Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 051319 AAA
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
619 AM PDT THU MAY 5 2016
An slow-moving upper level low pressure system will bring a chance
of showers and a slight chance of thunderstorms to southwestern
California from Thursday through the weekend. Below normal
temperatures can also be expected during the period. High pressure
will then bring mostly clear skies with a warming trend to the area
Monday through the middle of next week.
The forecast will be a bit tricky over the next few days. Latest
water vapor imagery indicated a slightly positively tilted
elongated trough just off the central coast. Some showers were
still off the coast. A relatively strong jet will nose in from
the south-southwest this morning over LA/VTU counties. a very
deep moist layer which is currently around 4000 ft, and will
likely continue to deepen to 5000 ft across all areas to the
coastal slopes of the mountains. There will be enough lift from
the upper jet to allow drizzle to develop across Los Angeles,
Ventura, Santa Barbara coast and valleys and to the coastal
mountain slopes this morning...then turn to showers in the
afternoon. Recent model runs have increased the instability
across the coast and coastal waters later today so have added
slight chance for thunderstorms for all coastal waters and coast
and valleys. Confidence is higher for thunderstorm development
across the mountains today...especially the Ventura County and
Santa Barbara County Mountains. The model soundings were quite
impressive with CAPE over 1200 j/kg and Lifted Index readings of
over -7 over the Ventura County Mountains. It is also fairly
unstable across the LA Mountains as well...but went with scattered
thunderstorms over the Ventura and southern SBA county mountains
and isolated thunderstorms for the LA county Mtns. Precipitable
Waters are expected to reach around three quarters of an inch
which could bring some local flooding with some storms today.
However not too concerned with flash flooding as steering winds
will be quite strong out of the south today. There will be some
concern if storms start training across the same areas. Will
continue to monitor the situation this afternoon. Storms that do
fire up will move north today. There is a decent dry punch in the
mid levels, so small hail will be possible with storms today. One
caveat will be if the cloud shield is too thick over the
mountains...then this could inhibit the convective temperature and
Today`s storms will be more dynamically driven due to the
placement of the upper jet and trough. So some thunderstorms could
continue into this evening. Still impressive instability off the
SoCal coast into Friday morning...so have continued with slight
By Friday...the upper low will be overhead. So, if there are some
breaks in the clouds...the atmosphere will become unstable once
again due to colder air aloft and warming at the surface. Since it
is May...there will be longer daytime heating, which should help
thunderstorms to get started. Although there is a slight chance
for thunderstorms to develop across the entire forecast
area...best chance will once again be in the mountains. There will
be a bit more concern for flash-flooding in the mountains due to
the steering winds being much weaker with the low overhead. In
fact...if storms fire up along the mountains, there will be a
chance that these storms will drift to the southeast into the
adjacent valleys and possibly coastal areas of Los Angeles and
Ventura Counties. Can`t rule out storms drifting into the coastal
areas of SBA county as well. The cutoff low will slowly move
eastward...but become more elongated and the possibility of wrap
around showers across the entire forecast area is there. At this
time it looks like thunderstorm chances will be limited to the
mountains and interior locations in the afternoon and early
evening hours. Precip chances will be lower than on Friday with
less areal coverage expected.
As far as rainfall totals...it is very difficult to pinpoint where
and how much will fall due to the convective nature of this
system. Some areas can get less than a tenth of an inch...while a
few locations could get over an inch in and around thunderstorms.
Snow levels will range from between 6500 to 7500 ft. But a few
snow showers could lower to 6000 ft with any heavier showers.
Expect high temperatures to be a bit below normal over the next
few days with the upper low hanging around.
By Sunday, the upper low will have ejected into the plains where
severe weather is likely to occur next week. However, SoCal will
be under generally more stable northerly flow aloft. there will
continue to be some lingering moisture around the region on
Sunday...especially in the mountains and foothills...so a few
showers are not out of the question across the higher terrain.
By Monday a fairly high amplitude ridge will begin to build in
from the west. expect a gradual warming trend through Wednesday
with mostly clear skies. there could be night through morning low
clouds developing once again across some coastal areas.
At 12Z, the marine layer depth at KLAX was around 5000 feet. The
top of the inversion was around 6300 feet with a temperature
around 7 degrees Celsius.
Low-to-moderate confidence in the latest forecast. MVFR
conditions will become prevalent through 16Z. There is a chance
of IFR conditions at coastal and valley terminals in rain/drizzle.
KLAX and KBUR...MVFR conditions will become prevalent through
14Z. There is a 30 percent chance of IFR conditions.
High confidence in winds and seas remaining below advisory
criteria through Friday. There is a 30 percent chance that winds
could reach advisory criteria across the Southern California Bight
on Friday evening. Beyond Friday evening, no significant wind or
sea conditions are expected through the early half of next week.