Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Los Angeles/Oxnard California
850 PM PDT Friday Jul 29 2016
A cooling trend is expected through the weekend as high pressure
weakens aloft, with temperatures returning to near normal by
Sunday. Seasonable weather will then continue through the end of
next week, with little day-to-day change. Night through morning
low clouds and fog will continue to affect some coastal areas.
The latest water vapor imagery indicates an upper-level ridge of
high pressure centered over the lower Colorado River Basin. Weaker
onshore flow today produced a warmer day today with record high
temperatures occurring at Paso Robles, Sandberg, and Lancaster.
High pressure aloft will begin to weaken over the weekend and
bring a slight cooling trend across the area as onshore flow
strengthens. With 500 mb heights declining and 1000-500 mb
thickness values thinning, the marine layer depth should deepen
slightly through the weekend. A cooling trend is reflected in the
current forecast with expanding marine layer stratus coverage
through the weekend. No updates are planned at this time.
Short term...fairly strong upper level ridging will persist over
srn California thru Mon. 500 mb heights of around 594 dm today will lower
slightly to 591 to 592 dm for sun and Mon. 1000-500 mb thicknesses
will range from 584 to 588 dm this afternoon then drop to 580 to
585 dm by sun and continue on Mon. The marine inversion which is
below 1000 ft today should gradually increase to near 1500 ft over
the weekend and into Mon, with very good onshore gradients
forecast by the NAM up to +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 Mon. 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, which is too small a chance to mention in the
zones. Otherwise, mostly clear skies will prevail thru Mon 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 the weekend and into early next week. Even so,
highs will remain several degrees above normal away from the coast.
Highs by sun in the warmest vlys and foothills will be in the upper
80s and 90s, except upper 90s to 103 in the Antelope Vly. Temps will
then change little for Mon.
Long term...ec/GFS in good agreement during the extended period.
The upper ridging will continue over srn California Tue thru Thu, then
weaken slightly Fri as upper troffiness develops along and off the
central coast. Some monsoonal moisture will affect mainly eastern
l.A. County Tue into Wed, with some afternoon cu buildups possible
in the mtns and deserts, but no thunderstorms are expected. 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. Little change in temps are forecast Tue thru
Fri, with highs several degrees above normal away from the coast.
The warmest vlys and foothills each day should reach the upper 80s
and 90s, with upper 90s to 103 expected for the Antelope Vly.
At 23z, the marine layer was around 800 feet deep at klax. The
top of the inversion was near 4500 feet with a temperature around
29 degrees celsius.
LIFR to IFR conditions will likely spread into coastal terminals
through 13z. The highest likelihood on LIFR conditions exists for
coastal terminals north of koxr. Conditions should improve to VFR
between 15z and 19z.
Klax...low confidence with return of IFR cigs between 10z and 16z.
There is a 30 percent chance that VFR conditions could persist
through entire forecast period.
Kbur...high confidence in VFR conditions through period.
Moderate to high confidence with the forecast through this
weekend. A Small Craft Advisory has been added for the southern
outer waters tonight, then winds will diminish and remain light to
occasionally moderate through the weekend. A shallow marine layer
will continue widespread dense fog with visibility 1 nautical
mile or less into Saturday morning.
A small but long period southerly swell will develop Sunday into
Fire weather...29/110 PM.
Hot and dry conditions continue with widespread temperatures
currently in the 90s and 100s and relative humidities in the 10s to
low 20s across interior areas. Locally gusty onshore winds will
continue during the afternoon and evening hours across portions of
the Los Angeles mountains into the Antelope Valley through at least
early next week. Gradually improving relative humidities and cooler
temperatures will help to reduce fire weather concerns including
over the sand fire this weekend into early next week as high
pressure aloft weakens and is gradually replaced by a weak low
pressure aloft off the coast of northern California.
Pz...Small Craft Advisory in effect until 3 am PDT Saturday for
zones 673-676. (See laxmwwlox).