Scientific Forecaster Discussion
Southwest California area forecast discussion
National Weather Service Los Angeles/Oxnard California
325 PM PST Wednesday Dec 4 2013
..updated aviation discussion...
A cold upper low will maintain well below normal temperatures with
hard freezes overnight across inland areas through Friday. Another
cold weather system will move in by Friday night and bring rain and
low Elevation Mountain snow into Saturday. High pressure will move
in next week and warm temperatures to near or slightly above normal
by middle week.
Short term (today-sat)...clear and cool through Friday with record
overnight lows in some places. Probability of precipitation for the next system Friday night
and Saturday have been raised across the board as models continue to
indicate a shallow but high water content moisture field associated
with a fast moving cold front. Low level winds never really get much
of a southerly component to them so orographic enhancement will be
confined to the central coast and eastern la County. The front
itself is pretty dynamic with an decent thermal boundary and if it
wasn't moving so fast precipitation amounts would be more impressive
despite the rather nominal precipitable waters (only .5-.6"). Instead we're
looking at a fairly low end rainfall system but possibly a decent
snow producer. Given the low snow levels to start out and even lower
with and behind the front there is a rather deep layer of moisture
in the ideal snow growth snow around -10c. So we should see much
better than usual snow/water ratios with this event and may need to
consider a Winter Storm Watch in the next day or two if the models
don't reverse course and take the system further east. At the very
least it does look like a solid advisory event especially given the
impacts to Interstate 5.
Precipitation expected to start up along the central coast Friday night and
taper off early Saturday morning. Further south the timing will be
about 6 hours later, so starting around or perhaps just before
sunrise Saturday morning and tapering off by around noon. A 150 knots
northwest jet is motoring this thing through so we should see fairly
rapid clearing behind the front with the exception of the mountains
where upslope flow there will maintain clouds and a threat of
lingering precipitation and low snow levels through Saturday night.
Long term (sun-wed)...another round of hard freezes expected behind
the front Sunday and Monday mornings, though since interior areas
will have already experienced at least two hard freezes this week
the only areas we likely will need warnings or advisories for will
be the coast and coastal valleys. Gradients will 2-3mb offshore
lax-dag during that time so there will be some northeast breezes
that may moderate temperatures a bit in the more exposed locations across
Ventura and la counties. Daytime highs will warm slowly through the
period, but probably not getting back to normal levels until Tuesday
or Wednesday. Dry weather through the period with no other
significant weather concerns expected the rest of next week.
Overall...high confidence in 00z taf package. VFR conds expected all
sites through the period due to cold and dry air mass. No significant
Klax...high confidence in 00z taf.
Kbur...high confidence in 00z taf.
freeze warning (see laxnpwlox).
Frost advisory (see laxnpwlox).
Small Craft Advisory (see laxmwwlox).
Marine weather statement (see laxmwslox).