Weather Service Report


767 
FXUS66 KLOX 220342
AFDLOX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA 
842 PM PDT Sat Oct 21 2017

.SYNOPSIS...
Gusty northerly winds focused across the mountains of Southwest 
California will turn northeast and become more widespread by 
Sunday. Moderate Santa Ana winds Sunday and Monday may become 
strong by Tuesday. The offshore flow is expected to bring record 
heat to some areas Monday and Tuesday. Weakening offshore flow 
will provide a cooling trend for the middle of next week.

&&

.SHORT TERM...(TONIGHT-TUE)

Clear skies covered the forecast area this evening and will continue 
overnight. N to NE gradients have increased over the forecast area, 
and as of 02Z the LAX-DAG gradient was -3.2 mb. The gradients will 
continue offshore and increase tonight, which will help to support 
gusty N to NE winds over portions of the Central Coast, the VTU 
County mtns and vlys, L.A. County mtns, Santa Clarita Vly and San 
Fernando Vly. Winds have diminished some early this evening over 
VTU/L.A. Counties, but are expected to increase again overnight to 
advisory levels in the wind prone areas. A Wind Advisory is in 
effect for many of these areas tonight into Sun.

An upper level ridge along and off the CA coast this evening will 
expand over CA Sun thru Mon then persist over the region thru Tue. 
500 mb heights over swrn CA are forecast to increase to around 592-
593 dm Sun, and to 595-596 dm for Mon and Tue. 1000-500 mb 
thicknesses are expected to top out in the 579-581 dm range Mon thru 
Tue. 

Offshore gradients will increase substantially thru Tue as well, 
with the latest NAM predicting the 12Z Mon LAX-DAG gradient lowering 
to -6.1 mb, and the 12Z Tue LAX-DAG gradient lowering to a whopping
-7.5 mb! Gusty offshore winds will persist on many areas Sun thru 
Tue, with the strongest winds expected Mon night into Tue morning. 
850 MB winds over the L.A./VTU mountains by early Tue morning are 
forecast by the 00Z NAM to be in the 40-45 kt range, while 950 mb 
winds are expected to be up to 45-50 kt over srn VTU County and 
western L.A. County. For Sun night into Mon morning, it looks like 
NE winds should reach around advisory levels for the L.A./VTU County 
vlys except San Gabriel Vly and possibly Santa Monica Mtns. For Mon 
night and Tue morning, NE winds will be at hi-end advisory-levels 
for these areas as well as the mtns and coastal areas, with some 
gusts in the mtn areas possibly reaching low-end warning threshold 
at times (60 mph). 

The strong offshore flow, warming boundary layer and 950 mb temps, 
and high 1000-500 mb thicknesses will result in record-breaking heat 
across much of the forecast area Mon and Tue. Mon may actually be 
slightly hotter overall than Tue, but there is hi confidence both 
days will be record-breaking. For downtown L.A., the record high for 
Mon is 98 degrees set in 1965, and the forecast high is 101, while 
the record hi for Tue is 99 set in 1909 and the forecast hi is 102. 
Even Sun should be significantly above normal for many coastal and 
vly areas. Overnight lows in the windy vly and foothill areas are 
forecast to be in the 70s to near 80 Sun night and Mon night. The 
forecast hot weather has resulted in Excessive Heat Warnings for the 
VTU/L.A. County coast and vlys for Sun and Mon. There is also an 
Excessive Heat Watch for the SBA County coast during the period. 
Please see the latest Non-Precipitation Weather Message (LAXNPWLOX) 
for the latest details on the Wind Advisories and Excessive Heat 
Warnings and advisories.

***FROM PREVIOUS DISCUSSION***

.LONG TERM...(WED-SAT)

The high pressure ridge looks slower to break down and
temperatures will likely stay very warm Tuesday night and
Wednesday. Highs will drop 5-8 degrees from Tuesday, but we could
still need heat advisories for the unusually hot temperatures over
the region. Winds will be much lighter by Wednesday afternoon and
may start to switch back to onshore late in the day. 

High pressure is shoved to the west by Thursday as a large upper
trough drops into the middle of the country. Heights fall and
upper winds turn more northerly for a cooler flow across the
region. Temperatures will continue to moderate, but remain well
above normal in most areas. For the end of the week, a high
pressure ridge will start building back across the west coast 
for continued warm and dry weather in store.

&&

.AVIATION...21/2350Z...

At 2324Z, there was a marine inversion at KLAX at around 600 ft 
deep. The top of the inversion was at 1700 ft with a temperature of 
23 deg C.

Hi confidence in the 00Z TAFs with VFR conditions at all airfields 
thru Sun afternoon. There will be gusty NW to NE winds at KSBP and 
KSMX thru early this evening and again Sun afternoon. Gusty N winds 
can also be expected at KVNY at times thru Sun. 

KLAX...Hi confidence in the 00Z TAF with VFR conditions expected 
thru Sun evening.

KBUR...Hi confidence in the 00Z TAF with VFR conditions expected 
thru Sun afternoon. There is also a 20 percent chance of low level 
wind shear at times this evening and again Sun morning due to gusty 
N winds just above ground level.

&&

.MARINE...21/800 PM.

For the Outer Waters...Northwest to north winds will remain at 
Small Craft Advisory (SCA) level through Sunday night, and 
possibly through Monday. The winds may shift to northeast over
portions of the outer waters at times through Tuesday.

