749 
FXUS66 KLOX 150418
AFDLOX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA 
918 PM PDT Sat Oct 14 2017

.SYNOPSIS...

Offshore flow beneath high pressure aloft will bring warm 
temperatures, gusty winds, and fire danger to portions of the area
through the weekend. The high pressure ridge should weaken by 
midweek for a cool down, then a low pressure system will approach
on Thursday for below normal temperatures and possible 
precipitation into Saturday.

&&

.SHORT TERM...(SAT-TUE)

Offshore flow continues to develop tonight as surface pressure
gradients are trending more strongly offshore. KLAX-KDAG surface
pressure gradient trends are hovering near -6 mb offshore and
KLAX-KTPH to near -8 mb offshore. Linear extrapolation should 
push KLAX-KDAG surface pressure gradients in excess of -7 mb by 
Sunday morning. Winds, slower to develop earlier today, should 
start to increase over the next couple of hours and gradients 
tighten and better upper-level support arrives. No changes have 
been to the current Wind Advisories in effect at this time.

With offshore flow developing, a warming trend will continue for
Sunday. Temperatures have been nudged a little high for Ventura
and Los Angeles Counties for Sunday. KLAX AMDAR sounding have a
thin but strong inversion, which should get wiped out over the 
coming hours as offshore develops.

In wind-sheltered locations, clear skies and calm winds will allow
for radiative cooling processes to be a bit more efficient 
tonight. A Frost Advisory has added for late tonight and into
Sunday morning for the Antelope Valley and San Luis Obispo County
Interior Valleys. 

*** FROM PREVIOUS DISCUSSION ***

Offshore gradients weaken quite a bit Monday and Tuesday but 
still remain offshore with a very warm air mass in place and the 
high pressure ridge aloft hanging tough across So Cal. So temps 
will remain well above normal with just some minimal cooling near 
the coast. Warmer valleys will threaten triple digits but most 
areas in the 90s.

.LONG TERM...(WED-SAT)

Weak offshore flow now expected to remain in place through
Wednesday and still pretty decent ridging aloft so again only
minimal cooling Wednesday with highs still 3-6 degrees above 
normal. 

After Wednesday forecast confidence drops quite a bit as models 
have been really bouncing around with the strength and timing of 
the next trough hitting the west coast as well as how much 
moisture it pulls in from the southwest. It looks like we should 
see at least one more dry day Thursday but cooler as gradients 
trend strongly onshore. And likely some marine lyr clouds
increasing as well. Both the GFS and ECMWF 12z solutions show the
trough axis moving onshore over Nrn CA around 18z Friday and
passing through the rest of the state Friday afternoon and
evening which is a good 12 hours delayed from earlier runs. 
Today's solutions are also noticeably less moist and don't drop 
quite as far south as previous runs showed and resulting precip 
forecasts are lower as well. In fact they are basically dry for 
most of LA/Ventura counties. But these solutions have varied 
considerably run to run and and it still looks like some chance of
rain late next week, though just not confident on timing and 
strength.

&&

.AVIATION...15/0137Z.

At 00Z, the marine layer depth was around 600 feet deep at KLAX. 
The top of the inversion was around 1200 feet with a temperature 
near 27 degrees Celsius.

Moderate-to-high confidence in VFR conditions throughout the
period. Moderate-to-strong wind shear and turbulence is possible
at all terminals through 16Z. 

KLAX...VFR conditions are expected throughout the period, except
for a 30 percent chance of MVFR conditions between 12Z and 16Z.

KBUR...VFR conditions are expected throughout the period. There is
a 30 percent chance of moderate wind shear and turbulence between
02Z and 12Z.

&&

.MARINE...14/800 PM.

No issues with wind or seas expected over the outer waters
through Tuesday, except local gusts to 25 kt between San Nicolas 
Island and Santa Cruz Island late tonight through Sunday morning.
Increasing NW winds with possible SCA possible as a frontal system
approaches Wednesday into Thursday. 

Gusty northeast winds across the nearshore waters will develop 
this evening and reach Small Craft Advisory (SCA) levels south of
Point Conception, mainly from Ventura to Santa Monica. The winds 
will diminish Sunday afternoon. 

There is a 20% chance that northeast wind gusts to 25 kt and 
short period northeast swell of 2 to 4 feet will affect Avalon 
Harbor Sunday morning.

&&

.FIRE WEATHER...14/205 PM.

Overall, everything still on track with regard to Red Flag
conditions across Ventura and Los Angeles counties. Offshore
winds are expected to increase this evening and overnight as
the surface pressure gradient increases. So, wind gusts between
35 and 55 MPH will be likely across Ventura and Los Angeles
county tonight and Sunday morning. The winds will diminish
gradually Sunday afternoon. As for relative humidity, very 
dry air will remain in place over the area through Sunday. So,
widespread and long-duration critical fire weather conditions
are expected and the Red Flag Warning will remain in effect
until 600 PM Sunday for Ventura and Los Angeles counties (except
for the Antelope Valley).

For Santa Barbara and San Luis Obispo counties, the offshore 
winds will be much weaker and more localized through Sunday.
However, the air mass will still be as warm and dry. So, elevated
fire weather conditions can be expected across these areas 
through Sunday evening.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Frost Advisory in effect from 2 AM to 9 AM PDT Sunday for
      zones 37-59. (See LAXNPWLOX).
     Wind Advisory in effect until 2 PM PDT Sunday for zones
      40-41-44>46-53-54-88-547. (See LAXNPWLOX).
     Red Flag Warning in effect until 6 PM PDT Sunday for zones
      240-241-244>246-253-254-288-547-548. (See LAXRFWLOX).
PZ...Small Craft Advisory in effect until 2 PM PDT Sunday for
      zones 650-655. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(TUE-SAT)
Elevated surf and strong rip currents are possible at area 
beaches Tuesday through Thursday.

&&

$$

PUBLIC...Hall/MW
AVIATION...Hall
MARINE...Smith
FIRE WEATHER...Thompson
SYNOPSIS...STu/Hall

weather.gov/losangeles