Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 240400
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
900 PM PDT Sun Oct 23 2016
Isolated to scattered showers will continue through Monday
morning, with the best chance for rain in Los Angeles County,
where there is also a slight chance of thunderstorms tonight.
Slight warming is expected on Tuesday and Wednesday, then a much
stronger storm system could bring widespread rain on Thursday and
Friday. Unsettled weather may continue into next weekend.
Broad upper level trough of low pressure continuing to pull in
subtropical moisture into Southern California tonight. Satellite
imagery showing a broad area of precipitable water values of 1 inch
or greater off the coast, with a max of 1.4 inches off the coast of
San Diego. Most of the shower activity so far has been focused from
Los Angeles county south and east, with a few light showers and
sprinkles over Ventura county. Earlier this afternoon and evening,
there was an active line of thunderstorms across Orange County and
the Inland Empire, extending into the coastal waters. During the
past hour, isolated thunderstorm activity has also been reported
across portions of Los Angeles county.
Influx of mid level moisture and some upper level instability
will continue to keep scattered showers and isolated thunderstorm
across much of Los Angeles county tonight into early Monday
morning, with just a slight chance of showers for Ventura and
southern SBA counties. Much of the LA Basin and Ventura county
will see less than a tenth of an inch with this storm, however
eastern portions of Los Angeles county could see amounts in the
quarter to half inch range, with local amounts between one half
and one inch in the eastern San Gabriels. Since the upper level
flow is fairly strong, any thunderstorms that develops tonight
should be fast moving, producing rainfall rates up to one quarter
inch per hour(with most of the rainfall likely occurring in
a 15 minute time frame). If a thunderstorm happens to develop
over a recent burn area, there is the potential for minor mud
and debris flows. With the strong winds aloft and a rather dry
layer near the surface, gusty and erratic winds will also continue
to be a threat with thunderstorms tonight. We have already seen
localized gusts over 40 mph across the region.
*** from previous discussion ***
Winds will ease and precipitation will end on Monday afternoon.
Skies will remain partly to mostly cloudy as there is no notable
break coming behind this system. Afternoon temperatures will
remain cooler but the cloud cover will keep morning temperatures
from dropping much.
Temperatures will warm a bit Tuesday and Wednesday and partly
cloudy skies will come and go as a sign of a few wiggles in the
atmosphere moving through the region.
A much stronger low pressure system will move toward the Pacific
Coast on Thursday. There remains a fair amount of uncertainty how
far south the system will move and hence what impact will be felt
in Southern California. Of most consequence would be a system that
slides to down off the Southern California coast prior to moving
inland over the region. In addition to being more directly
impactful to the region... it brings a better chance of tapping
into a tropical system currently off the Mexican coast that would
provide a good amount of warm and moist air.
For now the forecast package will keep a more conservative
approach. However a close eye will be kept on how the low develops
and moves as any notable precipitation would have a significant
impact on recent burn areas due to potential flash flooding and
Any precipitation looks to end by late Friday with some possible
slop over into early Saturday morning. Afternoon temperatures will
be on the cool side throughout the event then slowly warm with the
sunnier weather on Saturday.
At 0000Z, the marine layer was about 900 feet deep. The top of
the inversion was near 2800 feet with a temp of 22 degrees C.
Low confidence in TAFs tonight due to interaction between the
marine clouds and mid level subtropical clouds which may prevent
solid marine cloud coverage from forming overnight. Scattered
showers across portions of LA and Ventura counties this afternoon,
with heaviest showers and isolated thunderstorms extending off the
coast of Long Beach and adjacent coastal waters to the south.
For now, have included VCTS for KLGB through this evening, but
may need to include for KBUR, KLAX, and KVNY at some point
tonight as there is a 10-20 percent chance of tstms for these
areas. Due to the dry air at low levels, there could be locally
gusty and erratic winds near showers and thunderstorms through
the evening hours. As low level moisture increases, looking for
many coastal/valley TAF sites to have IFR/MVFR cigs overnight into
Monday morning, possibly extending in some areas into Monday
KLAX...Low confidence in 00Z TAF issuance. Another burst of
southeast winds associated with band of convective showers/tstms
to the south off the coast of Long Beach and Orange county.
Expecting these SE-E winds 8-10 knots with occasional gusts to 15
knots to continue through 03z-04z...becoming variable afterwards.
Than lighter SE-E winds expected to develop overnight into Monday
morning, in the 5-7 knot range. There is a 30 percent chance of
showers and a 10 percent chance of thunderstorms tonight that
could be associated with gusty and erratic winds. Low confidence
in lower level cigs developing tonight and Monday morning due to
influx of mid level subtropical moisture.
KBUR...Low confidence in the 00Z TAF issuance. There is a 30
percent chance of showers and a 20 percent chance of
thunderstorms that could be associated with gusty and erratic
winds. Low confidence in lower level cigs developing tonight and
Monday morning due to influx of mid level subtropical moisture.
There is a 20 percent chance of some local SCA level winds gusts
early tonight across the northern and outer waters. Otherwise,
fairly high confidence that winds and seas will remain below
SCA levels through at least Wed night.
There is a slight chance of thunderstorms across the southern
waters through early Monday morning.
Isolated thunderstorms will be possible for much of LA County
through late tonight. Isolated dry lightning strikes will be a
possibility, as there is a fair amount of dry air at low-levels,
and thunderstorms may produce less than 0.10" of rain. There have
already been several cloud-to-ground lightning strikes in LA
County this evening, with less 0.10" of rain. Any thunderstorms
that develop could also produce gusty and erratic winds.
Isolated showers could continue through Monday morning,
especially for LA and Ventura counties. A broad trough of low
pressure will persist off the West Coast on Tuesday and Wednesday,
producing gusty onshore winds for the mountains and interior
areas, where minimum daytime humidities could fall into the teens
and 20s. There is the potential for a significant storm during the
Thursday-Friday time frame, that could produce moderate to heavy
rain, thunderstorms, and gusty winds. Debris flows on recent burn
areas could be a concern.
CA...Beach Hazards Statement in effect until 9 PM PDT this evening
For zones 34-35. (See LAXCFWLOX).