Scientific Forecaster Discussion
Southwest California area forecast discussion
National Weather Service Los Angeles/Oxnard California
1030 PM PDT Monday Oct 12 2015
New aviation discussion
A low pressure system to the south-southwest of the area will bring
a cloudy and humid weather pattern along with possible shower and
thunderstorm activity through Thursday. A cooling trend will occur
this week with near normal temperatures by early next week. A second
low pressure system to our north will arrive Saturday and bring
possible showers from late Saturday into early next week.
Southeast flow aloft around a low pressure system to the south-
southwest of the area will continue to wobble around over the
offshore coastal waters through Wednesday before coming ashore
Thursday. Radar imagery continues to indicate isolated showers
streaming over the area this evening near the Los Angeles and
Ventura County line. A thick cloud shield in place over the area on
satellite imagery this evening will inhibit radiational cooling
processes tonight. Some locations south of Point Conception are
running about 5-7 degrees warmer than this time last night. Lax
international Airport is still 84 degrees...downtown Los Angeles 87
degrees...and Long Beach Airport 88 degrees. A warmer night looks on
tap due to the moisture moving into the area from the south.
Model solutions are indicating moisture continuing to the move over
the area into Tuesday. Model solutions keep a moist air mass in
place...with 850 mb mixing ratios in excess of 12 g/kg over the
Southern California bight. Precipitable water values...near 2 inches
over the coastal and offshore waters...are almost five Standard
deviations above normal for this time of year. Model solutions do
indicate lifted index values turning negative mainly south of Point
Conception Tuesday. A slight chance of showers and thunderstorms was
added for Tuesday and Tuesday afternoon with the moisture and lift
in place. The only thing that is missing would be a lifting
mechanism to work with for better precipitation formation.
With the cloud shield moving the area...stratus formation will be
difficult and mentions of fog have been removed from the forecast.
MOS guidance is starting to increase the temperatures and dewpoint
spreads for coastal terminals. A patch of stratus cannot ruled out
or some limited visibility due to haze...but for the most part...the
main story is the moist air mass in place.
An update was issued earlier to warm temperatures tonight...cool
temperatures Tuesday...and increase probability of precipitation and add weather through
Short term...as the upper low gets closer to the region on
Wednesday...NAM and GFS models in agreement on increasing southerly
flow and associated moisture/instability across the forecast area.
Best threat of showers and thunderstorms with this system seem to be
focused on Wednesday into Thursday. In afternoon forecast...have
bumped up probability of precipitation to chance category for a large portion of the
forecast area from Wednesday into Thursday...especially interior
sections. Also...expanded the threat of thunderstorms to now include
coastal/valley areas...with an increased threat over the mountains.
12z and 18z NAM continues to ramp up convective potential with this
system Wednesday into Thursday. Very unstable conditions being
depicted over the mountains(with lifted index values up to -
10)...where there will be the best chance of thunderstorms. As the
southerly moist flow increases across Los Angeles and Ventura
counties and the adjacent coastal waters on Wednesday and Wednesday
night...NAM model increases the most unstable cape values with a
large area of 500-1000j/kg readings...with isolated readings as high
as 1500 j/kg across the coastal waters. One other factor promoting
convective potential with this system is the veering wind profile
depicted in the model soundings.
Precipitable water values will be on the rise Wednesday into
Thursday...with values generally ranging between 1.3 and 1.6 inches
with this system. This kind of moisture and instability brings the
potential for heavy downpours with any thunderstorms that do
develop. However...on Wednesday...steering flow from the south will
be fairly strong so storms should be moving fairly quickly.
However...will have to keep an eye for potential storm training on
Wednesday. On Thursday...the steering flow is expected to be
weaker...thus increasing the flash flood potential over the
mountains and deserts.
Long term...still some timing differences for next weak weather
system for the weekend. Ec model continues to trend faster and digs
trough further south giving US a better chance of showers for the
weekend...especially in the mountains. The GFS is slower and not as
aggressive with the trough. For now...will keep extended forecast as
is with just a slight chance of showers for Saturday night into
There was no marine layer at klax. There was a surface based inversion
with a top of 850 feet with a temperature of 31 degrees celsius.
Good confidence in tafs. There is a 20 percent chance of a thunderstorm
at all sites Tuesday from 18z-04z.
Klax...good confidence in taf. There is a 20 percent chance of a
thunderstorm from 18z-04z.
Kbur....good confidence in taf. There is a 20 percent chance of a
thunderstorm from 18z-04z.
Mainly light winds are expected across the waters through Wednesday.
Northwest winds will then increase over the outer waters Thursday
through Saturday. An upper closed low west of Baja California California will
continue to drift northward through Tuesday...bringing isolated
showers and thunderstorms to the coastal waters Tuesday through
Special Weather Statement (see laxspslox).