Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Los Angeles/Oxnard California
1100 PM PDT Mon Oct 24 2016
New aviation discussion
the onshore will linger into Wednesday with near normal temperatures.
Then a low will arrive Thursday and with subtropical moisture
from the south to mix in should peak the precipitation amounts
from Thursday night into Friday. Another low will bring possible
precipitation into early next week and temperatures should be
Earlier this afternoon, had an active area of showers and thunderstorms
develop across the southern coastal waters extending from near
Catalina Island through the San Pedro Channel into the Orange
County coast. This activity has diminished this evening, and
not expecting rainfall across our forecast area for the remainder
of the night. There is another weak cold front to the north
across Monterey County, but a building ridge of high pressure will
keep this front to the north of US. In the meantime, considerable
low level moisture will linger across the forecast area tonight
into Tuesday morning, which will generate areas of low clouds and
patchy fog across coastal/valley areas. Marine layer cloud
formation is expected to be rather chaotic tonight, but already
starting to see some low clouds develop across the central coast
and la County coast as of 830 PM.
With an upper level ridge of high pressure building over the
region the next couple of days, look for a warming and drying
trend. There will continue to be some periodic high clouds
drifting across the area during the next couple of days.
Taking a glance at the 00z NAM model, storm still on track
for later in the week. Still looking like highest probability
of rain will be in the Thursday night/Friday time frame.
The 00z NAM model is showing rather impressive subtropical
moisture feed with this system, with precipitable water values
of 1.5 inches or greater. This model also showing strong
vertical lift and some elevated instability which could
support a slight chance of embedded thunderstorms with the
front, especially sba and slo counties.
*** From previous discussion ***
Radar this afternoon showing that the shower and thunderstorm
activity has moved east of the forecast area, but can't rule out
a brief shower could form in far eastern la County in the next
couple of hours. Meanwhile, satellite imagery is showing an
impressive upper-level low off the coast of the Pacific northwest.
Tonight through Tuesday, an upper-level shortwave will rotate
around the western periphery of the upper low, causing the long-
wave trough to deepen off the coast through Wednesday. As the
long-wave trough deepens, the ridge over the western US will
amplify, causing 500 mb heights to rise over socal. As a result,
slight warming is expected Tuesday and Wednesday, but high
temperatures will still be slightly below normal or near normal,
with highs in the 60s near the coast to low 80s in the valleys.
After a mix of mid and high clouds this afternoon and tonight,
most areas should see more sunshine Tuesday and Wednesday. The
low clouds tonight and Tuesday morning will probably be rather
chaotic with the mid and high-level clouds moving through the
area. Better chance for a more defined marine layer Tuesday night
into Wednesday morning, with the marine layer inversion
strengthening. It should be a fairly shallow marine layer though,
so probably confined to the coastal areas.
Wednesday night into Thursday, the previously mentioned short-wave
trough is expected to develop into a closed low at 500 mb several
hundred miles to the west-southwest of pt. Conception. The GFS and European model (ecmwf) both
show a weak surface low developing as well. This will likely allow
this system to tap into some subtropical moisture. Some remnant
moisture from Hurricane Seymour could also get drawn northward
into our region.
There continues to be some timing differences between the various
models. The European model (ecmwf) brings precip into the central coast by
Thursday morning and for areas south of pt. Conception by Thursday
afternoon. On the other hand, GFS and the NAM hold off until
Thursday night for all areas. Tried to split the difference
between the models, with chance pops from Ventura County north
and slight chance for most of la County by Thursday afternoon.
It looks like the best chance of rain for most of the area will be
Thursday night through Friday morning, as this is when both the
GFS and European model (ecmwf) show the potential for moderate to heavy rainfall.
This is where the biggest change was made to the forecast, with
pops increased to 70-80% from sba County north, and 50-60% for
Ventura and la County. The chance of slo and sba County getting
rain with this system is near 100%, but didn't go that high yet
for any given period due to timing concerns.
The GFS ensembles suggest that the best chance for heavy rain will
be north of pt. Conception. Heavy rain is also possible for
Ventura and la counties, but not as likely as up north. A couple
of inches of rain are possible on the central coast, with Ventura
and la counties likely seeing less than an inch. The exception
will be the upslope areas and anywhere that a thunderstorm
develops, where higher amounts will be possible. Thunderstorms
have not been included in the forecast yet, but there is a good
chance that they will be added in the next day or two. The best
chance for thunderstorms will be Thursday night through Friday
morning. Flash flooding and debris flows could be an issue,
especially for the burn areas.
Gusty south winds will also be likely late Thursday into Friday,
especially for the mountains. Wind advisories may eventually be
needed in some areas. Snow will not be an issue, with snow levels
during the bulk of the precip around 10,000 ft or higher, as the
result of the deep southerly flow.
Rain chances will start to decrease Friday afternoon into
Saturday, but here could be some lingering showers in a few
places. Another frontal system could bring another round of rain
for Sunday or Monday, but it looks much weaker than the previous
At 0545z at klax...there was no marine layer nor an inversion.
Low confidence in coastal and vlys tafs as low clouds will likely
randomly form and dissipate overnight and through mid morning. If
clouds do form they will be in the MVFR hgt range.
Klax...low confidence in taf. Bkn015 cigs are possible anytime 08z-
Kbur...low confidence in taf. Bkn015 cigs are possible anytime 08z-
Shower and thunderstorm activity has ended across the coastal
waters this evening. There is high confidence that winds and seas
will remain below Small Craft Advisory levels through at least Wed night. South to
southeast winds will likely reach Small Craft Advisory levels by Thursday night for
the outer waters as the next Pacific storm moves into the region.