Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 211949
AFDLOX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
1149 AM PST Sat Jan 21 2017

Updates Aviation, Marine, and Beaches Discussions

.SYNOPSIS...
Isolated showers are possible today, but generally dry conditions
can be expected. A strong storm system is forecast to move into
southwestern California late tonight then persist into Monday. The
storm will peak on Sunday, with rain, heavy at times, a slight
chance of thunderstorms, and strong gusty southerly winds. Lingering
showers and mountain snow showers will prevail over the region
Sunday night into Monday, with a slight chance of showers on
Tuesday. High pressure will then bring dry and milder conditions to
the area Wednesday through Friday.

&&

.SHORT TERM...(TDY-MON)

Models still hitting the area really hard with storm #3 on Sunday.
With snow levels initially being above 8000` with this evening
I`ve issued a high wind warning for the mountains for Sunday into
Sunday evening, then as the colder air moves in Sunday night a
winter storm warning will take over and run through Monday. Wind
advisories have also been issued for areas north of Pt Conception
and some may be needed for southern areas as well though it`s more
marginal there. Models still showing 40-60kt of south to southeast
wind during the peak of the event Sunday which will provide
significant orographic enhancement to rainfall rates even without
any convection. Rainfall rates of 1 to 1.5" per hour are expected
at times with this event, mainly south of Pt Conception due to the
more favorable orographics, and for this reason a flash flood
watch will be posted for all of LA and Ventura Counties, the SB
Mountains, and SB South coast.

***From previous discussion***

And then its Sunday and game on! All mdls agree and have been
consistent in forecasting the biggest storm to hit Srn CA since
2010. It all starts with a cold 507 DM upper low a couple hundred
miles west of Portland moving and pushing the flow pattern from NW
to SW. The thing that is making this storm special is a 160-170kt
jet that will align itself parallel to the front which will slow
it down and intensify it (rather than the usual orthogonal jet to
front orientation that shoots the front into and through the area
in just a few hours) This intense jet driven storm will also be
working with an impressive 1.2 to 1.4 inch precipitable water
column.

Light rain will overspread the Central Coast later tonight/pre
dawn Sunday morning. There is a chc some overrunning rain will
develop ahead of the main front over VTA and LA county after
midnight. Still all of this will be light rain (OK the NW tip of
SLO county may see moderate to heavy rain by dawn) Heavy rain will
affect SLO and Western SBA county from Dawn til noon with moderate
rain falling through the afternoon. Heavy rain will move into srn
SBA county and maybe western VTA county later Sunday morning.
During the afternoon there will be heavy rain falling everywhere
south of Point Conception. There will be 30 to 40 and maybe 50 kts
of south winds ahead of the front. These south winds will create
extreme orographic uplift (which will only add to the large storm
dynamic uplift) over the south facing slopes from Point Conception
to San Gabriel vly foothills. The front will exit LA county in the
early evening but there will be numerous showers continuing
overnight in the very moist and unstable WSW flow behind the
front.

Right now it looks like snow will not be a factor since the warm
south flow will boost snow levels to over 8000 feet. Snow levels
will however fall quickly behind the front and will reach at least
4500 feet by dawn Monday. This looks like a fairly stable system
in terms on convection and do not see a TSTM threat on Sunday.

Flash flooding is a distinct possibility with this system and not
just the burn areas but everywhere. This is because this system
will be able to produce greater than 1 inch per hour rainfall
rates without the help of convection. Also the previous two storms
have pretty much saturated everything so run off will start up
pretty quickly. This is going to be quite the rainmaker with the
csts and vlys looking to recieve 2 to 4 inches of rain and the
coastal slopes set to receive 3 to 6 inches. There will likely be
local amounts much greater than 6 inches above the city of SBA
and the coastal slopes above the San Gabriel Vly. A Flash Flood
watch will almost certainly be issued later today for the entire
area.

All of the mtns will very likely see warning level wind gusts and
a few of the interior vly locations may also. Every other area in
the forecast area will need a wind advisory for gusts between 45
and 55 mph. A High Wind watch will almost certainly be issued
later today for the mtns and maybe the Antelope vlys and SLO
interior vly.

While Monday will not be nearly as wet as Sunday it will still
have its fair share of weather hazards. Chief among them will be
the threat of TSTMs as the whole atmosphere over the area looks
unstable. This along with a few vort maxes that are forecast to
swing through will bring a slight chc of a TSTM to the entire
area. Any TSTM that forms will be more than capable of producing
more flash flooding. The next problem will be low snow levels
which will be near 4000 feet and down to 3000 feet in heavier
bursts. These low snow levels will likely create hazardous
traveling in and thru the mtns including I-5 and highway 14. Its
hard to estimate how much snow is going to fall as it depends on
when the cold air arrives and how much moisture is left over but
moderate snowfall amounts are not out of the question.

.LONG TERM...(TUE-FRI)

On Tuesday the upper low that is spinning off of the Oregon coast
from now until Monday will be incorporated into a large scale
trof rotating out of Canada. The latest mdls are taking this
system further to the west than previous runs and if this trend
continues there will be far less shower activity than currently
forecast.

