesday morning...there is a 20%
chance of MVFR ceilings/visibilities developing across coastal and valley taf

Klax...high confidence in 12z taf. There is a 20% chance of MVFR
ceilings/visibilities developing after 08z tonight.

Kbur...high confidence in 12z taf. There is a 20% chance of MVFR
ceilings/visibilities developing across 09z tonight.



13/200 am

For the outer waters...high confidence in forecast. Winds are
expected to remain below Small Craft Advisory levels through
Thursday. By Friday and Saturday...northwest winds will increase
with a good chance of reaching Small Craft Advisory levels.

For the inner waters...high confidence in forecast. Winds are
expected to remain below Small Craft Advisory levels through
Thursday. On Friday and Saturday...there is a chance of west to
northwest winds reaching Small Craft Advisory levels north of Point
Sal as well as across the western half of the Santa Barbara Channel.

There is a slight chance of thunderstorms Wednesday and Thursday as
an upper low drifts north to northeast across the coastal waters.


Lox watches/warnings/advisories...
Special Weather Statement (see laxspslox).





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