FXUS66 KLOX 201803

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1003 AM PST Tue Nov 20 2018

Updated Aviation Section

.SYNOPSIS...20/754 AM.

Dry and cool weather is expected through early Wednesday. Rain is
likely Wednesday afternoon through Wednesday nigh with a chance
of showers Thanksgiving morning. For Friday into early next week,
dry weather is expected, except for a slight chance of showers
across northern sections Friday and Friday night. Temperatures
will cool through Thursday then will gradually warm this weekend
into early next week.


.SHORT TERM (TDY-THU)...20/939 AM.

Generally clear skies across the area today ahead of our storm for
tomorrow. Air mass is a little cooler overall today but the added
sunshine should more than offset that and result in highs a couple
degrees warmer than yesterday in most areas. No other significant
issues for today. Looking at the morning model runs it looks like
we will probably see some light showers developing well out ahead
of the main frontal band tomorrow morning, including areas south
of Pt Conception so will likely be increasing pops during the
daylight hours prior to the main event Wed night and early Thu.
Earlier rain amount and rain rate forecasts look on track. Not
super impressed with instability, particularly southern areas.
LI's are ok, generally -1 to -3 across SLO/SB Counties but CAPE
values are minimal so at this point no plan to expand thunderstorm

***From previous discussion***

Weak ridging will develop briefly across the region tonight as an
upper low currently just west of northern Baja opens up and moves
eastward into Arizona. As the weak ridge moves east of the region
late tonight, southwesterly flow will develop aloft ahead of an
upper trough moving through the eastern Pacific. Upper level
moisture will increase across the region, so expect mid and high
level clouds to increase across the region overnight and early
Wed. There could be some low clouds in coastal areas as well, but
confidence in the low cloud pattern is low due to the abundant
high cloudiness.

A fairly sharp upper trough will approach the West Coast on Wed.
Models are in fairly good agreement with the overall upper
pattern, and have all trended a bit slower with the system. They
have also all trended toward bringing less in the way of rainfall
from the associated surface front, especially across Los Angeles

Skies should generally be mostly cloudy across the region on Wed,
although it could be partly cloudy to start the day across much of
Los Angeles County. There is a slight chance of rain north of
Point Conception in the morning. Models keep all other areas dry
in the morning, although the WRF hints at a weak warm front
extending into southern SBA and western VTU Counties. At this
point, do not expect much from this, and will leave pops confined
to areas north of Pt Conception in the morning. Rain will become
likely in the afternoon across SLO and northern SBA Counties, with
a chance of rain extending southward into Ventura County by late
in the afternoon. It appears that Los Angeles County will mostly
remain dry during the day Wed.

Models are in fairly good agreement in showing rain spreading into
Ventura and Los Angeles Counties Wed evening. Steady rain will
turn to showers across SLO and SBA Counties around midnight, but
not until early Thu morning across Los Angeles County.

Models continue to show fairly good precipitable water values of
about 1.1 inches, but all models have trended a bit farther north
with a very strong jet stream and the strongest and deepest lift
and moisture. This would favor the heaviest rain across SLO and
SBA Counties and the mountains of western Ventura County, with
somewhat lower totals across the remainder of Ventura County and
especially Los Angeles County.

There will also be an area of increased instability, across the
coastal waters north of Pt Conception Wed evening and across SLO
and SBA Counties Wed night. A slight chance of thunderstorms was
added to the forecast for these areas during those time periods.
A case could be made to extend the slight chance of thunderstorms
into Ventura County, but will let the day shift look at one more
model run for this.

There will also be a period of weak to moderate low level south
to southwesterly flow which could provide some orographic
enhancement of the rain on and below south and southwest facing

Models generally show rainfall totals of 0.5 to 0.75 inches
across SLO and SBA Counties, with local totals of 1 to 1.50
inches in the foothills and mountains, especially across
northwestern SLO County. Across Ventura and Los Angeles Counties,
models show rainfall totals of generally between 0.30 and 0.60
inches. Local totals of 0.75 inches to 1.00 inch are possible in
the foothills and mountains of VTU and L.A. Counties, especially
in western Ventura County and in the Santa Monica Mountains.

The rain will probably still be significant enough to cause
some issues in and around the recent burn areas, especially the
Woolsey and Hill burn areas, with mudslides and rock slides
possible. At this point, it appears that there is just a 10%
chance that rainfall intensities will meet USGS debris flow
thresholds across the Woolsey and Hill burn areas. Those
thresholds are 0.50 inches in one hour, 0.30 inches in 30 minutes,
and 0.20 inches in 15 minutes. Still, residents in and near the
burn areas should stay tuned to the latest forecasts, and may want
to be prepared with sandbags.

Any steady rain across eastern and southern L.A. County should
turn to showers early Thu morning. Moist broad cyclonic west
northwest flow across the region will keep a chance of showers
across the region through the morning, but skies should become
partly cloudy in most areas in the afternoon. Temperatures will be
several degrees below normal Wed and Thanksgiving Day.

.LONG TERM (FRI-MON)...20/422 AM.

Broad west to northwest flow pattern will set up across the
region Thu night through Fri night. The tail end of the frontal
system may bring a few showers to SLO County Fri, which could
spread into northern SBA County Fri night. If any rain falls, it
looks as though it will be very light. Elsewhere, expect partly
cloudy skies Thu night through Fri night. There should be a
couple of degrees of warming in most areas Fri.

A strong short wave trough will drop south southeastward from
southwestern Canada into the Rockies on Sat. The EC is now in
better agreement with the GFS with this system, whereas on
previous runs the EC showed it digging much farther to the west.
Max temps should rise a couple of degrees in most areas Sat,
generally to near normal levels. The GFS and EC show some
differences in the upper pattern Sun and Mon, with the EC showing
a ridge amplifying across the region thru the period. The GFS
shows ridging on Sun, then shows a very weak trough developing
just off the coast. Both models show decently high heights for
late November, along with some offshore flow. Expect max temps to
be above normal in most areas Sun/Mon with dry weather expected.



At 17Z at KLAX...A weak marine layer was 1400 ft deep. The
inversion top was at 2700 ft and 14 degrees Celsius.

High confidence in forecast through 03Z with minimal aviation
impacts. MVFR HZ could form anywhere due to mist and smoke. There
is a 30 percent chance that a shallow marine layer will bring
MVFR -SHRA will likely start to form as early as 10Z north of Los
Angeles County. Widespread RA expected Wednesday night into

KLAX...High confidence in TAF through 03Z. 30 percent chance of
IFR CIGS 03-12Z. 70 percent chance of several hours of east winds
8 kt or stronger sometime between 10-20Z Wednesday. -RA likely to
not begin until after 03Z Thursday, but there is a 10 percent
chance of a stray -SHRA as early as 15Z Wednesday

KBUR...High confidence in TAF through 03Z. 10 percent chance of
IFR CIGS 03-12Z. 10 percent chance of stray -shra after 15Z


.MARINE...20/905 AM.

High confidence in winds staying under Small Craft Advisory (SCA)
with relatively small seas through tonight. A storm system will
impact the area Wednesday into Thursday. Thunderstorms are
possible northwest of the Channel Islands. Gusty SE winds will
impact most waters, with a 70 percent chance of SCA conditions
north of Point Conception by Wednesday afternoon. Gusty west to
northwest winds will follow Thursday and Friday focused south and
east of Point Conception, including the Santa Barbara Channel and
Santa Monica Basin. SCA conditions are likely with building short
period seas.





No significant hazards expected.




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