Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 241822
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
1122 AM PDT Mon Oct 24 2016
Thunderstorms will flow across Los Angeles County and the adjacent
coastal waters through midday today. The weather will be dry
Tuesday and Wednesday with some warming by Wednesday. A stronger
storm system will bring widespread rain to the region Thursday and
Friday and unsettled weather will continue into the weekend.
Radar and satellite imagery this morning showing most of the
shower and thunderstorm activity over eastern LA County and areas
to the east. These areas have had as much as 0.50-1.00" of rain
since yesterday, with several hundredths of an inch as far west as
the San Fernando and Santa Clarita valleys. Most of the moisture
and instability will get pushed east of LA County by late morning
as a weak upper-level trough axis moves through the region. By the
afternoon there will just be a slight chance of showers for far
eastern LA County. Forecast for today calls for mostly cloudy
skies everywhere, but there should be at least some breaks of
sun. Temperatures will be several degrees below normal, with highs
in the 60s and 70s. Besides monitoring the radar this morning, the
main focus for today will be the Thu-Fri storm system.
***FROM PREVIOUS DISCUSSION***
The upper low off the Oregon coast this morning will lift
northward toward the coast of southwestern British Columbia
tonight and Tuesday. As this happens, some weak ridging will build
into the the forecast area from the south, and heights and
thicknesses will begin to rise. However, there will still be a
fair amount of low and mid level moisture, so expect skies to
remain partly cloudy. Max temps will probably rise a few degrees
on Tuesday, especially away from the coast.
The models are still consistent in showing a strong short wave
wrapping southwestward around the upper low off British Columbia
evolving into a strong upper low well to the west of the northern
CA coast late Tue and Tue night, with a deepening trough
extending southward through the eastern Pacific. To the east of
this, an upper ridge will amplify across Southern California.
Heights and thicknesses across the region will cont to rise, and
max temps should up several degrees in most areas, to above
normal levels for late October, probably into the mid 80s
in the warmer valley locations. It will be a dirty ridge, so
there will likely be at least some mid level clouds across the
region on Wed, along with a fair amount of sunshine.
Upper level energy dropping southward through the eastern Pacific
will cause an upper low to pinch off several hundreds of miles
west of the West Coast, with the center of the upper low as far
south as the latitude of northern Baja CA late Wed night. This
system will track eastward toward Srn CA on Thu, with abundant
moisture streaming into the region well ahead of the upper low.
Expect clouds to increase Wed night and Thu, with a chance of rain
developing during the day, most likely across SLO and SBA County.
However, there are enough timing, strength and track differences
between the models to dramatically decrease the confidence in the
The GFS is slower with the eastward progression, and if true,
southern and eastern sections of the forecast area would remain
dry on Thu, with well above normal temps continuing. The EC shows
rain overspreading much of the region during the day, possibly as
early as before daybreak on the Central Coast. The models have
also been flip-flopping with respect to the strength of the
frontal band and how well it will hold together as it moves south
of Point Conception. Both the EC and GFS show the upper low
opening up, lifting newd and weakening as it moves into Central
California late Thu night and Fri, but the EC shows the upper
system and the associated frontal band holding together much
better as it moves across the region. It brings significant
rain to the entire area, with heavy totals across SLO and SBA
Counties. The GFS shows decent rain across SLO and SBA Counties,
but much less in the way of rain farther south and east. With
so much uncertainty, have kept high chance pops in most areas,
although they will probably have to be bumped up later today, once
the WRF weighs in on the system.
There is still the potential for significant rain of an inch or
more across parts of the forecast area from this system, with
the threat of flash flooding and debris flows in recent burn
areas. This situation will be monitored very closely in the
Rain Thu night and Fri morning should turn to showers later Fri,
and it will be much cooler. The models show an unsettled weather
pattern continuing through the weekend, with broad and moist
cyclonic southwesterly flow aloft lingering. The GFS keeps a
slight chance of rain Fri night through the weekend, while the EC
shows some drying. Both models show another upper low approaching
the West Coast on Sunday, with an increasing chance of additional
rain for the area.
At 1626Z at KLAX...there was just a hint of a marine layer, but a
more apt description would be a deep moist layer.
The model data shows ample moisture lingering in the forecast area
for the next 24 hours. Skies will continue to range from partly
to mostly cloudy. There is moderate confidence in the forecast
with the primary difficulty in specifying the exact height of the
KLAX and KBUR...Moderate confidence in L.A. county coast and vly
TAFs with varying ceiling heights over the next 24 hours. The
coverage will also vary from sct to bkn.
There is a slight chance of thunderstorms across the southern
waters through the morning hours.
There is high confidence that winds and seas will remain below
SCA levels through at least Wed night.