Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Southwest California area forecast discussion 
National Weather Service Los Angeles/Oxnard California
1040 am PDT Friday Sep 19 2014

Synopsis...
weak upper level low pressure will bring below normal temperatures
to much of the region through the weekend. Temperatures will likely
warm to near normal early next week as the upper low moves east and
high pressure briefly returns. Another trough of low pressure may
bring a cooling trend for the second half of next week.

&&

Short term (rest of today-sun)...with the closed upper low
continuing to spin its way down the central coast...a weak onshore
gradient and a moderate to strong eddy...there's been quite a bit of
stratus coverage inland this morning with around a 3k feet marine
layer. Cooling trend also on track with many locations as much as 10
degree cooler than yesterday morning's readings at this time. So cooler
high temperatures forecast for today look good. Visible loop showing
stratus deck is trying to thin out so expect most areas to clear out
by late morning but possibly not by early afternoon. Will tweak the
forecast for less clouds this afternoon over all but the Ventura and
sba County mountains where there still appears to be at least a
slight chance of convection this afternoon. Latest model convective
parameters show some instability with lifted indicies around -3.0
centered over the vtu County mountains north into sba mountains...a
pool of low to middle level moisture indicated by a broad area of 850
dewpoints between 10 and 12c...and finally some dynamic help from a
favorable position of the upper low. What does form should be moving
fast enough to limit any flash flooding. Overall will leave thunderstorm
chances in the forecast for this afternoon over the areas of vtu and
sba County mountains. Convective indicators hint at possible
activity over the la County mountains but will keep chances out for
now and monitor through the afternoon.

Will likely go ahead and add thunderstorm chances for Saturday afternoon in
this afternoon's update. Moisture appears to be a little more
limited...but models forecasting the upper low to eject out over the
area tomorrow...so moisture deficiency should be made up for by
better dynamics. At first glance believe better chance will again be
over vtu and sba County mountains due to that area's relatively
better moisture availability...but dynamics appear better south over
la County. Will take a closer look this afternoon. The morning's
marine layer stratus will be a tricky forecast so persistence looks
to be the best option.

***From previous discussion***

Latest fog product imagery indicated plenty of low level moisture
lingering across coast and valleys north of Point Conception...while low
clouds were more random in nature for areas south of Point
Conception. Low clouds should fill in across coast and valleys of
Los Angeles...Ventura and Santa Barbara counties by dawn. There will
be enough low level lift to cause some patchy drizzle to occur just
about anywhere west of the mountains this morning. Latest acars
sounding near lax showed a very weak inversion in place which makes
sense why clouds are not more uniform S of Point Conception this
morning. Wind patterns are interesting this morning. Over the
coastal areas into la County the flow is mostly cyclonic northwest becoming
SW over la County. However...northerly winds were occurring over the
mountains and San Joaquin Valley. This northerly flow was causing
additional clouds to pile up along the northern slopes of the local
mountains this morning including the I-5 corridor.

Synoptically...a closed 578 dm upper low currently off the central
coast will drift slowly south just west of Point Conception by this
afternoon. The proximity of the upper low will allow favorable cool
cyclonic flow over most of Los Angeles...Ventura and Santa Barbara
counties today. There is not much in the way of moisture associated
with the upper low...but there will be plenty low clouds lingering
across the coast and valleys through this morning and into the
afternoon across most coastal areas. Rap and NAM-WRF models
indicated that there would be enough instability and limited middle
level moisture to warrant a slight chance for showers or
thunderstorms to develop over the Ventura County mountains as well
as portions of the Santa Barbara County mountains this afternoon and
evening hours. Temperatures will continue to trend cooler through
Saturday as the upper low moves directly over sba/Ventura
counties Saturday afternoon. There is not much diffluence associated
with the upper low for Saturday and models were not picking up on
any additional moisture or convective parameters across the local
mountains. Will let later shifts look closer to see if slight chance
probability of precipitation will be needed for Sat in the local mountains once again.Expect high
temperatures to be near or just below normal today and Saturday.

By Sunday...the upper low will fill in and move NE into Nevada
allowing heights and thickness levels to rise. There will be night
through morning low clouds but a quicker burnoff and high temperatures will
be a few degrees higher...especially across interior
locations...with warmest valley locations in the middle 80s...and middle
to upper 70s along the coast.



Long term (mon-thu)...both GFS and European model (ecmwf) stay in pretty good
agreement over the long term period. A weak ridge will begin to
build over Baja California California while much of northern California remain
under a broad upper trough through Wednesday. Expect night through morning
low clouds for coastal areas and possibly some coastal valleys
through Wednesday morning. Temperatures will continue to rebound Monday and
Tuesday with highs reaching the lower 90s across warmest valley
locations. For Wednesday and Thursday...a broad upper trough will begin to dig
farther south. The GFS is showing a sharper trough which would mean
more significant cooling by Thursday...where the European model is
less pronounced. At this point have trended high temperatures down slightly
for Wednesday...with a few more degrees of cooling for Thursday to
slightly below normal for this time of year.



&&

Aviation...19/1800z.

Middle/upper level low pressure system centered southwest of Point
Conception will shift northeast. Upper level strong southwest winds
will become moderate southwest by 21/00z while middle level light
southeast winds become moderate southeast after 20/08z over the
area. Isolated thunderstorms between 19/23-20/03z over Ventura and
Santa Barbara County mountains with tops 45kft and storms will move
north approximately 15kt. Low level cloud tops were approximately
3kft along the central coast to 3kft over the bight. The capping
inversion was weak and will likely differ little Saturday morning
from this morning. The cloud field was disorganized...broken over
the bight and scattered on the central coast and will likely
continue disorganized Saturday morning. Weak onshore pressure
gradient becoming moderate onshore after 20/19z.

Klax...likely scattered 035 by 19/20z. Chance ceilings 015 between
20/09-20/17z and ceilings 027 between 20/17-20/19z.

Kbur...likely scattered 025 by 19/20z. Chance ceilings 017 by 20/10z.

Virtually certain - 95-100%
very likely - 80-95%
likely - 60-80%
chance - 30-60%
very unlikely - 20% or less

&&

Marine...19/900 am.
There is a chance Small Craft Advisory conditions in the vicinity of
the northern Channel Islands will develop this afternoon and end
late tonight. There is a chance Small Craft Advisory conditions will
redevelop each evening and exist several hours each day
Sunday through Tuesday. There is a chance the northwest winds will
increase to gales Thursday afternoon in the vicinity of the northern
islands to San Nicolas Island. A mixed west northwest swell and
south southwest swell will continue to subside and linger through
Saturday. Swells originating from a fast moving and short lived Gulf
of Alaska storm on Thursday are expected to arrive Sunday from
295-305. There is a chance the swell will peak Sunday night and
linger through Tuesday. A low pressure system is likely to develop
over the eastern Pacific next week Monday through Tuesday and
guidance had trended deeper with the system. Swells generated by
storm force winds are expected to arrive from 300-310 degrees
Wednesday evening and peak Thursday and linger through Friday.

&&

Lox watches/warnings/advisories...
Small Craft Advisory (see laxmwwlox).

&&

$$

Public...Jackson/Kaplan
aviation/marine...30
synopsis...sukup

Www.Weather.Gov/losangeles

National Weather Service Glossary of Abbreviations