Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 030509
AFDLOX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
1009 PM PDT MON MAY 2 2016

.SYNOPSIS...

Night through morning low clouds and fog are expected for many
areas west of the mountains through Wednesday, followed by
partial afternoon clearing. A slow moving low pressure system
will bring a chance of showers and thunderstorms Thursday through
Sunday, along with cooler conditions. Temperatures will likely
return to near normal early next week as high pressure returns.

&&

.UPDATE...

Latest sounding data (VBG and AMDAR) indicates marine inversion
ranging from 1200 feet deep north of Point Conception to about
800 feet south of Point Conception. With weaker onshore gradients,
slightly increasing 500 MB heights and no forecasted eddy, do not
expect much deepening of the marine inversion overnight. So,
will expect stratus/fog to become rather widespread across the
Central Coast and Santa Ynez Valley. For areas south of Point
Conception, will expect stratus to develop across the coastal
plain, but should be rather limited in the coastal valleys (due to
no expected deepening of the marine inversion). Otherwise, some
high level clouds will drift in overnight.

Earlier, an update was issued to scale back stratus/fog coverage
across the coastal valleys of Ventura and Los Angeles counties.
Otherwise, current forecast looks to be in good shape in the
immediate short term.

&&

.SHORT TERM...(Tonight-THU)

Forecast area generally under weak high pressure aloft for the
next couple of days. A remnant of the last trough to move through
the area has left enough cold air aloft to induce some cumulus
over the mountains and extending into the Antelope Valley.
Showers developed over the eastern San Gabriels by late morning
but they have recently decreased due to warming aloft and weak
height rises. The overnight stratus has cleared out over coastal
areas of Los Angeles County and for the most part north to Point
Conception. However, stratus has held on over the immediate
central coast due to a slightly stronger onshore pressure gradient
versus southern coastal areas. Expect a similar stratus pattern
tonight as pressure gradients and marine layer depth change little
from last night.

Heights and thickness values continue to increase tomorrow to
bring another few degrees of warming nearly all areas. High clouds
will move across the area in advance of the next trough. Some mid
level clouds may sneak in by early evening. These shouldn`t deter
the warming trend. Another round of overnight coastal stratus for
Tuesday night. But with an increase in onshore flow and resultant
increase in the marine layer depth expect it to penetrate further
inland compared to last night and tonight.

By Wednesday 12z the upper trough axis is about 500 miles west of
LAX with fairly good agreement among models. Upper level
southwest flow increases as it gets closer along with general
cooling through the column. High temperatures on Wednesday will be
around 5 to 10 degrees cooler than Tuesday`s highs. By late in the
day Wednesday the upper trough is close enough to possibly produce
some showers over northern San Luis Obispo County extending westward
offshore.

By late in the day Thursday the upper level trough becomes a well
defined closed low, the center of which by 00Z Friday is around or
just west of Point Conception.  Mid level temperatures of -23C
near the core of the low will produce a marginally unstable
conditions. The NAM indicates lifted indices around -3.5C over
the Ventura County Mountains and CAPE values around 600 J/kg.
These numbers justify the the possibility of thunderstorms mainly
over the mountains for late Thursday into Thursday night. The GFS
swings a pocket of unstable air across southern sections Thursday
afternoon. This along with the favored dynamics south of the low
center would put the best chance of thunder over the San Gabriel
Mountains. At lower elevations, expect isolated to scattered
showers on Thursday but at this time do not expect any thunder
these areas. Moisture is lacking a bit at the onset of the
system`s arrival on Thursday. The precipitation will be highly hit
and miss on Thursday. Some areas may not receive any measurable
precipitation at all. Of course temperatures continue to cool on
Thursday when highs will be as much as 10-15 degrees below normal.

.LONG TERM...(FRI-MON)

Models generally agree in putting the center of the upper closed low
by Friday morning somewhere just south of Los Angeles, the ECMWF a
little further southwest compared to the GFS. Friday represents
the best chance of thunder lower elevations mainly south of Point
Conception. Increasing moisture that models show wrapping around
the low`s center could be a factor in chances for thunder on
Friday. The GFS places the upper low`s center around the Calif-
Ariz border south of Vegas by 00Z Saturday which keeps the
stronger dynamics well east of the forecast area. Again,
difficult this far out to identify any areas that have better
chances of precipitation over others. Friday will be the coolest
day of the 7-day forecast.

By around midday Saturday models put the upper low`s center over
western Utah with moist and cool northernly flow behind it
sweeping across southern California. Both the GFS and ECMWF
indicate a weak vort lobe rotating around its back side but any
precipitation from this at this time looks to stay well offshore.
Despite the low`s position well to the north and east there`s just
enough weak instability and moisture around to keep at least a
slight chance of showers on Saturday. Some slight warming expected
on Saturday despite the cloudiness although high temps still quite
a bit below normal.

Total QPF from this system between Thursday and late Saturday
should stay below a quarter most areas, less than a tenth more
likely. The one exception would be higher amounts under any
thunderstorms that develop. Overall this will not be a big rain
maker. Snow-wise, minimum snow levels with this system could be
as low as around 5700 to 6000 feet on Friday night.

The low continues to lift out through the intermountain west on
Sunday. Some additional warming is expected along with clearer
skies. High temperatures inch a few degrees higher on Sunday but
still not enough to get back to normal. Have backed off on
precipitation chances on Sunday as the low is just too far
removed. Additional warming for Monday when upper level high
pressure builds in earnest to help get high temps around or just
below normal.

&&

.AVIATION...03/0507Z...

At 0507Z, the marine inversion at KLAX was based at 800 feet. The
top of the inversion was 1800 feet with a temperature of 21
degrees Celsius.

Overall, moderate confidence in 06Z TAF package. Moderate
confidence in stratus/fog returning to coastal TAF sites this
evening and overnight, but low confidence in timing. There is a
10% chance of stratus reaching into the coastal valley TAF sites
overnight.

KLAX...moderate confidence in 06Z TAF. Moderate confidence in
stratus returning overnight, but low confidence in timing (could
develop 2-4 hours earlier than current 12Z forecast).

KBUR...high confidence in 06Z TAF as VFR conditions are
anticipated through the TAF period. There is a 10% chance of IFR
conditions 09Z-17Z.

&&

.MARINE...02/830 PM...

For the Outer Waters, moderate to high confidence in current
forecast. High confidence in winds and seas remaining below SCA
levels through Tuesday night. From Wednesday through Friday, winds
will increase and there is a chance of SCA level winds developing
(especially in the afternoon and evening hours).

For the Inner Waters, high confidence in current forecast. High
confidence that winds and seas will remain below SCA levels
through Friday (although there is a 20% chance of SCA level gusts
Thursday and Friday afternoon across the waters north of Point Sal
as well as western sections of the southern Inner Waters).

Given the limited depth of the marine inversion, there could be
some patchy dense fog across the coastal waters late tonight and
Tuesday morning.


&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.

&&

$$

PUBLIC...RAT/MJ
AVIATION...RAT
MARINE...RAT
SYNOPSIS...Sukup

weather.gov/losangeles



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