Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 271848
AFDLOX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
1148 AM PDT Thu Jul 27 2017

Updated aviation section

.SYNOPSIS...

Partly cloudy skies will persist into next week, with an overnight
marine layer along the coast. Early next week, monsoonal moisture
should bring possible thunderstorms and showers in the afternoon and
evening for the Ventura mountains and Los Angeles valleys and
eastward. The temperatures are forecast to be around normal this
week, with a warming trend next week as the high builds in.

&&

.SHORT TERM...(TDY-SAT)

Overall, 12Z models in good synoptic agreement through the period.
At upper levels, ridge remains centered over the Texas Panhandle
area through Saturday as weak trough settles over the West Coast.
On Sunday, the center of the ridge moves westward, settling a bit
closer to the Four Corners area. Near the surface, generally
moderate onshore flow will prevail through the period.

Forecast-wise, no major issues through Saturday. With upper trough
settling over West Coast, there will be some lowering of H5
heights which should allow for some slight marine inversion
deepening. High resolution models indicate weak eddy circulations
developing the next couple of nights. So, will expect stratus to
be rather widespread across the coastal plain the next two
mornings and could squeeze into some of the lower coastal valleys.
Otherwise, skies should remain mostly clear through Saturday. As
for temperatures, there should be a degree or two of cooling for
most areas Friday and Saturday. The only other wrinkle for tonight
will be the possibility of some locally gusty northerly winds
across the Santa Ynez range and I-5 corridor. Nothing approaching
advisory-levels, but probably just enough to notice.

For Sunday, upper flow pattern turns a bit more southeasterly as
upper ridge moves westward and weak low develops over northern
Baja Mexico. So, some mid-level moisture could filter into the
area. With decent instability over the mountains, could be enough
to generate an isolated thunderstorm. PWATs are not too
impressive, but definitely enough to warrant continuation of
slight chance POPs over the Antelope Valley and Los Angeles county
mountains. Otherwise, there will be some stratus Sunday morning
across the coastal plain with increasing high clouds through the
day for all areas. As for temperatures, will expect some slight
warming for most areas on Sunday with some increase in H5 heights
and thicknesses.

.LONG TERM...(SUN-WED)

Should be an interesting long term forecast with some
uncertainty in respect to monsoonal moisture. The dry SW flow
aloft will begin to shift to the south to southeast by Sunday. At
this point there will be no remnants from Hurricane Hilary or from
what is now Tropical Storm Irwin. However, the GFS and ECMWF are
in agreement of the upper ridge will strengthen and set up across
the Four Corners region allowing south to southeasterly flow to
advect some mid level moisture out of nrn Mexico and southern
Arizona into the far southern portion of Calif Sunday afternoon.
Most of the activity will remain to the south and east of LA
County, but there will be enough mid level moisture and some
limited instability across the San Gabriel Mtns to justify a
slight chance for showers for the LA County Mtns and eastern
Antelope Valley. High to mid level clouds should increase across
LA County Sunday night into Monday. By Monday, there will be
increasing clouds and more widespread mid level moisture moving
into the mountains and deserts. Slight chance for thunderstorms
for the LA/VTU County Mtns and Antelope Valley. By Monday evening,
an easterly wave sets up over LA County and areas to the south.
In coordinating with the San Diego NWS office, have added slight
chance of thunder for the LA County Valleys for now, but see no
reason based on latest models why showers wont reach the coastal
waters Monday evening through the overnight hours. If models do
not change over the next 24 hours, expect that showers or slight
chance thunderstorms will be added to the forecast. This easterly
wave will continue into Tuesday and should bring additional
chances for showers or slight chance thunderstorms for the same
areas and possibly into the Ventura County valleys and Coastal
areas as well Tue evening. As we get closer to next Mon/Tue, POPs
might have to be increased for the interior areas including the
LA/VTU Mtns and Antelope Valley.

As far as temps go, the stronger ridge aloft will allow for some
additional warming, although with the added humidity and cloud
cover temps might be a few degrees too warm for LA/VTU inland
areas including the valleys. The added humidity will make it quite
uncomfortable at night as well for the lower mountain elevations,
Antelope Valley and possibly the elevated valley locations. It is
hard to pin down when and where showers will occur, so blanketing
LA/VTU counties with slight chance makes the most sense at this
point.

&&

.AVIATION...27/1848Z.

At 18Z, the marine layer depth was around 1100 ft. The top of the
marine inversion was around 1950 ft with a temperature of near 23
degrees Celsius. There was another inversion above up to around
4000 ft.

North of Point Conception...LIFR to IFR conditions will spread
into coastal terminals as soon as 03Z or as late as 10Z.
Conditions will become primarily LIFR between 07Z and 11Z, then
linger through 15Z before improving to VFR conditions between 17Z
and 18Z Friday.

South of Point Conception...Conditions should remain VFR through
at least 03Z, except for a chance of IFR to MVFR conditions at
KOXR this afternoon. Otherwise, IFR to MVFR conditions will
spread into coastal terminals between 03Z and 13Z. Conditions
should improve to VFR between 16Z and 19Z Friday.

KLAX...There is a 70 percent chance of IFR to MVFR conditions
will spread into KLAX as soon as 06Z or as late as 13Z. VFR
conditions could delay until 20Z Friday, but more likely 17Z.

KBUR...VFR conditions are expected throughout the period. There
is a 30 percent chance of IFR to MVFR conditions between 13Z and
16Z but could occur as soon as 10Z.

&&

.MARINE...27/900 AM.

For the Outer Waters, moderate-to-high confidence in current
forecast. A small craft advisory was added for the waters near
Point Arguello and Point Conception from late this afternoon
through late this evening. There is a 30-40 percent chance that
Northwest winds could reach small craft advisory levels each
afternoon through Saturday evening. Otherwise, winds and seas
expected to remain below advisory levels through Monday.

For the Inner Waters, high confidence in current forecast. For
all the waters, winds and seas are expected to remain below SCA
levels through Monday, except for local small craft advisory level
gusts across the Inner Waters each afternoon and evening.

A long-period southerly swell will spread into the coastal waters
on Friday, then a moderate-to-occasionally large swell with a
rather long period will affect the waters from Friday night
through weekend. The Santa Barbara Channel will likely be blocked
from the swell energy due to the Channel Islands.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement in effect from Friday morning through
      Tuesday evening for zones 40-41-87. (See LAXCFWLOX).
PZ...Small Craft Advisory in effect from 3 PM this afternoon to 3
      AM PDT Friday for zone 673. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(SAT-WED)

A high surf event should peak over the weekend as a long-period
southerly swell arrives at the Southern California beaches. High
surf and strong rip currents will occur through the weekend and
probably into late next week.

&&

$$

PUBLIC...Thompson/Kaplan
AVIATION...Hall
MARINE...Hall
SYNOPSIS...STu

weather.gov/losangeles


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