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Southwest California area forecast discussion 
National Weather Service Los Angeles/Oxnard California
815 PM PST Thursday Feb 26 2015

Synopsis...

Friday looks to stay mostly dry and several degrees cooler. A cold
low pressure system will bring even cooler temperatures...rain
showers and mountain snow to our region over the weekend. A second
weaker system is expected Monday through Tuesday. The rest of next
week will be dry and warmer with light offshore breezes.

&&

Update...
overall...rather quiet evening across the district. Latest satellite
indicates mostly clear skies across the area as some cirrus drifts
overhead. Surface observation indicate some gusty northerly winds in the
mountains this evening...but nothing strong/widespread enough to
warrant any advisories at this time. With the gale force northwest
winds over the outer waters...a decent eddy should spin up
overnight. This should allow stratus to reform over the bight and
spread into the coastal areas of Ventura/Los Angeles counties and
even some of the coastal valleys. Additionally...expect stratus to
become rather widespread across the central coast overnight although
confidence is only moderate at best for this scenario as northwest
flow could limit stratus formation.

Overall...current forecast looks to be in decent shape. Will issue
an evening update to make some slight adjustments to cloud wording
and tweak some wind wording.

Quick look at initial 00z model guidance indicates that unsettled
weather is still anticipated across the area as a cold upper level
low impacts the area.

&&

Previous discussion...

Short term (today-sun)...a pretty quiet couple of days today and
Friday. Onshore trends will be on the increase tomorrow and we
should see slightly better marine layer coverage in the morning than
today, and even more Sat morning. Daytime highs expected to drop
about 5 degrees on average Friday and another 5 Saturday.

Another interesting little weather system headed our way this
weekend. The upshot is that the probability is high that all areas
will get at least some precipitation at some point over the weekend.
However, trying to nail down the when and how much has and will be a
significant challenge. The system will be coming down from the north
and as is usual with storms from this direction will be somewhat
moisture starved. The other challenge with this system is that at
least for the first part of the weekend we're going to be under the
influence of a pretty strong northwest flow pattern which creates a
drying, downsloping effect to areas south of pt Conception,
especially Ventura and southern sb counties. By Saturday night into
Sunday the system starts to pick up a little more over-water
trajectory and our northwest flow decreases. So that period should
present the best chance for showers, plus the air mass will be at
its coldest and most unstable by that time. Still though, this will
be a showery system and we'll likely see some placed getting
drenched while other areas get very little. Can't argue with
thunderstorm chances, though the most unstable air is across sba/slo
counties and adjacent waters Sat afternoon through Sunday.

Snow levels will be in the 4000-5000' range for most of this event
so lower elevations roads will likely see some accumulating snow,
including Interstate 5 over the Grapevine. Again, snow amounts will
vary with the randomness of the heavier showers, but the 8-16"
mentioned in the current Winter Storm Watch is within the realm of
possibility but not likely the norm as those higher numbers would
likely require a few good storms to develop over the same areas.
3-6" is a more likely areal average, but given the impact on lower
elevation driving and the outside chance of those higher numbers
hitting popular spots the watch seems warranted at this time.

Rain amounts similarly difficult but a tenth to half seems like a
good areal average (lowest in eastern sba County and coastal Ventura
county). And convection could locally boost totals well over an
inch. Burn area problems would likely require one of these
convective cells to move overhead and while the chances are low over
any one location they aren't zero.

Long term (mon-thu)...the trough axis should be just east of the area
Monday morning but there is enough lingering moisture and cold air
aloft to warrant at least some precipitation chances through the afternoon,
highest over the mountains a second and fast moving system will follow
quickly on the heels of the weekend storm Monday night into early
Tuesday but models aren't showing too much moisture with this one as
it's taking an even more inside track. This could change though and
can't argue with small rain chances through Tuesday. We should see
rapid clearing by Tuesday night as we shift to offshore flow with a
significant warming trend through the end of the week.

&&

Aviation...26/2310z...

At 2220z...the inversion at klax was based at 1400 feet. The top of
the inversion was 4500 feet with a temperature of 14 degrees
celsius.

Overall...moderate confidence in 00z taf package. Strong northwest
flow over the coastal waters is expected to spin up an eddy tonight
which will bring stratus/fog into klax/klgb/koxr late tonight. Also
will anticipate some stratus/fog developing at ksmx and kprb
overnight.

Klax...moderate confidence in 00z taf. High confidence in VFR conds
through this evening. Overnight...high confidence in return of MVFR
cigs/vsbys...but low confidence in timing (return could be +/- 2
hours of current 11z forecast). There is a 30% chance that MVFR ceilings
will remain through Friday afternoon.

Kbur...moderate confidence in 00z taf. High confidence in VFR conds
through this evening. Low confidence in MVFR visibilities 11z-18z. There is a
20% chance of IFR ceilings 11z-18z.

&&

Marine...26/815 PM...

For the outer waters...high confidence in the continuation of gale
force northwest winds through Friday night. The northwest winds will
diminish on Saturday and Sunday...but there is a chance of winds
remaining around Small Craft Advisory levels.

For the inner waters...other than some local gusts to 25 knots
across the western Santa Barbara Channel...winds are expected to
remain below Small Craft Advisory levels tonight. By Friday
afternoon and continuing through Saturday...moderate confidence
in west to northwest winds increasing to Small Craft Advisory levels
especially across western sections. Expect winds to increase Friday
afternoon across the Santa Barbara Channel...so will issue Small
Craft Advisory for this evening.

Showers are expected to develop this weekend over the coastal waters
as a low pressure system impacts the area. By Saturday afternoon
and continuing through Sunday...there is a slight chance of
thunderstorms which could produce gusty winds...choppy seas...and
possibly an isolated waterspout.

&&

Lox watches/warnings/advisories...
Winter Storm Watch (see laxwswlox).
Gale Warning (see laxmwwlox).
Small Craft Advisory (see laxmwwlox).
High rip current risk (see laxsrflox).

&&

$$

Public...mw
aviation...rat
marine...rat
synopsis...bartling

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