Scientific Forecaster Discussion
Southwest California area forecast discussion
National Weather Service Los Angeles/Oxnard California
920 am PDT Monday Jul 14 2014
Weakening high pressure and an increase in clouds will bring a
cooling trend through Tuesday. Monsoonal moisture will bring threat
of showers and thunderstorms to the region through
midweek...especially for the mountains and desert.
Short term (today-wed)...a little subtropical surge going on today
with precipitable waters around or in excess of 1.5". Also have a couple little
embedded vorts that bear watching but given the thick middle/high level
cloud shield it's unlikely we'll get much, if any significant
convective development. Best instability is over the eastern san
Gabriels 21z-00z as usual, but vorts are west at that time and that
area is actually under negative vorticity advection so the timing is not good. Plus we're not
expecting much in the way of clearing today so it's just not a good
setup unless much more clearing takes place than expected. Will
leave the forecast as is but for coastal zones probably the most
we'll see is a few big drop sprinkles, mainly in the morning. The
air mass is certainly juicy enough such that if convection does
develop there would be a threat of significant rain, but steering
flow generally in excess of 15kt so storms not expected to linger
too long. For the vast majority of our population mainly just warm
and muggy today with a few sprinkles at times. Highs will likely
take a hit from yesterday's levels, especially in the valleys due to
the thick clouds but compensated by the higher humidities.
***From previous discussion***
The thunderstorm activity should continue into the evening but will shut off
around midnight. All of the monsoon moisture and east flow should
prevent the marine layer stratus from forming. Some overnight lows
will be rather warm and will feel warmer with the increased humidity.
Tuesday was looking pretty convective with sundays model runs but now
models are turning the flow in a more southerly and drier direction.
This is not good for thunderstorm development and decreased thunderstorm chance and
coverage to reflect this. The steering flow is stronger as well and
this will reduce the chance of flash flooding. Lower heights and cooler bl
temperatures will trigger a several degree cool down.
Skies will clear Tuesday night and will along enough radiational
cooling to form a decent marine inversion which in turn will allow
for some stratus to form. It will likely be pretty shallow and will
only cover the beach areas and a few miles inland.
The stratus should only last until middle morning. Skies will be mostly
sunny. The monsoon flow will be marginal at best and while all the
sunshine will nicely destabilize the higher elevations of the mountains
there will not be that much moisture to work with and there is only
a slight chance of thunderstorms near the higher peaks.
Long term (thu-sun)...
the upper high dissolves on Thursday and is replace by an enormous
ridge which will cover the western third of the nations. The flow
will be from the south-southwest to the SW and will be dry so the convective
threat will come to an end. The marine inversion will strengthen and
this will lead to an increase in stratus coverage for the coasts
(the heights will be high enough to keep the marine layer smooshed
enough to keep it out of the vlys). Maximum temperatures will slowly warm each
day and by Sunday will be slightly above normal everywhere except
the coast which will be right near normal.
moderate confidence in 12z tafs... with the reduced confidence
due to uncertainties with the marine layer... the impact of the
incoming middle level clouds on the marine layer... and the timing and
extent of shower and thunderstorm development. The marine layer
will be patchy and weak and ceilings are most possible at coastal taf
sites when the skies are clear above and then lessening when middle and
high level clouds move overhead. Showers and thunderstorms are
possible in Los Angeles County by late morning but again the chance
of strong development increases with clearer skies and lessens if
too much cloudiness precedes the instability and moisture moving in
from the east. All taf sites have at least a slight chance of
showers and/or thunderstorms in the afternoon through midnight then
the possibility lingers in the Ventura/Los Angeles County mountains
and the Antelope Valley into the early morning.
moderate confidence in 12z taf... with the reduced confidence due to
uncertainties with the marine layer... the impact of the incoming
middle level clouds on the marine layer... and the timing and extent of
shower and thunderstorm development. A patchy and weak marine layer
with occasional MVFR ceilings through 15z. A slight chance of showers
and thunderstorms by late morning and continuing through midnight.
Otherwise VFR conditions.
moderate confidence in 12z taf. A slight chance of showers and
thunderstorms by late morning and continuing through midnight. VFR
conditions are expected throughout the period.
subtropical moisture is moving across the area this
morning...bringing light to locally moderate southeast winds and a slight
chance of showers and thunderstorms to southern sections of the
coastal waters. The thunderstorm risk will slowly spread northward
across the waters today into this evening. Otherwise...seas will
diminish steadily through the week. Mainly light and variable winds
are expected across the waters through Wednesday. Thursday and
Friday west winds 10 to 20 knots will affect the outer
waters...otherwise light winds and small seas will continue.
Patchy dense fog will be possible at times Tuesday and Wednesday
during the overnight to morning hours.
Small Craft Advisory (see laxmwwlox).
Special Weather Statement (see laxspslox).