Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 171810
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1010 AM PST Sat Feb 17 2018

...Aviation discussion updated...

.SYNOPSIS...17/810 AM.

Above normal temperatures are expected today. An upper level
trough will bring a sharp cooling trend to the area Sunday through
Monday, with areas of gusty winds, night through morning low
clouds, and some light measurable rain and mountain snow across
Los Angeles County and northern Ventura County. Monday night and
Tuesday night will be very cold. Cool and mostly dry weather is
expected Tuesday through Friday.

&&

.SHORT TERM (TDY-MON)...17/939 AM.

Skies were sunny across the forecast area this morning and will
continue thru the afternoon. Gradients were weaker offshore this
morning (bottoming out at -2.7 mb LAX-DAG at 15Z) and there have been
some offshore flow noted over the area, but rather weak with highest
gusts near 25 mph for wind-prone areas such as the hills of
southeastern VTU County and in the L.A. County mtns. Gradients will
trend quickly onshore thru the afternoon, with an earlier return to
the seabreeze along the coast. Temps today will be several degrees
above normal across the forecast area, with highs in the warmest
vlys and inland coastal areas expected to reach into the mid 70s to
around 80.

Weak upper level ridging will persist over the forecast area through
early this evening. A large upper level trough that will move into
the Pac NW late today and tonight is expected to sag S into nrn CA
for Sun, with lowering 500 mb heights and a broad NW flow aloft
developing over srn CA.

Onshore flow will increase tonight into Sun, and a return to marine
layer clouds is expected along many coastal areas into some of the
adjacent vlys. Some of these clouds may persist into Sun afternoon
as well especially along the coast. Otherwise, mostly clear skies
can be expected tonight with sunny to partly cloudy skies on Sun.

Gradients will be strongly onshore Sun afternoon, with the NAM
forecasting LAX-DAG to be +7.5 mb at 00Z. Gusty NW winds near
advisory levels will affect the Central Coast Sun afternoon, with
gusty SW winds possibly to advisory levels expected over the L.A.
County mtns and deserts. Temps are forecast to cool substantially
for Sun back to near normal to slightly below normal, with highs in
the warmest vlys and inland coastal areas in the mid 60s to near 70.

The large mainly inside slider upper trof will push into CA Sun
night and Mon. The 00Z EC and 12Z NAM have a slightly more over-
water trajectory with this trof and a wetter solution, while the 12Z
GFS continues to have the more inland trajectory and drier. Thinking
here is a slight edge toward the NAM/EC with slightly more precip
potential Sun night and Mon, but precip amounts will still be rather
light. An eddy will try to develop Sun evening over the near shore
waters of the SoCal bight, but it is looking more like a surface
cold front will move into the area and disrupt this development with
increasing NW flow over the coastal waters thru the night. There
will still be a SW lower level flow into srn VTU County and L.A.
County Sun evening before the flow turns NW later Sun night.

Deep marine layer clouds should move into mainly L.A. County Sun
evening and linger into early Mon morning along with a slight chance
to chance of rain from the coast to the coastal slopes. Snow levels
will drop quickly by Mon morning down to 3500 to 4500 ft, with some
snow possible in the San Gabriel mtns. The snow level will drop even
more Mon afternoon, down to 2500 to 3000 ft, and possibly to as low
as 1500 to 2000 ft over SLO County (950 mb temps there down to 1 to
2 deg C). Upslope flow will bring clouds and a chance of snow
showers for the N mtn slopes and possibly the eastern Cuyama Vly
foothills on Mon. There is the outside chance snow showers could
affect travel on Interstate 5 in NW L.A. County and over the
Grapevine late Sun night and Mon with an inch or two of snow. The
latest model runs were also showing some precip moving into the
Central Coast on Mon, and with extremely low snow levels, there is
the potential of light snowfall in the hills along the Central
Coast. Will take a closer look at this for the afternoon zone
package.

Strong and gusty NW to N winds are expected Sun evening for the
Santa Ynez Mtns and S SBA County coast, with strong and gusty NW
winds for the mtns and deserts. Gusty NW to N winds will affect much
of the forecast area on Mon. Wind Advisories will likely be needed
for Sun evening in some areas including the SBA County S coast and
adjacent mtns, and the L.A. County mtns along the I-5 Corridor. Wind
Advisories will also possible on Mon especially for the Central
Coast, foothills, mtns and deserts.