For the Inner Waters north of Point Sal...north to northwest winds
are increasing this evening and are expected to remain at SCA
through late Sunday night.  

For the Inner Waters south of Point Conception...from Sunday 
night through Tuesday, there is a 70 percent chance of SCA level 
northeasterly winds nearshore from Ventura to Santa Monica. For
the San Pedro Channel out to Catalina Island, SCA level northeast
to east winds will be most likely Tuesday morning. 

A large long-period northwest swell will move through the coastal
waters through the weekend. Seas up to 15 feet are likely across 
the outer and northern waters. There will be dangerous breaking 
waves on the Central Coast through tonight with dangerous 
conditions in and near harbors.

&&

.BEACHES...21/200 PM.

A large long-period northwest swell will move through the coastal
waters through Sunday morning. 

North of Point Conception...A High Surf Advisory continues 
through Sunday morning. Surf will continue between 11 and 16 feet 
through midday then will begin to diminish this afternoon but 
remain above 10 feet through Sunday morning. Some impacts from 
these large and powerful waves and strong currents are the risk of
ocean drowning and sneaker waves that can suddenly overrun 
previously dry beaches and jetties. For Monday...the surf will 
linger around the minimum criteria for High Surf of 10 feet.

South of Point Conception...A Beach Hazards Statement continues 
through Sunday afternoon for surf of 4 to 7 feet with local sets 
to 8 feet. Dangerous rip currents are a primary impact of this 
elevated surf.

&&

.FIRE WEATHER...21/430 PM...

Gusty north to northeast winds and a drying trend will continue 
to bring elevated fire danger to much of the region through 
tonight. The strongest winds during this period will remain 
focused across the mountains and Santa Clarita Valley, where a Red
Flag Warning is already in effect. Brief critical fire weather 
conditions will also be possible through tonight across the 
valleys of Los Angeles and Ventura counties as well as the Santa 
Monica Mountains and Santa Barbara south coast.

From Sunday through Tuesday, a prolonged duration of gusty Santa 
Ana winds with hot and very dry conditions is expected. Northeast 
winds are expected to gust between 30 and 45 mph across wind 
favored passes and canyons of Los Angeles and Ventura counties on 
Sunday. The Santa Ana winds are expected to increase in areal 
coverage and intensity Monday into Tuesday, with peak gusts 
between 45 and 60 mph on Tuesday across wind favored passes and 
canyons. The hottest and driest conditions are also expected on 
Monday and Tuesday, when triple digit heat and humidities lowering
into single digits and lower teens will be common across coastal 
and valley areas. Northeast winds gusting between 30 and 40 mph 
will also bring elevated fire weather concerns to portions of the 
Central Coast and higher terrain in San Luis Obispo County through
Monday, however humidities will not be sufficiently low to 
warrant Red Flag Warnings. 

Very warm and dry conditions during the overnight hours in the 
mountains, foothills, and wind prone areas will add to the fire 
weather threat. A Red Flag Warning is in effect Sunday through 
Tuesday for much of Los Angeles and Ventura counties. The 
duration, strength, and widespread nature of this Santa Ana wind 
event combined with extreme heat will bring dangerous fire weather
conditions to Southwest California Sunday through Tuesday. If 
fire ignition occurs, there will be the potential for very rapid 
spread of wildfire with long range spotting and extreme fire 
behavior that could lead to a threat to life and property.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory in effect until noon PDT Sunday for zones
      34-35. (See LAXCFWLOX).
     Wind Advisory in effect until 5 AM PDT Monday for zones
      34-35-51. (See LAXNPWLOX).
     Excessive Heat Watch in effect from Monday morning through
      Tuesday evening for zone 39. (See LAXNPWLOX).
     Beach Hazards Statement in effect through Sunday afternoon
      for zones 40-41. (See LAXCFWLOX).
     Excessive Heat Warning in effect from 10 AM Monday to 8 PM
      PDT Tuesday for zones 40-41-44>46-87-88-547. (See LAXNPWLOX).
     Wind Advisory in effect until 2 PM PDT Sunday for zones
      44-45-88-547. (See LAXNPWLOX).
     Red Flag Warning in effect from 6 AM Sunday to 6 PM PDT
      Tuesday for zones 240-241-244>246-547-548. (See LAXRFWLOX).
     Red Flag Warning in effect until 10 AM PDT Sunday for zone
      252. (See LAXRFWLOX).
     Red Flag Warning in effect until 6 PM PDT Tuesday for zones
      253-254-288. (See LAXRFWLOX).
PZ...Small Craft Advisory in effect until 3 PM PDT Sunday for
      zones 670-673. (See LAXMWWLOX).
     Small Craft Advisory in effect until 9 AM PDT Monday for zone
      676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(TUE-SAT)
Near record high temperatures with excessive heat and critical 
fire weather conditions will continue on Tuesday, especially 
across valleys and coasts. Gusty northeast winds could impact
mountain areas Tuesday.

&&

$$

PUBLIC...Sirard/Boldt
AVIATION...Sirard
MARINE...Smith
BEACHES...Sweet
FIRE...Gomberg
SYNOPSIS...Munroe

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Western Regional Climate Center, wrcc@dri.edu