Both the GFS and EC agree at least in principle that east pac
ridging will build into the area Wednesday and remain through at
least the weekend. While it will almost certainly be dry the mdls
do not show much in the way of warming with this ridging which is
odd given the higher hgts and offshore flow.

&&

.AVIATION...21/1845Z.

At 18Z at KLAX there was no marine layer or inversion.

High confidence in VFR conditions everywhere through 06Z...with
sct-bkn035 possible in the afternoon. Powerful storm system still
on track to affect region on Sunday...with moderate confidence in
timing. IFR-LIFR conditions will be common for several hours
during peak of storm. Thunderstorms are not expected on Sunday,
but there is a chance on Monday.

KLAX...High confidence in VFR conditions through 06Z. sct-bkn035
likely 20-00Z today with a 10 percent chance of BKN025. High
confidence in southeast winds well over 10 KT on Sunday for
several hours...with an onset window 09-16Z. IFR conditions likely
after 18Z with +RA...with a 30 percent chance of occasional LIFR.

KBUR...High confidence in VFR conditions through 06Z. sct-bkn035
likely 20-00Z today with a 10 percent chance of BKN025. IFR
conditions likely after 18Z with +RA...with a 40 percent chance of
occasional LIFR. Gusty southeast winds likely as well.

&&

.MARINE...21/1045 AM.

Very dangerous sea conditions are expected through at least Sunday.

Gusty west winds and choppy seas are likely to continue in the
Santa Monica Basin through this afternoon.

The storm system is still on track. High confidence for
widespread Gale Force south winds on Sunday...likely exceeding 40
KT. This will result in very steep seas on top of the large long
period west swell that is peaking today but will still be large
tomorrow. Breaking waves should continue over Morro Bay and
Ventura Harbor entrances...as well as near the shoreline and
breakwalls. Waves like these have a history of capsizing boats of
all sizes. While the swell will remain large likely through
Tuesday, it will be highly variable after today due to the changes
in winds and additional swell energy moving in. Low confidence
therefore on the precise timing of the swell heights, but
confidence is high in the general large swell picture.

High confidence in Small Craft Advisory (SCA) conditions to follow
the gales Sunday night through Monday night. Offshore flow is
expected Tuesday or Wednesday.

Moderate confidence in thunderstorms forming late Sunday night and
Monday...with a better chance than from Friday`s storm.

&&

.BEACHES...20/1100 AM.

A very large long period west to northwest swell will continue to
impact the region through at least Sunday...peaking today. The
current high surf warning for the Central Coast will not be
changed, as buoys nearshore are reporting 20 to 25 feet. The
Ventura County coast warning will also remain as 12-20 foot surf
is being reported. The warning for Los Angeles County may be
canceled soon as surf is large but not quite to warning
level...but an advisory would replace it. Those areas that have a
warning now will be replaced with an advisory after they end...and
advisory level surf looks likely through Tuesday night and
possibly into Friday. Coastal flood advisories will likely be
adjusted as well, as the threat tomorrow morning is highly
localized mainly to the areas under a high surf warning.

This is a dangerous situation for people near the surf zone.
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.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Warning in effect until 6 AM PST Sunday for zones
      34-35-40-41-87. (See LAXCFWLOX).
     Coastal Flood Advisory in effect until 6 AM PST Sunday for
      zones 34-35-39>41-87. (See LAXCFWLOX).
     Wind Advisory in effect from 2 AM to 2 PM PST Sunday for
      zones 34>38-51. (See LAXNPWLOX).
     High Surf Advisory in effect until 6 AM PST Sunday for zone
      39. (See LAXCFWLOX).
     Flash Flood Watch in effect from Sunday morning through
      Sunday afternoon for zones 39-40-44-45-52-53. (See LAXFFALOX).
     Flash Flood Watch in effect from Sunday afternoon through
      Sunday evening for zones 41-46-54-59-88-547. (See LAXFFALOX).
     High Wind Warning in effect from 4 AM to 7 PM PST Sunday for
      zones 52>54-59. (See LAXNPWLOX).
     Winter Storm Warning in effect from 7 PM Sunday to 6 PM PST
      Monday for zones 52>54. (See LAXWSWLOX).
     Wind Advisory in effect until 3 PM PST this afternoon for
      zone 59. (See LAXNPWLOX).
PZ...Small Craft Advisory in effect until 9 PM PST this evening
      for zones 645-670-673. (See LAXMWWLOX).
     Gale Warning in effect from 9 PM this evening to noon PST
      Sunday for zones 645-670-673. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PST Sunday for
      zones 650-676. (See LAXMWWLOX).
     Gale Watch in effect from late tonight through Sunday
      afternoon for zones 650-676. (See LAXMWWLOX).
     Small Craft Advisory in effect until 9 AM PST Sunday for zone
      655. (See LAXMWWLOX).
     Gale Watch in effect from Sunday morning through Sunday
      afternoon for zone 655. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(MON-FRI)
Lower snow levels Monday could bring snow down to the Grapevine.

&&

$$

PUBLIC...MW/Rorke
AVIATION...Kittell
MARINE...Kittell
BEACHES...Kittell
SYNOPSIS...Sirard

weather.gov/losangeles



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