Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 171813

1013 AM PST Tue Jan 17 2017

A series of storms will bring precipitation...heavy at times...
and gusty winds through early next week. The first will arrive
Wednesday night into Thursday with a chance of thunderstorms. The
second storm from Thursday night into Friday will have heavy rain
especially around the foothills and lower snow levels. The third
storm on Sunday and Monday should bring the heaviest precipitation
amounts. The temperatures will be below normal into next week.



Sunny skies this morning across the region with temperatures
generally similar to those yesterday... a few degrees warmer or
cooler depending upon the location. This should hold true with the
afternoon temperatures as well.

The forecasting focus for the next week will be the three storms
that will be sweeping through the region Wednesday through Monday.
The current forecast package looks to be have the storms handled
well with only tweaks needed as we get a clearer picture of how
the systems are developing. In particular... will be looking at
whether the first storm may be coming in a bit sooner... the
timing and areal extent of the thunderstorms on Thursday... and
whether the third storm may be coming in slower. If it stalls enough
that the heaviest precipitation from that system falls on Monday
morning it would have a greater impact on the morning commute than
currently expected.



Mostly clear skies across the forecast area early this
morning, although a few low clouds have developed just south of
the L.A. County coast. Do not expect these clouds to move onshore
this morning, so today should be a sunny day across the entire
forecast area. Pressure gradients were weakly offshore, about 2
mb offshore between KLAX and KDAG. There were some locally gusty
winds in the mountains early this morning, and these could become
more widespread and push into the valleys, but winds will not be a
significant factor today. Max temps today should be fairly similar
to those reached on Mon, generally not too far from normal for
mid January.

A positively titled upper ridge across the region today will be
forced southeastward tonight and Wednesday as a large full
latitude trough approaches the West Coast of North America. The
WRF suggests there might be some low clouds late tonight and Wed
morning across coastal sections of L.A. County. Otherwise, as the
jet stream sags southward on Wed with the approaching trough, high
clouds will begin to increase across the region. Clouds will lower
and thicken north of Point Conception and low level moisture will
increase as a surface cold front approaches the region. There is a
slight chance of rain north of Pt Conception late in the
afternoon, but it looks as though most of the daylight hours will
be rain-free. Max temps should be down a few degrees in most areas
on Wed.

The southern end of the trough will sharpen up and take on a
negative tilt as it moves across the region Wed night and Thu
morning. This should help to energize the surface front as it
pushes across the forecast area. Rain will overspread SLO and SBA
Counties Wed evening, and likely become moderate to heavy at times
by late evening or just after midnight. A potent jet, increasing
upper level diffluence, and cooling aloft could destabilize the
atmosphere to produce a few thunderstorms along or just off the
Central Coast after midnight. Rain will overspread Ventura and Los
Angeles Counties overnight, and moderate to heavy rain will
linger through the morning commute across most of Los Angeles
County. The best instability and threat of thunderstorms late Wed
night and Thu morning will shift into Los Angeles County and the
adjacent coastal waters. Steady rain should end by late morning
across Los Angeles County, but there will be enough moisture to
keep plenty of clouds and at least a slight chance of showers
through the day across the entire forecast area. Thu will be a
very cool day across the region.

This will be a rather fast moving system, but good jet and mid
level dynamics will likely allow for some decent rainfall
intensities on the order of one quarter to one half inch per hour
at times, possibly briefly higher in any thunderstorms. This could
be intense enough to generate at least minor mud and debris flows
in and around the recent burn areas. In addition, there will be a
period of decent southerly low level flow, which should provide
some enhancement of the rain on and below south and southwest
facing slopes. Expect widespread rainfall totals of one half to
one inch across the forecast area, with local totals of 1.5 to 2
inches possible in favored terrain. Snow levels will be mostly
above 6000 feet to start, but should lower to around 5000 feet
late Wed night and early Thu. Snow accumulations of at least 4 to
8 inches look to be a good bet, with greater totals possible in
higher terrain. Winter Weather Advisories or Winter Storm Warnings
may be needed with this event.


A very active and wet pattern will continue across the region Thu
night through Monday. There will be somewhat of a break in the
rain Thu afternoon into early Thu night. A very strong and cold
upper low will drop quickly southeastward from the southern Gulf
of Alaska toward the Pac NW Thu night, with a powerful 175 kt
west-northwesterly jet aimed into the forecast area by Fri
morning. Expect another period of steady rain to develop on the
Central Coast by late Thu night, then quickly overspread the
region Fri morning. While there is somewhat less in the way of
southerly low level flow, this system will have a good deal of
energy fueled by that powerful jet. With deep moisture lingering
into Fri night or early Saturday, the duration of the rain may
actually be a bit longer than with the first storm, especially
across L.A. County, where low level flow will be most favorable.
It will also be a colder storm with snow levels possibly down to
4000 feet or slightly below that Friday night. Additional rainfall
totals of 1 to 2 inches are expected, with local amounts to 3
inches in some foothill areas. In the mountains, significant
snowfall is expected. Given the strength of the jet and dynamics,
and the cold air aloft, thunderstorms will probably have to be
added to the forecast for Fri at some point.

