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Southwest California area forecast discussion 
National Weather Service Los Angeles/Oxnard California
830 PM PDT Sat Apr 18 2015

Synopsis...
expect a steady cooling trend through the middle of next week with
low clouds and fog in the coastal and valley areas. There will be a
slight chance of thunderstorms over some of the mountains early in
the week. Persistent low pressure will bring a slight chance to
chance of showers and mountain snow showers to much of the area by
Wednesday then persist into Saturday.

&&

Short term (tonight-tue)...
upper level low pressure system forming off the coast of northern
California is forecasted to dive southeastward and become centered
off the coast of Southern California by Monday afternoon. Already
seeing substantial increase in onshore flow this evening with
current lax-Daggett gradient at +5.2 mb. As a result...increasing
onshore winds across interior sections this afternoon and
evening...as evident by the wind gust of 45 miles per hour being observed at
Lake Palmdale at 8 PM. Satellite imagery showing a solid blanket of
low clouds and fog beginning to spread inland across the central
coast. For areas south of Point Conception...low clouds and fog are
expected to fill in across most coastal areas by Sunday
morning...spreading locally into some of the valleys. Latest acars
sounding showing marine layer depth around 900 feet across la
basin...so there could be some patchy dense fog with initial onset
of stratus. However...by Sunday morning...marine layer is expected
to deepen to around 1500 feet.

As the low pressure system becomes centered off the central California
coast...increasing instability will likely generate some afternoon
cloud buildups over the mountains of Ventura and Santa Barbara counties
on Sunday. While moisture is quite limited tomorrow...there are some
rather impressive convective parameters...with a broad area of
lifted index values between -3 and -5 over the sba/Ventura County
mountains and cape values around 1000 j/kg. While moisture is
limited...cannot totally rule out an isolated storm over sba/Ventura
mountains late Sunday afternoon...which is reasoning for 10 percent
thunderstorm probability for this area. Model sounding shows
inverted-v structure with moisture around 600 mb and dry at low
levels...so if any storms were to develop...they would produce
minimal rainfall with gusty winds and isolated dry lightning being
the threats.



00z NAM continues to show Monday and Tuesday as being potentially
more active with thunderstorms across some of the local
mountains. Once again...it appears that the Ventura and Santa Barbara
County mountains will have the best threat as these are the areas
with best available moisture and instability. NAM-12 lifted index
values are a rather impressive -6 over the Ventura County mountains
with a projected cape value around 1000 j/kg. With better available
moisture and instability (due to cooling temperatures aloft)...there
would certainly be an increased threat of hail with any thunderstorm
development on Monday afternoon...along with potential for gusty
winds. Any storms that do develop on Monday and Tuesday also look to
be slow moving due to fairly weak winds aloft.

*** From previous discussion ***

Second impact of this trough will be chances for some late afternoon
and evening convection Monday and Tuesday over the Ventura County
mountains and bleeding over into the sba County mountains. Models
continue to show some instability for these areas and enhanced by a
pool of middle level moisture. Lifted indicies forecast Tuesday by the NAM
are between -4 to -5 degree c...so slightly more instability forecast
for Tuesday compared to Monday. GFS and European model (ecmwf) also in agreement as far
as tr chances. Not a lot of moisture in the column to work with so
do not expect any significant precipitation amounts with anything
that does develop. The GFS produces quite a bit of precipitation over the
Ventura County mountains...around an inch and a half run total
through Tuesday night which looks to be well overdone. The NAM on
the other hand shows a more reasonable third of an inch. At this
time. Lower elevations and the san Gabriels will stay dry through
this period.

Temperatures cool at least several degrees each day through the
short term. By Tuesday highs will be 10 degrees below normal in some
areas...but with the deserts still around normal. Highs will be in
the 60s most coastal areas and only in the low 70s nearby valleys.

Long term (wed-sat)...
a more complicated forecast for the extended period. A second upper
low drops southeast through the eastern Pacific...and by midday Wednesday the
center of this low will be around 500 miles to the southwest of
lax...the European model (ecmwf) further away than the GFS. First the easier part of
the forecast...temperatures will continue to cool each day through
at least Thursday with partly to mostly cloudy skies. Also expect
another healthy marine layer Wednesday morning. Model probability of precipitation are not
in very good agreement for Wednesday. Most deterministic model probability of precipitation
are restricted to over the vtu County mountains while statistical
and ensemble guidance spreads probability of precipitation into the valleys. Current
forecast has at least slight chance probability of precipitation everywhere and chance probability of precipitation
over vtu County mountains and over l.A. County where dynamics may be
better. Not enough clarity in the latest models to really change
official forecast much if at all. Some lower elevations may not see
any measurable.

