Scientific Forecaster Discussion
Southwest California area forecast discussion
National Weather Service Los Angeles/Oxnard California
900 am PDT Wednesday Aug 20 2014
A slight chance of thunderstorms today in Los Angeles County and in
the Ventura County mountains. A slight chance of thunderstorms
tomorrow in the Los Angeles and Ventura County mountains and the
Antelope Valley. Afternoon highs below normal through the end of the
week with low clouds along the coast and in adjacent valleys.
Clearer skies and warmer temperatures are on tap for the weekend.
Short term (today-fri)...
*** middle-morning discussion update ***
big question today will be whether and when thunderstorms slide into
Los Angeles County today and if any development makes it into the
Ventura County mountains as well. The primary rain band continues
to slide southeastward across San Diego... Riverside... and San
Bernardino counties this morning. However as the center of the low
drops southward indicates continue to show a likelihood of the
thunderstorm band wrapping around into Los Angeles County this
afternoon. With good instability stringing across the region into
the mountains of San Luis Obispo County it appears that development
will not be a problem but the best moisture appears to stay to the
southeast so this could be a limiting factor especially for anything
west of the San Gabriel range. Looks better tomorrow... but then
the low may be out of position.
Not anticipating any notable changes to the forecast package at this
time other than possible tweaking for thunderstorm development today
*** Early morning discussion ***
what a difference 60 miles makes. All of l.A. Counties coasts and
valleys as well as the coastal slopes are covered with low clouds. 60
miles south at the northwest tip of San Diego County a persistent line of
thunderstorms is rumbling on. The marine layer is 3000 feet deep but the
cloud layer is pretty thin and expect decent clearing today...except
for the central coast where the beaches will see slow to no
clearing. The coastal/valley forecast says mostly sunny this
afternoon but this deep a marine layer might produce some afternoon
Strata cumulus. Thunderstorms are the real story of the day. All brought to US
courtesy of 576 dm up low just to the west of Catalina Island. A
vorticity maximum whipping around the base of the low is providing the focus
for the San Diego County thunderstorms. There is a slight chance that a thunderstorm
could form over l.A. County or the adjacent waters early this
morning but for the most part the convection will stay to the south.
Things change later this morning however. The upper low drop a
little more southward and the flow pattern shifts from the SW to the
south-southwest. This will push the moisture and lift form the upper low
northward into eastern la County. By early afternoon the low has settled
further south and the flow pattern GOES from near San Clemente to
Pasadena to Lancaster. This flow pattern looks pretty conducive for
convection and should last into the evening. The la coasts and valley
will have a slight chance of thunderstorms while the mountains and deserts (as well
as the vta County mtns) will have a chance. There is not much moisture
to work with and any storms that do form will move at a pretty good
clip so the threat of flash flooding is fairly small. 578 heights...
the deep marine layer and good onshore flow will all contribute to a
much cooler than normal day.
Unless some really huge thunderstorms develop today with outflow powerful
enough to disrupt the marine inversion (unlikely) there will be
another round of low clouds covering the coasts and valleys tonight.
Expect good clearing Thursday and almost all areas should be sunny
by noon. There is just enough wrap around moisture associated with the
exiting upper low to allow for a slight chance of afternoon thunderstorms over
the la/vta mountains maximum temperatures will rise some due to higher heights and
weaker onshore flow but all temperatures will still be below normal.
The upper low will open up and move eastward into Arizona by
Thursday night and will no longer influence our weather. Heights rise
and the onshore flow decreases the marine layer will shrink. It will
most likely shrink out of the valleys but there is a 30 or 40 percent
chance that some valleys will have one more night of low clouds. More
warming will bring maximum temperatures back close to normal.
Long term (sat-tue)...
extended models have been terrible all Summer and tonights run are
equally troubled. Yesterdays building high pressure was different
from mondays cull area forecast and todays forecast calls for 4 days of
a long wave trough. Did not make a bunch of changed to the long range
forecast. Lowered maximum temperatures a few degrees since the the ridge is no
longer in the picture...but did not mess with the marine layer
clouds which if this scenario comes true will be deeper than forecast
and will affect the valleys.
Latest amdar soundings indicate marine inversion around 3000 feet
and satellite/surface observation indicate coastal/valley taf sites
blanketed in MVFR ceilings. Expect MVFR conds to continue through early
this afternoon...then low clouds should clear to the coast. Return
tonight should be very similar with regard to ceiling category and
stratus coverage. However...any impacts from possible convection
could greatly change the stratus picture.
As for thunderstorms...currently activity is south of Orange County.
However as the upper low wobbles...this activity could move into Los
Angeles County today. At this time...chances for any thunderstorms
at taf sites is low...so only about a 10% chance through this
evening although will carry a cumulonimbus cloud group this afternoon for kpmd
Klax...moderate confidence in 12z taf. High confidence in MVFR ceilings
through this morning. Confidence in clearing this afternoon is
low...and may not occur. There will also be a 10% chance of a thunderstorm
impacting the airfield through this evening. Will monitor closely.
Kbur...moderate confidence in 12z taf. High confidence in MVFR ceilings
through this morning. Low confidence in current forecast/timing of
clearing this afternoon. There is a 10% chance of thunderstorms impacting
the airfield through this evening.
No significant wind issues anticipated across the coastal waters for
the next several days.
There is the possibility of some isolated thunderstorms across the
southern waters through this evening...with gusty winds and choppy
seas likely with any thunderstorms.
marine weather statement (see laxmwslox).