Some showers may linger Sat and Sat night, but the bulk of the
rain should be over by late Sat morning. The third storm will
affect the region Sunday into Monday, and has the potential to
produce the most rain and wind. The 00Z runs of the GFS and EC
were in good agreement in showing a deepening trough approaching
the area Sunday, with a moist and strong southwesterly jet aimed
into the region, and strong to very strong sly low-level flow
developing during the day and continuing at night. Both models
were similar in showing rain spreading from nw to se across the
forecast area Sun afternoon, with rain, heavy at tines and gusty
winds Sun night and early Mon. The 06Z GFS shows the system
digging more sharply with the jet becoming more parallel to the
surface front, slowing it down significantly. This would keep much
of Sunday evening dry, with rain overspreading the area Sun night
and early Monday. The heaviest rain in this scenario would be late
Sunday night across SLO and SBA Counties, and during the day
Monday across VTU and L.A. Counties. At this point, have decided
not to buy off on the 06Z run just yet, but it will certainly have
to be considered if later runs start to trend that way.



At 1704z at KLAX...there was no marine inversion.

Good confidence in the 18z TAFs. Weak low level offshore flow
this morning should keep TAF sites VFR through this evening.
In coastal areas S of Point Conception MVFR cigs should develop
after 09z-11z Wed morning. There is a 30% chance that cigs will
not occur before 12z Wed. There is a 40% chance of low clouds and
fog possibly spilling into KPRB from the east by 12-15z.

KLAX...Good confidence in the 18z TAF. VFR conditions are expected
through this evening followed by MVFR ceilings after midnight.
Only a 30% percent chance of MVFR cigs not developing after 09z
Wed morning. Very light east wind expected Wed morning, but
speeds expected to remain below 7 knots.

KBUR...Good confidence in VFR conditions through this
evening followed by except MVFR/IFR cigs after 10z Wed. There is a
40% chance it will remain VFR.


.MARINE...17/900 AM.

No SCA level wind gusts expected through Wed morning. the first of
several storms will approach the coast by Wed night. Ahead of the
front, southerly winds will increase through the afternoon where
SCA level winds will be likely for all coastal waters north of Pt.
Conception. SCA level winds will then spread the entire outer
waters. A slight chance of thunderstorms and rain will accompany
the front. South winds will increase to SCA levels again late
Thursday night into Friday morning along with another frontal
passage...with gale force gusts possible over the northern waters.
Moderate to strong northwest winds will follow behind the front
through Saturday.

For the Inner Waters south of Pt Conception...Light winds are
expected through Wednesday afternoon then southerly winds will
increase on Wednesday ahead of a frontal system. There is a 60%
chance that a SCA will be needed for Wednesday night into
Thursday. Slightly stronger southerly winds for Thursday night
into early Friday ahead of another front. Increasing northwest
winds behind the front Friday afternoon into Saturday will likely
reach SCA levels. Slight chance of thunderstorms expected for
THursday morning.

A fairly large long period swell will affect the waters through
today. Another large long period swell will arrive early
Wednesday... then a very large swell is forecast to arrive Friday
night into Saturday. This third swell will be the
largest...potentially reaching heights over 20 feet at 18 seconds
over the northern outer waters late Friday night into early
Saturday. Breaking waves will become possible across west facing
harbors on Saturday.


.BEACHES...18/900 AM.

A long period westerly swell will bring elevated surf to the
Central Coast, L.A and Ventura beaches through today. Another
larger swell will bring rising surf later in the week with High
Surf conditions expected Thursday through Monday for all areas.
There is a good chance of very large westerly swell developing
Friday into Saturday...potentially resulting in very large,
damaging surf for area beaches. The best chance of damaging surf
will be on the Central Coast late Friday and Saturday, where surf
between 20 to 30 feet is likely. Less certain is damaging surf
south of Point Conception, but surf heights may rise to near 15
feet on west-facing beaches. During this time...expect strong rip
currents in all areas and dangerous, rough surf due to strong
south wind waves and large westerly swell. Coastal erosion will be
likely, with a possibility of sneaker waves and coastal flooding.

Beach goers should use the highest level of caution during this
time. Much larger waves could wash over rocks, jetties and beach
areas near the water`s edge, potentially sweeping you into the
water. West facing harbor entrances will be dangerous with large
breaking waves. Breaking waves may also occur in shallow water
near the outer edge of the surf zone.


CA...Beach Hazards Statement in effect from late Wednesday night
      through Monday evening for zones 34-35. (See LAXCFWLOX).
     Beach Hazards Statement in effect from Thursday morning
      through Monday evening for zones 39>41-87. (See LAXCFWLOX).
     Beach Hazards Statement in effect until 10 PM PST this
      evening for zones 40-41. (See LAXCFWLOX).


A wet and unsettled weather pattern is likely Wednesday night
through Monday. The heaviest rain during this period will probably
fall on Friday and again later Sunday and Monday. There is a chance
of damaging warning level surf and coastal flooding issues starting
Friday and continuing through Saturday, especially along the
Central Coast.



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