Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 232316
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
416 PM PDT SAT JUL 23 2016
Weakening high pressure aloft will shift to and persist over the four
corners next week. This will lead to one more hot day today with
temperatures cooling to near normal Sunday with little change in
temperatures for much of next week. High pressure across the four
corners area may support an influx of monsoon moisture with
showers and thunderstorms possible at times next week.
Sunny skies covered much of the forecast area early this afternoon,
except for patchy low clouds and dense fog at some of the L.A.
County beaches and lingering smoke from the Sand Fire over portions
of the L.A. basin, Santa Clarita Vly, and San Gabriel mtns. Strong
upper level high with 500 mb heights around 595 to 596 dm will
persist over SoCal this afternoon, helping to build the heat to
dangerous levels again for the vlys and lower mtns of VTU/L.A.
Counties. Temps from the upr 90s to 108 were noted in these areas
early this afternoon, with expected highs of 100 to 110 looking
good. Heat index values should reach 100 to 105 at times this
afternoon. Altho temps will slowly cool toward evening, the
Excessive Heat Warning will continue thru 8 PM for the vlys/lower
mtns of VTU/L.A. Counties.
The upper high will weaken slightly tonight, with weak upper
troffiness expected to move into swrn CA on Sun. Upper level ridging
should edge back into the area Sun night and Mon then linger over
the region thru Tue.
A shallow marine inversion is forecast to deepen slightly tonight
thru Tue. Varying amounts of night and morning low clouds and fog
are expected to affect much of the coastal plain and some of the
adjacent vlys later tonight thru Tue morning. Otherwise, mostly
clear skies will prevail during the period, except for some
afternoon cu buildups over the mtns and Antelope vly Tue afternoon.
Stronger onshore flow can be expected each afternoon thru Tue as
well, with gusty s to w winds expected over the foothills, mtns and
deserts. The re-organization of the marine layer and stronger
onshore flow will result in cooler temperatures. Even so, highs will
still be several degrees above normal each day with the warmest vlys
and foothills reaching the 90s to around 100, except 102 to 106 in
the Antelope Vly.
EC/GFS in generally good agreement during the medium range. An upper
high will build over NV into UT by Wed and remain centered over this
area for the most part thru Sat. The position of the upper high will
open the door for some monsoonal moisture to start to creep into srn
CA during the period, but most of the moisture should remain e of
the forecast area thru Thu. The GFS was indicating a bit deeper
moisture intrusion from the e on Fri and Sat, with PWAT values
around 1.15 to 1.25 inches moving inland over VTU/L.A.Counties. A
slight chance of afternoon and early evening showers and
thunderstorms can be expected for the L.A./VTU mtns and Antelope Vly
for Fri and Sat, altho there could be an isolated thunderstorm as
early as Wed or Thu afternoon, but chances of that occurring is less
than 15 percent. Otherwise, some afternoon cu buildups can be
expected over the mtns and deserts each day Wed thru Sat. Marine
layer clouds will continue to affect the cst and some adjacent vlys
night and morning hours thru the period as well. Temps are forecast
to change little Wed thru Fri, then cool slightly for Sat, but highs
will remain several degrees above normal for most areas. The warmest
vlys and foothills Wed thru Fri should reach the 90s to 104, except
the Antelope Vly will be in the 102 to 107 deg range each day.
Mid upper/level ridge of high pressure centered east and west of
the area and trough of low pressure north and south of the area
will change little through the period. Mid/upper level light and
variable winds will become light southwest after 24/20z. Moderate
to weak onshore pressure gradient through 24/03z and after 24/20z
otherwise weak onshore gradient. isolated low level cloud field
had moved north in southern L.A. county this morning and capping
inversion was below 1kft or on the surface. there is a chance
the cloud field will become broken to overcast with tops
approximately 1kft Sunday morning. Smoke trajectory is expected to
shift NE after 24/06z.
Marine layer at LAX at 1700Z is 0 feet deep and the inversion top
is at 2428 feet with a temp of 30.1 degrees C.
KLAX...chance smoke layer 070 through 24/04z. chance cigs 006
KBUR...chance smoke layers 040 and 070 through 24/01z. then chance
vsby 5sm fu after 24/10z.
virtually certain - 95-100%
very likely - 80-95%
likely - 60-80%
chance - 30-60%
very unlikely - 20% or less
Northwest winds will likely diminish tonight and small craft
advisory conditions will end from Piedras Blancas to Point Sal
0-10nm this evening and 10-60nm early Sunday morning and from
Point Sal to San Nicolas Island late tonight. There is a chance
northwest winds will increase Sunday afternoon from Piedras
Blancas to Point Sal 0-60 nm and there is a chance SCA conditions
will exist for several hours through the evening. SCA conditions
are then not expected through the remainder of the period. Patchy
dense fog with visibility 1 nm or less is expected tonight.
Red flag warning remains in effect for the mountains in Santa
Barbara, Ventura, and Los Angeles Counties, as well as the Santa
Barbara County South Coast in the foothill areas through Midnight
tonight due to gusty winds, extremely low relative humidity, and
hot temperatures. Widespread humidities in single digits and teens
Low level winds around the Sand Fire will continue to be south to
southwest gusting 20 to 25 mph through this evening, and will
be slightly stronger on Sunday afternoon/evening. The upper level
winds are from the north which is currently transporting
considerable smoke and ash into the valleys and coast of Los
Angeles County and eastern Ventura county. As of 4 pm, humidities
around the Sand Fire will continue to range between 10 and 15
percent this afternoon with temperatures near 100 degrees.
Humidities will recover slightly later tonight, generally in the
20 to 30 percent range. With humidities lowering back to between
10 and 20 percent on Sunday, there will be the potential for brief
critical fire weather conditions near the Sand Fire on Sunday
afternoon and evening as the gusty onshore winds increase.
CA...Excessive Heat Warning in effect until 8 PM PDT this evening
For zones 44>46-53-54-547-548. (See LAXNPWLOX).
Red Flag Warning in effect until midnight PDT tonight For
zones 239-252>254-259. (See LAXRFWLOX).
PZ...Small Craft Advisory in effect until 9 PM PDT this evening
For zone 645. (See LAXMWWLOX).
Small Craft Advisory in effect until 3 AM PDT Sunday For
zones 670-673-676. (See LAXMWWLOX).