Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 221254
AFDLOX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
454 AM PST Sun Jan 22 2017

...Aviation and marine discussions updated...

.SYNOPSIS...

A strong storm system will move into southwestern California today.
The storm will bring periods of heavy rain and strong southerly winds.
Lingering rain and mountain snow showers will prevail over the
region on Monday, along with a slight chance of thunderstorms.
High pressure will bring dry and milder conditions to the area
Wednesday through Saturday.

&&

.SHORT TERM...(TDY-TUE)

Cloudy skies prevail and light rain is falling across the area.
The warm south winds have not kicked in yet and the snow levels
remain fairly low and there is some snow falling on the north
slopes. The snow levels will rise quickly starting around dawn
and snow will not be much of a factor with todays storm.

Satellite shows a textbook powerful winter storm with a cold low
to the west of Oregon with an occluded front wrapped around it
and extending into Oregon. From the occlusion a cold and a warm
front split off. The cold front extends from Modoc county to the
Bay and then out to the SW over the east Pac. Behind the front
there is plenty of convective open cell cu with a few lightning
strikes.

The 06Z GFS and NAM runs remain in remarkably good agreement with each
other and are consistent with their previous runs. The forecast has
not been changed must at all. This will be the most significant storm
system for Southwestern California.

The heaviest rains associated with the main cold front will likely
affect SLO County between 5 AM and 1 PM, SBA County between 7 AM
and 3 PM, Ventura County between 9 AM and 5 PM and Los Angeles
County between 11 AM and 8 PM.

Just ahead of the front there will be a period of gusty se to s
winds across much of the region. Wind advisories have been expand
to include most all of the coastal and valley areas south of Point
Conception save for the San Gabriel. Gusty west winds behind the
front will likely affect coastal sections of LA/VTA Counties tonight.

The models do not show a great deal of instability with this
system. But there is a serious amount of jet energy and while the
chc of TSTMs is under 20 percent it is not zero either. There is a
better chc of TSTMs Monday.

Rain amounts will likely be 2-4 coast/valleys (highest near the
foothills), 3-6 foothills/mtns. Would not be surprised to see more
rain in the foothills above the city of Santa Barbara and the San
Gabriel Mtn Cstl Slopes. Peak hourly rainfall rates will be in
the 1-1.5" range (mainly south of Pt Conception due to the better
orographics)Rates this are capable of producing flash flooding
not only in the burn areas but across the entire forecast area
south of Pt Conception where a flash flood watch is in effect.

This is a special storm due to three factors. There is a 160-170kt
jet that will align itself parallel to the front which will slow
it down and intensify the from (rather than the usual
perpendicular 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. Finally, a 8mb LAX to BFL onshore will
develop and this will bring very strong south sfc winds to the
area which will greatly enhance the rainfall on the south facing
slopes.

Snow levels will rise quickly to 8000 feet with the warm
southerly flow. They will then fall just as quickly this evening
in the wake of the cold front. Available moisture drops off
significantly but there will still be enough moisture to combine
with the colder air to generate decent snowfall amounts across
the mountains. A winter storm warning will go into effect just as
the high wind warning expires early this evening. Snow levels
will fall to between 4000 and 5000 ft by late tonight and continue
near there through Monday. These low snow levels will bring some
snow to the Grapevine but there will not be that much
accumulations.

Broad cold cyclonic flow will dominate the area on Monday. It will
be a showery day and the entire area will see some rainfall. The
rainfall with this storm will be much less homogeneous. The
atmosphere will be very unstable and there will be a slight chc of
TSTMs. These TSTMs will be able to put out some good rainfall
rates and another flash flood watch just for the burn areas may be
needed. It will be  very cool day with highs mostly in the 50s at
lower elevations and 20s/30s in the mountains.

On Tuesday the upper low that is spinning off of the Oregon coast
will become 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 it looks like the best chc of rain will be
over the outer waters.

.LONG TERM...(WED-SAT)

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 due to the large sfc cold
pool. There will be some warming over the weekend as the ridge
increases and the offshore flow increases.

&&

.AVIATION...22/12Z.

At 07z at KLAX... there was no notable inversion.