Skies will be partly to mostly cloudy across much of the region on
Mon. Temps will turn much cooler and several degrees below normal
with highs in the warmest vlys and inland coastal areas only in the
upper 50s to lower 60s.

***FROM PREVIOUS DISCUSSION***

.LONG TERM (TUE-FRI)...16/254 PM.

The large upper trof will move slowly E Mon night and Tue. Another
upper level trof should move into the area Tue night and Wed, with
the EC a bit stronger with this trof compared with the GFS. The EC
drops another, wetter upper trof into the region Thu and Thu night
while the GFS keeps this trof to the N and E. Broad upper troffiness
should persist in Fri as well, with the GFS a bit more bullish with
this system. With all the model timing and speed differences, went
generally with a model blend and low confidence in the day to day
fcst.

Kept the wx dry over the forecast area during the period, altho do
have ghost pops and some clouds on Thu due to the outside chance the
EC may have the better solution. Otherwise, mostly clear skies
should prevail over the forecast area Tue thru Wed, with partly
cloudy skies Thu and Fri. Very cold overnight lows are expected Mon
night and Tue night. Frost Advisories and Freeze Warnings will
likely be needed in the coldest wind-sheltered parts of the coast
and vlys, with the Ojai Valley expected to drop into the mid 20s Mon
night. Daytime highs will be several degrees below normal for the
most part on Tue then slowly warm to near normal in many areas by
Thu.

&&

.AVIATION...17/1807Z.

At 17z at KLAX... There was a surface-based inversion. The top of
the inversion was around 1400 feet with a temperature of about 19
degrees Celsius.

Overall... High confidence in the current TAFs through this
evening then low confidence in coastal and adjacent valley TAFs
due to uncertainty in the timing and intensity of the return of
low clouds and fog. Widespread VLIFR/LIFR conditions along the
coast and LIFR/IFR in adjacent valleys. Good chance of the low
clouds and fog lingering into late morning or later due to
increasing onshore winds. Elsewhere VFR conditions will prevail.

KLAX... High confidence in VFR conditions through this evening
then low confidence in the TAF due to uncertainty in the timing
and intensity of the return of low clouds and fog. There is an
eighty percent chance of VLIFR/LIFR conditions and if formed they
may linger into late morning or later due to increasing onshore
winds. East winds greater than seven knots are not expected
through the forecast period.

KBUR... High confidence in VFR conditions through this evening
then low confidence in the TAF due to uncertainty in the timing
and intensity of the return of low clouds and fog. There is a
sixty five percent chance of LIFR/IFR conditions and if formed
they may linger into late morning or later due to increasing
onshore winds.

&&

.MARINE...17/913 AM.

For the Outer Waters... Small Craft Advisory (SCA) level
northwest winds are likely to develop on Sunday and continue
through Tuesday. There is a twenty percent chance of Gale force
winds on Sunday and a forty percent chance on Monday.

For the Inner Waters... For the waters north of Point Sal, SCA
level northwest winds are likely each afternoon and evening on
Sunday and Monday with a twenty percent chance of Gale force
winds on Monday. For the waters south of Point Conception, SCA
level west to northwest winds are possible Sunday and likely
Monday with the strongest winds across western sections.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect from 9 AM Sunday to 3 AM PST
      Tuesday for zones 645-676. (See LAXMWWLOX).
     Small Craft Advisory in effect from 3 AM Sunday to 3 AM PST
      Tuesday for zones 670-673. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (MON-FRI).

Gusty and potentially damaging northwest winds are possible
Monday through early Tuesday. It will be very cold during the
late night through mid morning hours Monday night through
Wednesday, especially where the winds diminish. Frost or freezing
temperatures may occur in the valleys, in coastal areas of Santa
Barbara and San Luis Obispo Counties, and possibly across interior
coastal sections of Ventura County.

&&
$$

PUBLIC...Sirard
AVIATION...Kj
MARINE...Kj
SYNOPSIS...DB/B

weather.gov/losangeles



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