Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 310401
AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
901 PM PDT SAT JUL 30 2016
Near normal temperatures are expected through the end of next
week, with night through morning low clouds and fog for some
coastal and valley areas. Monsoonal moisture will bring a slight
chance of the thunderstorms to the mountains and desert through at
least Sunday. A weak trough of low pressure will develop over the
west coast by midweek, pushing any monsoonal moisture to the east.
High pressure aloft remains over the region this evening,
centered over the Mojave Desert. Some clouds remain from earlier
convection over the Southern California Mountains and Mojave
Desert. The southern part of the area remains on the edge of the
deepest monsoonal moisture this evening and should remain so into
Sunday evening. Model solutions suggest 850 mb mixing ratios
remaining above 8 g/kg into Sunday afternoon and evening, while a
shortwave disturbance clips the area. With a cap eroding in model
soundings by Sunday afternoon, slight chance PoPs were added to
the forecast for Los Angeles and Ventura County Mountains and the
Antelope Valley for the afternoon and evening hours.
A well-entrenched marine layer stratus deck is in place this
evening and should spread into the lower valleys by Sunday
morning. Should is the key word. Some concern exists that the
remnant middle and high level cloudiness could play tricks with
the marine layer tonight by altering the inversion. This would
prevent stratus formation. A marine layer depth near 1100 feet
deep this evening should deepen to near 1700 feet by late tonight.
Some NAM-WRF BUFR time height sections suggest some thinning of
the marine layer after midnight, but confidence is low in the
occurring for the moment. The shallowness of the marine could
continue to bring patchy dense fog most coastal areas and the
lower valleys through Sunday morning.
SHORT TERM...Fairly strong upper level ridging will persist over
srn CA thru Tue, with 500 mb heights around 591 to 593 dm.
1000-500 mb thicknesses will about 580 to 585 dm for Sun thru Tue.
The marine inversion which was about 1000 to 1300 ft deep this
morning should gradually increase to near 1500 ft thru Mon then
remain around this level for Tue. Very good onshore gradients are
forecast by the NAM up to around +9.0 mb LAX-DAG each afternoon.
Marine layer clouds and fog should expand along the coastal plain
and extend into some of the adjacent vlys night and morning hours
thru Tue. Afternoon clouds should also develop over the mtns and
deserts each day with marginal amounts of monsoonal moisture
filtering into the area from the e at mid levels. A 10 percent or
less chance of a thunderstorm cannot be ruled out as well in the
mtns and deserts for Sun and Mon, which is too small a chance to
mention in the zones. Otherwise, mostly clear skies will prevail
thru Tue across the forecast area. Gusty afternoon and evening s to
w winds will persist across the foothills, mtns and deserts during
the period. The onshore flow and gradually deeper marine layer will
help to lower temps some thru Sun, with little change in temps
expected Mon and Tue. Even so, highs will remain several degrees
above normal away from the coast. Highs for Sun thru Tue in the
warmest vlys and foothills will be in the upper 80s and 90s, except
100 to 104 in the Antelope Vly.
.LONG TERM...EC/GFS in good generally agreement during the
extended period, altho minor differences start to show up on Sat.
The upper ridging will continue over srn CA on Wed. The ridging
will weaken over the area Thu, allowing an upper level trof to
approach from the e Pac. This upper trof will move into central
and swrn CA for Fri. The GFS builds an upper ridge back into the
region for Sat, while the EC lingers the upper troffiness over the
area. Favored the GFS fcst of a slightly more progressive movement
of the upper trof. The marine layer pattern will continue thru the
week, with varying amounts of night and morning low clouds and fog
for the coast and some adjacent vlys. Otherwise, mostly clear
skies will prevail across the region. Temps are forecast to be
slightly above normal Wed, then cool to near normal overall for
Thu thru Sat.
At 00Z, the marine layer was around 1100 feet deep at KLAX. The
top of the inversion was near 4700 feet with a temperature around
26 degrees Celsius.
Moderate to high confidence in the current forecast, except low
confidence in Los Angeles County Valley terminals where there is
a chance the sites remain clear overnight. Coastal terminals will
see mainly in IFR to MVFR category, likely deteriorating to LIFR
north of Point Conception between 06Z and 15Z.
KLAX...MVFR conditions will spread in between 02Z and 06Z.
Conditions should remain MVFR but there is a 40 percent chance of
IFR conditions with an earlier arrival than forecast...and
possibly after 08Z. VFR conditions should develop around 17Z, but
there is a 20 percent chance that MVFR conditions could linger
KBUR...There is a 60 percent chance of IFR conditions between 10Z
and 16Z. VFR conditions should develop between 15Z and 16Z.
High confidence in the forecast through the weekend into early
next week. Winds will generally remain light to occasionally
moderate through the weekend...strongest over the outer waters.
Patchy dense fog will impact the coastal waters through Sunday
morning. A small but long period southerly swell will develop
Sunday into Monday.