Overall... Moderate confidence in the TAFs. The reduced confidence
is primarily due to the uncertainty in the timing of the storm.
MVFR conditions will continue to spread south and east with at
least MVFR conditions likely at all terminals by 19Z. IFR
conditions with a chance of LIFR conditions will develop around
the time of frontal passage. Gusty cross winds along with moderate
to strong wind shear and turbulence will spread south and east
through 02Z. There is a thirty percent chance of thunderstorms
after 08z.

KLAX... Moderate confidence in the TAFs. The reduced confidence
is primarily due to the uncertainty in the timing of the storm.
MVFR conditions will develop 13z-16Z with a seventy percent
chance of IFR conditions 20Z-01Z. East winds greater than 7 kts
will continue through 20z then shift to the southeast through 01z.
There is a forty percent chance of southerly cross winds greater
than 20 knots 22Z-02Z. There is a thirty percent chance of
thunderstorms after 08z.

KBUR... Moderate confidence in the TAFs. The reduced confidence
is primarily due to the uncertainty in the timing of the storm.
There is a seventy percent chance of IFR conditions 17Z-03Z.
There is a forty percent chance of southerly cross winds greater
than 20 knots 20Z-02Z. There is a thirty percent chance of
thunderstorms after 08z.

&&

.MARINE...22/300 AM.

Very dangerous sea conditions are expected today.

Breaking waves will continue over Morro Bay and Ventura Harbor
entrances as well as near the shoreline and breakwalls. There is
high confidence in a large swell through Tuesday but low
confidence on the timing of the swell heights.

High confidence for widespread gale force southerly winds and very
steep seas through midday for the northern waters and into the
evening for the southern waters. There is a thirty percent chance
that Gale Warning conditions across the southern waters may
continue into the overnight hours.

Moderate confidence in thunderstorms forming late tonight and
Monday.

&&

.BEACHES...22/300 AM.

A very large long period west to northwest swell will continue to
impact the region today. High surf warnings remain in effect
through 6 am this morning. Advisory level surf is likely through
Tuesday night and possibly into Friday. Coastal flood advisories
remain in effect through 9 am this morning.

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 and potentially sweep people
into the water. Large breaking waves will continue today at west
facing harbor entrances and in shallow water near the outer edge
of the surf zone.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Warning in effect until 9 AM PST this morning for
      zones 34-35-40-41-87. (See LAXCFWLOX).
     Coastal Flood Advisory in effect until 9 AM PST this morning
      for zones 34-35. (See LAXCFWLOX).
     Wind Advisory in effect until noon PST today for zones
      34>36-39. (See LAXNPWLOX).
     Wind Advisory in effect until 3 PM PST this afternoon for
      zones 37-38-40-41-44>46-51-87-88-547. (See LAXNPWLOX).
     High Surf Advisory in effect until 6 PM PST Tuesday for zone
      39. (See LAXCFWLOX).
     Flash Flood Watch in effect until 2 PM PST this afternoon for
      zones 39-52. (See LAXFFALOX).
     Flash Flood Watch in effect from 8 AM this morning to 4 PM
      PST this afternoon for zones 40-44-45-53. (See LAXFFALOX).
     Flash Flood Watch in effect from 10 AM PST this morning
      through this evening for zones 41-46-54-59-87-88-547. (See LAXFFALOX).
     High Wind Warning in effect until 7 PM PST this evening for
      zones 52>54-59. (See LAXNPWLOX).
     Winter Storm Warning in effect from 7 PM this evening to 6 PM
      PST Monday for zones 52>54. (See LAXWSWLOX).
PZ...Gale Warning in effect until noon PST today for zones
      645-670-673. (See LAXMWWLOX).
     Small Craft Advisory in effect until 6 AM PST early this
      morning for zones 650-655-676. (See LAXMWWLOX).
     Gale Warning in effect until 2 PM PST this afternoon for
      zones 650-655-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(TUE-SAT)
No significant hazards expected.

&&
$$

PUBLIC...Rorke
AVIATION...Kj
MARINE...Kj
BEACHES...Kj
SYNOPSIS...Smith

weather.gov/losangeles



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