Scientific Forecaster Discussion
fxus66 klox 131212
Area forecast discussion
National Weather Service Los Angeles/Oxnard California
412 am PST Wed Nov 13 2019
Temperatures will be cooler through the end of the week then warm
again this weekend. Skies will be mostly cloudy through Friday and
then will become mostly clear for the weekend.
Short term (tdy-fri)...13/333 am.
The marine layer is about 1100 feet deep near klax and probably
about half that amount over the central coast. Low clouds cover
most of the coasts and some of the vlys esp in la County. Enough
dense fog has formed across the central coast to warrant a dense
fog advisory and there is also patchy dense fog in the vlys of
la/vta counties. There is very weak onshore flow. Yesterday's
ridge is now tilting to the NE which is allowing a large swath of
mid and high level clouds to move into the area. These clouds are
not that thick so it will likely be a partly to mostly cloudy day
after the low clouds dissipate. There will not be much temp
change near the coast but there will be 3 to 6 degrees of cooling
everywhere else and even up to 10 degrees of cooling in the Santa
Clarita Vly and western San Fernando Vly. Max temps will still
come in a few degrees above normal.
Weak troffing moves into the state on Thursday and the mid and
high level clouds thicken up making for a mostly cloudy day. There
will also be morning clouds across most of the coasts to further
add to the gloom. The clouds and the lowering hgts will subtract
another 2 to 4 degrees from the Max temps.
Wednesday night into Thursday those high clouds are expected to
thicken up quite a bit and keep skies mostly cloudy. Between that
and the cooling aloft associated with the approaching trough
temperatures area-wide are expected to cool several degrees, but
more so across inland areas. Max temps everywhere will be a
couple degrees cooler than normal.
The mid and high clouds (as well as the coastal low clouds) will
clear out Friday morning leaving mostly sunny skies in the
afternoon. Synoptically an upper will pinch off from thursday's
trof and set up either 400 nm (ec) or 800 nm (gfs) to the SW of
klax. Either way the cut off will not have much effect on the
areas weather. Cooler air will advect into the interior sections
for another 3 to 6 degrees of cooling there. A little east flow
over the central coast will bring 3 to degrees of warming there
while the rest of the area will see little change from thursday's
Long term (sat-tue)...13/333 am.
An upper ridge slides over the cut off low pushing the low away
to the SW as it moves into the state. Offshore flow will set up
overnight and skies should be clear. There will be some morning NE
canyon winds in the la/vta vlys but nothing near advisory levels.
The big news of the day will be the warm up with almost all areas
slated to see 8 to 10 degrees of warming.
Offshore flow peaks on Sunday and there will be a little (very
little) upper and thermal support. This may be enough to create a
need for a few low end advisories in the la/vta vlys and it will
be gustier than Saturday across the la and vta coasts. Nothing too
remarkable though. Sunday should be the warmest day of the next 7
with 3 to 6 degrees of warming everywhere xcp for the Antelope
Vly and la mtns where a little cool air intrusion will put a
damper on things.
The ridge hangs on Monday but the offshore flow reverses. Still
not enough onshore push for marine layer clouds. The onshore flow
will bring an earlier sea breeze and the lack of offshore flow
will eliminate the compressional warming. The coasts and vlys will
cool while the interior sees little change in temps.
The ridge breaks down Tuesday as another trof moves into the
northern portion of the state. Onshore flow will increase and
marine layer stratus will likely develop. Look for 3 to 6 degrees
of cooling across the board.
All eyes still on Thursday the 21st as the ec and most of its
ensemble members are still bringing rain to the area mostly la/vta
counties. The GFS and most but not all of its ensemble members
are dry. It will be interesting to monitor the evolution of this
forecast which much to far away to have any real confidence in.
At 0952z at klax, the marine layer was around 1100 feet deep. The
top of the inversion was at 1600 feet with a temperature of 21 c.
Moderate confidence in coastal and valley tafs, especially with
regards to timing of flight category transitions. All sites that
do not have 1/4sm fog have a 20% chance of it through 15z. Lower
confidence on the timing of arrival of low clouds tonight.
Klax...moderate confidence in taf. Good confidence in IFR cigs
through 17z, but lower confidence on return of low clouds
tonight both for the height and timing. Good confidence that any
east wind component will be 5 kt or less.
Kbur...moderate confidence in taf. There is a 30% chance that
cigs remain vlifr. Clearing of cigs may occur as early as 14z.
For all of the coastal waters, high confidence that winds and
seas will remain below Small Craft Advisory (sca) level through
Thursday. Moderate confidence that winds will increase to Small Craft Advisory
level over the offshore waters along the central coast and
southward towards San Nicolas Island on Friday afternoon and last
through the night. Lower confidence on whether the Small Craft Advisory level winds
will continue into Saturday, as winds are likely to weaken.
Patchy dense fog with visibility of one nautical mile or less is
expected to impact all of the coastal waters through this morning.
A large and long period northwest swell is expected to arrive over the
coastal waters on Friday afternoon and last through the weekend,
with the largest impacts along the central coast.
California...dense fog advisory in effect until 9 am PST this morning for
zones 34-35. (See laxnpwlox).
Hazard potential outlook (fri-tue).
There is the potential for moderate Santa Ana winds next Sunday
with elevated to critical fire weather conditions possible.