Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 041612
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
912 AM PDT WED MAY 4 2016
The onshore flow will bring overnight clouds...fog and drizzle
into early Thursday. Then a low arrives on Thursday for a chance
of showers and thuderstorms into Saturday night... with showers
to linger in the mountains on Sunday. The temperatures will
remain below normal into early next week...then a warming trend
into midweek as a high builds in.
A strong marine incursion topped by increasing incursion of higher
clouds will keep afternoon temperatures cooler today and that is
reflected in the hourly readings. Winds will likely strengthen a
bit but are expected to remain below advisory levels at this time.
The incoming low pressure system and the timing and intensity of
showers and thunderstorms will be the focus of the shift today.
Overall the period of possible precipitation remains Thursday
through Sunday and am not anticipating any significant changes to
the forecast package. There is an increasing possibility of
convective activity on Saturday and that will be a particular
focus for today.
Southern California`s weather will be dominated by a cut off low
over the next four days. Currently the upper low is just a trof in
the process of cutting off about 400 miles west of Point
Conception...it will be a fully formed cut off later this
afternoon. Today will be a mostly cloudy day as mid and high level
clouds stream over a large marine layer created by increasing
onshore flow and the lift from the trof. There will be some
patchy fog this morning under the marine layer but nothing too
bad. By afternoon the onshore gradients will be over 7mb to KDAG
(or 9mb if you believe the NAM which is not a good idea) This
increase in the onshore push to the east will keep the low clouds
going near the coast. It will also create breezy conditions across
SLO county and the Antelope vly this afternoon. Max temps will
nose drive under the clouds...the falling hgts and the increasing
The extra lift and the strong onshore flow will likely create a
very deep marine layer and some drizzle over the coasts and vlys
and into the foothills tonight.
The upper low will move within 60 miles of SLO county on Thursday.
hgts will drop to 558 DM and there will be further cooling under
mostly cloudy skies. Small vort maxes will rotate into and out of
the area all day and hit or miss showers will likely develop
throughout the day. Convective parameters look decent across the
interior and there is a slight chc of TSTMs there. Mdl sounding
show good potential for hail as well. Rainfall amounts will be
highly variable and spotty with a tenth of an inch the best bet
and an outside chc of some quarter inch amounts. Snow levels will
fall to about 6500 feet.
The shower threat will persist overngiht but the TSTM threat will
On Friday the upper low moves from Point Conception over Srn SBA
county and then over and through VTA and L.A. counties. The
lift...vorticity...and cold pool assoc with low will bring good
convective parameters to VTA and LA county and there will be a
good chance of showers and a slight chc of TSTMs over both
counties. There will be a little more moisture to work with and
can see a little more rainfall over the area...esp over VTA and LA
county. The big question mark will be cloud cover. Right now it
looks like it will be a mostly cloudy day...but if the mid and
upper level clouds are a little less than fcst and it turns into a
partly cloudy day...then watch out. The greater SFC heating during
peak hours in the afternoon will match perfectly with the cold
pool aloft moving over the area and this would allow for a much
bigger convective outbreak.
Shower activity will diminish Friday night.
Saturday could also be an interesting day. In fact it may well be
the most convective day of the next four. The upper low will move
to the UT/NV/AZ border triple point where it will be in perfect
position to advect vort maxes into the interior sections of the
forecast area. There should be much less in the way of clouds so
look for much better sfc heating. This instability could well
interact with the incoming PVA and kick off a very nice convective
outbreak. The coasts and vlys will see much more sunshine than the
previous three days (or the next three days depending on you like
to look at things) and there will be a nice bump up in max temps.
By Sunday the upper low will be far enough away that the only
threat of showers will be confined to the interior sections and
esp the L.A. mtns. There will be noticeable decrease in clouds
and corresponding warm up in temps.
Ridging moves into the area on Monday and intensifies on Tuesday.
Max temps will warm each day and most areas will be above normal
by Tuesday. The high pressure aloft will create a strong enough
marine inversion that some night through morning low clouds will
likely form over the LA coast.
At 1131Z at KLAX the marine layer was 1500 feet deep. The top of
the inversion was 3100 feet with a temperature of 17 degrees
N of Point Conception...Moderate confidence that lifr cigs will
move up two categories to mvfr by 18z. 30% chance to remain in ifr
conds through 18z. 30% that cigs will scatter out after 20z.
Higher confidence for cigs to remain in mvfr category tonight into
S of Point Conception...Moderate confidence that ifr cigs will
become mvfr by 17z. 40% chance that cigs will scour out before
21z. 20 percent chance for cigs to go into vfr for a portion of
the afternoon hours. better chance for Thursday.
KLAX...Fair confidence in 12z TAF. There is a 30 percent chc that
MVFR cigs will scour out after 20z. There is a 20 percent chance
that cigs will get to vfr late this afternoon.
KBUR...good confidence that there will be mvfr cigs...with 30
percent chance for ifr cigs through 16z. 30 percent chance that
mvfr cigs will remain in place all day.
For the Outer Waters, good confidence in current forecast. Winds
and seas are expected to remain below SCA levels through the
weekend. However there is a 30% chance that some SCA level gusts
could develop Friday afternoon/evening across western sections of
the Outer Waters.
For the Inner Waters, good confidence in current forecast. Winds
and seas are expected to remain below SCA levels through the