Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Los Angeles/Oxnard California
300 PM PDT Wednesday Jul 27 2016
The high will bring above normal temperatures inland this week.
A flow of monsoonal moisture will move in on Friday and over the
weekend as the high moves east. This may bring a slight chance
showers and thunderstorms to the Ventura and Los Angeles County
mountains and deserts. Then early next week the onshore flow will
increase for a cooling trend.
Minimal changes to the forecast through Saturday. Models continue
to advertise a very slight warming trend through Friday as the 4
corners high pressure ridges strengthens while gradient trends
remain basically flat. Just a degree or two at most and mostly for
areas away from the immediate coast. Marine lyr will remain
shallow but still pretty solid along much of the coast during the
overnight and early morning hours. Dense fog will continue to pose
some issues for coastal areas during those times but should burn
off fairly early each day.
Moisture continues to be a limiting factor with thunderstorm
formation the next few days. Some instability but too dry. By
Friday we start seeing a tad more moisture and instability but
agree with previous shift that chances seem too low to include in
the forecast. Somewhat better over the weekend, enough to tip the
scales into the slight chance category, but again moisture is
scant and mostly at or above 600mb. Perhaps more concern for dry
lightning and gusty winds.
Ridge starts to weaken over the weekend allowing temps to drop a
few degrees, especially far inland areas like Antelope Valley and
interior slo County. However, marine lyr will remain shallow and
confined mainly to coastal areas. May have slightly later burn off
times but otherwise not much change. Will continue to leave
thunderstorms out of the forecast after Sunday, though there are
hints of additional moisture surges into southeast California early
next week so if thunderstorms do flare up over the weekend pops
may need to be added.
At 18z, the marine layer was around 600 feet deep at klax. The
top of the inversion was near 4000 feet with a temperature around
27 degrees celsius.
North of Point Conception...moderate to high confidence in all tafs
during the upcoming forecast period. Persistence in play with a
shallow marine layer generating LIFR/IFR ceilings and visible at times
near the ocean while remaining sky clear inland.
S of Point Conception...low confidence in the next 24-hours.
Shallow marine layer in place, but stratus/fog has not been as
consistent except closer to the south Santa Barbara coast.
Anticipate areas of LIFR/IFR ceilings and visible moving back to the
Ventura and Los Angeles coastal tafs later tonight. Extent of
coverage is the limiting confidence factor. A few high-based
cumulus will be possible over the San Gabriel Mountains between
21-02z due to weak instability.
Klax...18z taf...VFR conditions expected through 08z. Low
confidence after that time frame for possible LIFR/IFR ceilings and
patchy MVFR visible as stratus develops near the beach early Thursday
Kbur...18z taf... high confidence in VFR conditions throughout
the forecast period.
High confidence in the current forecast through Friday. The
marine inversion remains shallow which will result in lingering
dense fog at times and poor visibilities across the outer waters,
near the northern Channel Islands, and in the Santa Barbara
Channel through at least Thursday morning. Weak to locally
moderate northwest winds will kick up each afternoon near the
islands and across the outer waters, with gusts up to 20 kts
through the early evening hours. There is a better chance of
strengthening winds with Small Craft Advisory level winds
possible on Thursday afternoon and evening and again Friday
afternoon and evening across the southern outer waters.
A continued south swell mixing with northwest swell is forecast
with the a longer period southerly swell arriving over the weekend
and lingering into early next week.
Fire weather...27/200 PM.
Hot and dry conditions continued today with widespread temperatures
in the 90s and 100s across the interior and relative humidities in
the 10s to low 20s. A slight increase in onshore flow and
increasing north flow has resulted in winds starting early today
and about 5 mph stronger. These winds are expected to transition
to locally gusty northwest winds across the I-5 cooridor into the
western Antelope Valley tonight. Additionally, a weak Sundown
across the western Santa Barbara South Coast may bring northwest
wind gusts up to 25 mph tonight. A few degrees of warming with a 2
to 5 percent drop in minimum relative humidities is expected for
Elevated fire weather conditions across the interior mountains and
valleys including the location of the sand fire will persist each
afternoon and early evening through Thursday or Friday due to low
relative humidities and locally gusty onshore to northwest winds.
Northwest winds are expected to peak across the I-5 cooridor and
western Antelope Valley Thursday night with gusts up to 35 mph. A
weak to moderate sundowner is also possible Thursday evening for the
western Santa Barbara South Coast and the Santa Ynez range with
local wind gusts to 35 mph. Additionally, substantial drying aloft
may bring minimum relative humidities back down to 8 to 15 percent
to the interior mountains (also the Santa Ynez range) above about
2000 feet including far interior areas feet Thursday possibly
extending into Friday. This combination of low minimum relative
humidities and gusty winds across the I-5 corridor, western
Antelope Valley and western Santa Barbara South Coast may support
a few hours of critical conditions Thursday evening.
Elevated fire weather concerns extend into the upcoming weekend
due to the possibility of afternoon thunderstorms across interior
areas as high pressure weakens and shifts east, opening the door
for a push of monsoon moisture from the southeast.