Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 270021 AAA
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
521 PM PDT Wed Oct 26 2016
...update for new aviation discussion...
A low will arrive Thursday with subtropical moisture to create a
slight chance of thunderstorms and peak precipitation amounts from
Thursday into Friday. Another low will bring possible precipitation
from Saturday into early next week with temperatures below normal.
The forecast remains on-track with main impacts expected to be
our most significant widespread rainfall of the month and the
potential for heavy rainfall on recent burn scars from late
Thursday through midday Friday.
For tonight, the marine inversion will be about the same as last
night, roughly 1000 feet or lower with areas of dense fog possible
near the ocean. Not as confident in fog due to the higher clouds
overhead which can scramble fog formation. Today`s cirrus was
compliments of a 90+ kt subtropical jet originating well south of
20N in a plume northwest of even deeper moisture associated with
Hurricane Seymour. This plume will pull precipitable water values
upwards of 1.5 inches along our coast by Thursday afternoon with a
peak of 1.8 inches, a record high for this date.
Rain is expected to start along the San Luis Obispo County coast
perhaps before noon on Thursday, then increase later Thursday
afternoon as the eastern Pacific low pressure system and cold
front advance eastward. Overall timing with this storm has been
delayed by a few hours which seems to be associated with how the
main low pivots and lifts northward late tomorrow. Light sprinkles
or rain will begin across Santa Barbara and Ventura Counties
Thursday afternoon, but the bulk of the heavier rain potential
will arrive during the night then shift to Los Angeles County
before sunrise on Friday. As the cold front moves through and we
get stronger southerly flow with the jet there remains the
potential for heavy rain bands and embedded thunderstorms.
Although individual cells will be moving rapidly, downpours
exceeding rainfall intensities of 0.5 inch per hour could trigger
mud and debris flows for the recent burn areas, especially those
that have not received heavy rain yet this season. Will proceed
with a flash flood watch for all burn areas which covers most of
our county warning area, starting earliest in SLO Co. (3pm Thu)
and holding off until 3am Fri for southern Ventura and Los Angeles
Counties. All watches will go through noon on Friday, but this
will depend on how rapidly the threat of heavy rain ends.
Rainfall totals still look the same with most areas seeing 0.5-1
inch, but potentially higher where thunderstorms occur. Foothills
and mountains will have higher totals due to strong orographic
lift and the very moist subtropical airmass with 1-2 inches. The
coastal hills of SLO County will likely be the precipitation
winner due to the duration of rainfall there with 2-4 inches
Scattered showers will continue in the moist southwesterly flow on
Friday afternoon, but amounts should not be an issue for burn
areas. Precipitable water values remain elevated going right into
the weekend and there is potential that light rain will redevelop
in the moist flow by Saturday afternoon. Temperatures will be
impacted by the rain, but overall it should remain mild due to the
subtropical nature of this entire event. Snow levels will remain
well above most higher mountain peaks through Saturday.
Then a second upper level eastern Pacific trough slides across
the northern part of the state on Sunday which would give us
another chance of rain, mostly across northern counties. This
system should be a little cooler and will knock a few degrees off
afternoon high temperatures, especially in the mountains.
High pressure begins to ridge across the region by Halloween with
drier and warmer conditions rebounding across the state. However,
the storm train will continue with another system poised to move
across central and northern California on Tuesday. There are
typical timing and strength issues with medium range models, but
have brought a slight chance of rain into the central coast area
for Tuesday, then dry again by Wednesday. Lower confidence in this
scenario, but the storm next week does not indicate much of an
impact for southwest California.
At 2307Z, the marine inversion at KLAX was at 800 feet deep.
The top of the inversion was at 2000 ft with a temp of 23 deg C.
VFR Moderate to hi confidence in the 00Z TAFs.
Moderate to hi confidence in the 00Z TAFs. VFR conditions are
forecast at all the airfields tonight. However, there is a 30
percent chance of low clouds and LIFR cigs at KSBP and KSMX late
tonight into Thu morning. Low clouds and LIFR conditions are
expected to develop at KLAX and KLGB by 13Z Thu and persist thru
about 17Z, altho there is a 30 percent chance these low clouds may
not develop. MVFR cigs with rain should move into KPRB, KSBP and
KSMX for Thu afternoon. Otherwise, VFR conditions should prevail at
the airfields on Thu.
KLAX...Moderate to hi confidence overall in the 00Z TAF. VFR
conditions are expected at the airfield thru Thu evening, except for
a period of IFR conditions with low clouds from about 13Z to 17Z,
and MVFR vsbys 17Z to 20Z. However, there is a 30 percent chance the
low clouds may not develop., and the timing of the development of
any low clouds could be off +/- an hour or so.
KBUR...Generally hi confidence in the 00Z TAF with VFR conditions
expected at the airfield thru Thu afternoon.
There is moderate confidence that winds and seas will likely
remain below SCA levels through Saturday. However, brief localized
small craft wind gusts are possible along and just ahead of a cold
front moving through the waters from west to east late Thursday
into Friday. Patchy dense fog with vsbys 1 nm or less are possible
again tonight into Thursday morning. Patchy dense fog may
redevelop tonight and persist into Thursday morning. Moderate to
heavy rain with the slight chance of thunderstorms is expected
along and ahead of a cold front moving west to east late Thursday
A lingering long period south swell originating from Hurricane
Seymour combined with a building long period swell originating
from an elongated low pressure system over the North Pacific may
bring Small Craft conditions due to hazardous seas as early as
late Saturday and more likely Sunday continuing into Monday
across the outer waters.
CA...Flash Flood Watch in effect from Thursday afternoon through
Friday morning For zones 34-35-37. (See LAXFFALOX).
Flash Flood Watch in effect from Thursday evening through
Friday morning For zones 36-39-52-53. (See LAXFFALOX).
Flash Flood Watch in effect from late Thursday night through
Friday morning For zones 40-45-46-54-59-547-548. (See LAXFFALOX).