By Wednesday night the GFS forecasts the low to close off over socal
with a couple of vorticity lobes rotating around its perimeter to the
west and southwest. The European model (ecmwf) meanwhile closed it off further to the
south over northern Baja California. Then midday Thursday models are in better
agreement and generally agree on continuing to swing the low into
the Desert Southwest. Technically the trough axis is through our
forecast area by this time. Nothing is clear cut as far as the
precipitation forecast Wednesday night into Thursday. Again...wouldn't be
surprised if some areas don't receive any measurable precipitation
this period...which the chances of that are greatest if the European model (ecmwf)
verifies. For now will leave forecast as it stands with little
change from wednesday's forecast probability of precipitation...especially since ensembles
support at least a slight chance of measurable this period. With the
cooler temperatures there is some chance of some snow mixed in with the
precipitation down to the Grapevine but without much chance at all of any
accumulation.

Big differences between the European model (ecmwf) and the GFS for Thursday night
into Friday. The GFS keeps general troughiness over the area and the
European model (ecmwf) actually moves a shortwave ridge through the area. Current
forecast has slight to chance probability of precipitation restricted to mainly southern
areas and mountains on Friday and for now will leave that as is.
European model (ecmwf) is driest of course and this is sort of the direction the
ensemble probability of precipitation are taking.

The biggest question mark of the extended period is Friday night
into Saturday. The European model (ecmwf) brings a very sharp trough across the area
this period and including a well-defined trough passage. The higher
probability of precipitation are also supported by the ensemble solutions. And despite the
GFS deterministic run that keeps energy well to the south...its
ensemble mean hints at something closer to ec solution. The ec model
has also shown a little more run to run consistency. But definitely
not enough clarity to make sweeping changes to saturday's forecast
at this point. For now at least a mention of precipitation will suffice
until we get to say midweek when hopefully things will be clearer.
For now temperatures are forecast to moderate some on Friday and Saturday
but that could change.

As it stands now we do not expect much in the way of storm total
precipitation amounts for Wednesday through Saturday...lower elevations
possibly in the tenth to quarter and higher amounts mainly over the
vtu County mountains. A lot of this depends on the development of a
possible system on Saturday which could double these amounts.

&&

Aviation...18/2345z.

Marine layer at lax at 2330z is 900 feet deep and the inversion top
is at 1650 feet with a temperature of 20 degrees c.

Weak upper level trough developing off the California coast combined
with strengthening onshore flow will bring increased low clouds and fog
to coastal/valley areas overnight into Sunday morning. Initially...could
see some brief LIFR conditions. As marine layer depth gradually
increases...should see mostly IFR category by Sunday morning. High
confidence in ceilings returning to most coastal taf sites...with moderate
confidence in ceilings reaching valley tafs (burbank and Van nuys). Increasing
onshore flow should bring slightly stronger winds tomorrow afternoon
to kpmd and kwjf.

Klax...high confidence in ceilings developing late tonight into Sunday
morning...lower confidence in ceiling height and associated visibilities.

Kbur...moderate confidence in ceilings reaching kbur late tonight into
Sunday morning. Lower confidence in ceiling height and associated
visibilities.

&&

Marine...18/800 PM.

Northwest winds in the vicinity of the northern Channel Islands have
been gusting 20 to 25 knots...and appear to be widespread enough to
issue a marginal Small Craft Advisory for this evening through late tonight. After
that expect light winds across all the waters with subsiding seas. A
weak upper low will cross the waters on Tuesday...but expect winds
to remain light as this occurs. However...in the wake of this system
northwest winds will increase over the outer waters...and it seems
likely that a Small Craft Advisory will be needed.

Areas of fog with visibility 1 nm or less expected for much of the coastal
waters tonight. The layer will begin to lift with improving visibility
between midnight and 5 am PDT.

&&

Lox watches/warnings/advisories...
Small Craft Advisory (see laxmwwlox).
High surf advisory (see laxcfwlox).

&&

$$

Public...gomberg/Jackson
aviation/marine...gomberg/Smith
synopsis...sirard

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