Scientific Forecaster Discussion
Southwest California area forecast discussion
National Weather Service Los Angeles/Oxnard California
435 PM PDT Monday Mar 10 2014
..updated aviation discussion...
A weak upper trough will continue moving into the west and bring a
few high clouds and slightly cooler temperatures to the region. High
pressure will build from the west while an upper low approaches from
the northeast by Wednesday. This should bring Santa Ana winds to the
region by Tuesday and continue through Wednesday...which will raise
temperatures back above normal across coast and coastal valleys on
Short term (tue-thu)...
slightly cooler today but still above normal with an upper trough
sliding eastward into northern California...also pulling some high
level moisture across the area for some filtered sunshine in spots.
This trough will be the focus of our weather the next 48 hours or so
as it tightens up...closes off...and moves into the Desert
Southwest. For tonight...models in good agreement on the trough axis
running from Idaho southward into Nevada. Then by 12z Tuesday one
piece of the trough continues east into Colorado while another vorticity
lobe on the backside of the trough drops into Nevada. The associated
surface high pressure system behind the trough by this time is
nudging into Pacific northwest and helping to induce some offshore
surface pressure gradients by this time. The NAM forecasts slightly
stronger offshore winds than the GFS for Tuesday morning...but both
models at this time are in pretty good agreement. Tuesday
temperatures will be a little cooler than today's due to the
proximity of the upper trough to our northeast and lower heights.
Differences in the model solutions then appear for Tuesday night
into Wednesday. Not a lot of difference with the strength and
position of the surface high pressure over the northern Great Basin.
Its position results in lax to dag pressure gradients at 12z
Wednesday of -5.4 mb in the NAM and -3.4 in the GFS...a difference
that can also be attributed to resolution differences so in essence
very little difference in the pressure gradient between the two
models. The model discrepancy...and subsequent strength of this
offshore event...comes with the position and strength of what ends
up being a west-east oriented elongated upper low. Because the NAM
drops the upper low just southwest of Vegas by 18z Wednesday...thus
producing good northeasterly winds and thermal support at low to middle
levels over socal...this has the potential to be a High Wind
Warning-level event for the mountains and strong advisory and
moderate to strong Santa Ana event for the Santa Ana-prone foothills
and valleys/coasts. The NAM has been trending weaker with time...but
even the 18z run indicates a strong event. Meanwhile the GFS and
European model (ecmwf) forecast the same west-east elongation but further
north...resulting in weaker upper level support and weaker winds
down at the surface...more like an advisory level event. Models
agree on this being a quick shot with winds weakening Wednesday
afternoon. For now will issue a high wind watch for the vtu/la
mountains and the Santa monicas late Tuesday into Wednesday afternoon and
make the decision on mountain warning or mountain advisory tomorrow
as the event draws closer....either way advisories will be needed
for the coasts/valleys. Despite the recent rains...fuels are still
near critical so there will likely be a fire weather threat. See the
fire weather discussion below for additional details.
Temperatures will warm some on Wednesday with the offshore
flow...and cool a bit on Thursday as the offshore flow is weaker.
Long term (fri-mon)...
strong high pressure builds over California with long range models
centering a 587 dm high just off the socal coast Sunday morning.
Temperatures will be well above normal Saturday and Sunday...the
warmest day Sunday. Plentiful sunshine and even a chance at nearing
or even breaking some record highs. By Sunday expect highs at the
beaches in the 70s...80s coastal valleys...and some isolated 90
degree readings or above interior valleys. The ridge moves just to
our east on Monday for slightly cooler temperatures. It doesn't have
a big impact on monday's forecast...but for impacts beyond the
forecast period there are big differences between the GFS and the
European model (ecmwf) in the forecast upper level pattern. The GFS has a rather deep
closed upper low and associated deep surface low about 500 miles
west of sfo 18z Monday that eventually becomes a rain producer over
socal. Meanwhile the European model (ecmwf) puts the forecast area in weak to
moderate northwest flow at upper levels and shows a weaker closed
low further west than what the GFS forecasts. But...both models
struggling with this feature with very poor run to run consistency.
Overall...high confidence in 00z taf package. VFR conds are
anticipated for all sites through the period. There is a 30 percent
chance of some MVFR/IFR conds developing after 10z near the la
County coast....mainly klgb and a 20 percent chance for klax.
However the anticipated increasing offshore flow should limit that
Klax...high confidence in 00z taf. There is a 20% chance of MVFR/IFR
conds developing after 10z.
Kbur...high confidence in 00z taf. With developing offshore flow
there is the possibility of some low level wind shear/turbulence between 12z-17z.
Gusty offshore flow is expected Tuesday through Thursday...peaking
on Wednesday and focusing over Los Angeles and Ventura counties. Dry
air from the offshore flow this last weekend will linger through
Tuesday with minimum humidities mainly under 15 percent...which will
result in a number of hours of red flag conditions over much of the
wind prone areas of Los Angeles and Ventura County on Tuesday as the
As the winds increase further Tuesday night into Wednesday...cold
air from the low will bring increased humidities...especially in
the mountains...which will take time to lower through the day on
Wednesday...first near the coast Wednesday morning...then up through
the higher elevations in the afternoon. Despite the slightly higher
humidities...there is a potential for sustained winds above 40 miles per hour
and gusts above 60 miles per hour during the peak on Wednesday...which will
bring a potential for extreme fire behavior if a fire started. A
Fire Weather Watch will be in effect as a result.
Gusty northeast winds will continue Wednesday night through
Thursday...but will be significantly weaker on Wednesday. Humidities
however will lower some...and if Wednesday proves to be red flag
worthy...this period may need to be added as well.
Fire Weather Watch (see laxrfwlox).
High wind watch (see laxnpwlox).
Small Craft Advisory (see laxmwwlox).
High rip current risk (see laxsrflox).