Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 242355
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
455 PM PDT Mon Oct 24 2016
The current storm activity will end this evening...then a marine
layer will linger into Wednesday. A large low will arrive Thursday
and persist into the weekend for precipitation...with the peak
amounts from Thursday night into Friday. The onshore flow should
allow for a marine layer and temperatures below normal
temperatures into early next week.
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 NW.
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 WSW of Pt. Conception. The GFS and 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 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 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 2330Z at KLAX...there was just a hint of a marine layer.
The model data shows ample moisture lingering in the forecast area
for the next 24 hours, but only moderate confidence in cigs
developing due to weak marine inversion and satellite imagery not
showing much in the way of organized low clouds offshore.
If cigs develop, looking for mvfr conditions across most
coastal/valley areas, except ifr conditions north of Point
KLAX...Moderate confidence in mvfr cigs returning to KLAX
overnight into Tuesday morning, but confidence is low in timing.
There is a 20 percent chance that cigs could lower in IFR
KBUR...Moderate confidence in mvfr cigs returning to KBUR
Overnight into Tuesday morning, but confidence is low in timing.
There is a 30 percent chance that cigs could lower in IFR
There is a slight chance of thunderstorms across the southern
waters through the early evening hours.
There is high confidence that winds and seas will remain below
SCA levels through at least Wed night.