Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 03/19/17
AREA FORECAST DISCUSSION
NWS LOS ANGELES/OXNARD CA
835 PM PDT Sat Mar 18 2017
A prolonged cooling trend will begin this weekend as a trough of
low pressure builds into the West Coast. This pattern will
support low clouds and patchy fog gradually expanding into the
valleys. This persistent trough will bring rain and mountain snow
at times with continued cool conditions next week.
Latest fog product imagery indicated low clouds surging in across
the Central Coast this evening as well as the Santa Barbara
Channel filling in almost to the SBA South Coast. Low clouds were
also approaching LA/VTU coastal areas and should fill in over the
next few hours. High clouds have and will continue to move across
the region through Sunday as well. High temps average 3-6 degrees
cooler in most locations with a few coastal areas similar to
today where low clouds were persistent most of the day.
Synoptically, a broad west coast trough will persist through much
of next week with a few disturbances in the flow bringing periods
of rain and snow in the long term portion of the forecast (Tue-
Fri.) SW flow aloft with moderate onshore flow will continue to
cause high temperatures to trend downward with best cooling
likely for areas N of Point Conception with less in the way of
cooling for areas to the south for Sunday. Highs will be in the
lower to mid 60s along the coast with highs ranging in the 70s
away from the coast to around 80 in the warmest valley locations.
For tonight, low clouds will fill in across most all coastal areas
with some stratus sneaking into the coastal valleys. However,
latest AMDAR soundings were showing the marine layer shrinking to
around 700 ft from the 1300 ft there was earlier this morning.
Cross Sections from the NAM WRF were not indicating much in the
way of low clouds deepening overnight. There is not much of an
eddy this evening which is concerning if the marine layer stratus
is going to deepen since thickness and heights will remain the
same. For now have left patchy low clouds in the forecast for the
San Gabriel, San Fernando as well as the Ventura County Valleys,
but would not be surprised if there was less stratus in the
valleys compared to early this morning. Overall there will be
little change in the overall pattern through tomorrow with high
For Sunday night into Monday, low clouds are expected to deepen. A
weak vort lobe will move over the region which could cause some
drizzle or light rain to develop along the Coast and adjacent
foothills and possibly across coastal areas of LA/VTU counties as
well. Clouds will thicken up with a chance of light rain
developing across NW SLO county by Monday afternoon spreading
across much of the Central Coast and western portions of SLO/SBA
***From Previous Discussion***
A cold front will move into SLO/SBA County Mon night, with rain
spreading into much of SLO/SBA Counties and a chance of rain
developing over VTU/L.A. Counties. The cold front will dissipate on
Tue, while a second cold front moves in from the e Pac, bringing
mainly light rain to much of the forecast area. There will also be
breezy s to sw winds across the area, especially over higher terrain
and in the Antelope Vly.
Rainfall amounts Mon thru Tue should range from about a tenth to one
quarter of an inch for many areas, with local higher amounts in the
SLO County mtns. However, one quarter to one half inch or slightly
more will be possible for the SLO County Central Coast, particularly
in the far nwrn SLO County foothills. Snow levels will be quite high
through Tue, generally above 8000 ft, so no snow impacts are
expected in the mountains.
Temps will be on a continued cooling trend Sun thru Tue. Highs
should be down slightly on Sun, and down several degrees on Mon to
slightly below normal for the coast and vlys and slightly above
normal for the mtns and Antelope Vly. Then for Tue, highs will drop
to about 3 to 8 deg below normal across the region. Highs in the
warmest vlys will be in the mid 70s to near 80 on Sun, upper 60s to
mid 70s on Mon, and mid to upper 60s on Tue.
EC and GFS are in generally good agreement with regard to synoptic
scale features during the extended period. A sharper upper trof and
another surface cold front will impact the area on Wed with showers,
and even a very slim chance of thunderstorms, less than 15 percent,
so is not mentioned in the forecast. A chance of snow will be
possible as well in the mountains, generally above 5500 to 6000 ft,
with a few inches of accumulation not out of the question.
There should be some residual moisture and lingering showers over
the area into Thu, with mostly dry conditions expected Thu night
into Fri morning as a weak upper ridge moves over the area. A large
upper trof an surface cold front will approach from the e Pac Fri
afternoon with rain moving into nwrn SLo County. The front will move
into SLO/SBA Counties Fri night then across the rest of the forecast
area on Sat. There is the potential for significant rain, mountain
snow and gusty winds with this weather system.
Temps across the region will remain several degrees below normal
thru the extended period. Highs in the warmest vlys will be generally
in the mid to upper 60s each day, except upper 60s to low 70s on
At 00Z, the marine layer depth was 700 feet at KLAX. The top of
the inversion was around 1600 ft with a temperature of 21 degrees
Overall low to moderate confidence for coast and valley tafs in
respect to timing. higher confidence with vis categories IFR to
low MVFR through Sunday.
On the Central Coast, moderate confidence for LIFR cigs +/1 hour
from current 00z TAFs. 30% chance for IFR cigs early, then once
again by Sunday morning after 15z. There is a 30% chance that IFR
conditions will continue through the afternoon.
South of Point Conception, stratus is expected to remain between
IFR to MVFR categories for coastal areas...20% chance for brief
LIFR this evening. 30% chance for cigs to continue through 21z.
KLAX...Moderate confidence in 00Z taf. IFR to low MVFR conditions
could return as soon as 04Z or may be delayed by up to 2 hrs. IFR
conditions expected overnight...rising to MVFR after 12z but
timing could differ by +/- 2 hrs from taf time.
KBUR...High confidence in 00Z taf with VFR conds. Then lower
confidence due to stratus. Cigs should be IFR, but could quickly
go to low MVFR category by 13z. 40% that LIFR cigs could develop
from 07z to 10z. Timing may differ by +/-2 hrs from taf times.
.MARINE...18/800 PM. Patchy fog is expected over much of the waters tonight
into Sunday morning but no dense fog issues are expected.
High confidence in no significant winds or seas through
the weekend. Winds and seas will build Monday into Tuesday as a
storm system approaches the region and SCA for gusty winds and
short-period seas will likely be needed during this time. Another
storm system will approach on Thursday with stronger SCA
.HAZARD POTENTIAL OUTLOOK...(TUE-SAT)
Rain can be expected across the area on Tue, with showers and
mountain snow showers into Wed. Slick roads will impact much of
the area. A vigorous cold front will be possible Fri night and
Sat, with significant rain, mountain snow, and gusty winds
possible. Possible impacts include flash flooding and debris flows
near recent burn areas, urban and small stream flooding, and
wintry driving conditions in the higher mountains.