Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Southwest California area forecast discussion 
National Weather Service Los Angeles/Oxnard California
900 am PST Monday Jan 26 2015

Synopsis...
a couple of low pressure systems will impact the area this week. The
first will likely bring showers through early Tuesday. The next
could bring a small chance of rain Thursday or Friday. Temperatures
will remain next normal through the week.

&&

Short term (today-wed)...
the low pressure system that will impact US through Tuesday remains
450 miles southwest of Los Angeles. Southeast flow has settled into
the area...bringing a warm airmass up from the south. While maximum
temperates will be much lower than yesterday...they will be a few
degrees warmer than previously expected due to the localized
offshore flow and warm start to the morning. Updated forecast
accordingly. While no measurable rain is expected this
morning...added a slight chance to Los Angeles counties with a few
reports of light big drop rain. Rain chances for late this afternoon
and night look unchanged from previous forecast. Wish we could be
more definitive with the probability of precipitation...but uncertainty remains as to the
coverage of measurable rain. This is due to the flow looking more
easterly than southeasterly which would produce a drying effect at
lower levels. Regardless...any rainfall still looks light. The
moisture also looks to exit the area a little quicker...so Tuesday
morning probability of precipitation may need to be trimmed back with the afternoon package.

***From previous discussion***

563 dm upper low 400 miles South West of l.A. This low will slowly
make its way to the north-northeast and will be over pt Conception around dawn
Tuesday. As this low moves towards the region it will entrain a
large mass of moisture over southeast California and Arizona and advect this moisture
into Southern California. This flow trajectory will actually favor rain chances
for the l.A. Mountains and the Antelope Valley. This morning will be mostly
cloudy and there will be a big drop sprinkle or two here and there
as some precipitation falls from the middle level decks but nothing of note or
over a hundredth of an inch. There will be enough moisture over the
area mixed with a little positive vorticity advection to bring a chance of light rain showers to
most of l.A. County and a slight chance to vta County. The best chance of
rain will be this evening. The upper low will be 100 miles southwest
of pt Conception maximum moisture will be over the area and a vorticity
maximum will rotate over the forecast area from the south to the north.
Rain will be likely for l.A. County and most of the mountains due to the
southeast to northwest movement of the vorticity maximum there will only be chance of rain
over the waters and the valleys and coastal sections of vta County.
There will only be a slight chance of rain over the central coast. The
chance of precipitation will diminish after midnight as the vorticity lobe moves
off. By Tuesday morning the chance of rain will be confined to areas
north of pt Conception...all of the mountains and the Antelope Valley. By
Tuesday afternoon there will only be a slight chance of showers near
the Kern County border. Snow levels will be rather high...around
7000 feet. The ski resorts could see an inch or two of new snow.

Rainfall amounts will likely be the highest in the Antelope Valley
and the north facing slopes of the San Gabriel Mountains...where
0.25-0.50" of rain are possible. Areas west of the mountains will
generally receive 0.10-0.25" or less...but amounts will be highly
variable. Rainfall rates should remain below about 0.15" per hour.

While thunder is not mentioned in the forecast the chance of a thunderstorm is
not zero. The area near the low center will need monitoring.

A little pop up ridge (ahead of another upper low to the sw) is
slated for Wednesday. Skies will turn partly cloudy. Rising heights and
weak offshore flow will allow for nice bump up in temperatures.

Long term (thu-sun)...
another upper low to our SW pushes the ridge out as it moves towards
Southern California. This low is much weaker than todays and will likely open up
into a trough as it moves overhead. Only the ec sees this trough
producing any rainfall and even then just over the l.A./Vta mountains and
the av. During this time skies will be partly or mostly cloudy and
maximum temperatures will be pretty close to normal perhaps a degree or two
warmer due to the offshore flow to the low off the coast.

Both the ec and the GFS agree that another ridge is on tap for next
weekend. Skies should clear and maximum temperatures will rise several degrees
above normals with the higher heights and weak offshore flow.

&&

Aviation...26/12z...

At 0920z at klax... the inversion extends from the surface up to
1000 feet with a temperature of 23 degrees c at the top.

Overall...moderate confidence in 12z tafs. Reduced confidence due
to uncertainty in the timing and intensity of the incoming
precipitation and the effect on wind flow and visibility. An increasing
chance of precipitation across the region beginning in Los Angeles
County 22z-00z and moving north toward San Luis Obispo County by
06z. Conditions are most likely to remain VFR through the forecast
period though there is a thirty percent chance of MVFR conditions
after 23z.

Klax...moderate confidence in 12z taf. Reduced confidence due to
uncertainty in the timing and intensity of the incoming
precipitation and the effect on wind flow and visibility. An increasing
chance of precipitation beginning 22z-00z. Conditions are most
likely to remain VFR through the forecast period though there is a
thirty percent chance of MVFR conditions after 23z. A twenty
percent chance of east to southeast winds greater than 7 kts prior
to 19z.

Kbur...moderate confidence in 12z taf. Reduced confidence due to
uncertainty in the timing and intensity of the incoming
precipitation and the effect on wind flow and visibility. An increasing
chance of precipitation beginning 22z-00z. Conditions are most
likely to remain VFR through the forecast period though there is a
thirty percent chance of MVFR conditions after 23z.

&&

Marine...26/300 am...

An area of low pressure will move north over the region today...
bringing a chance of precipitation and shifting the winds to the
southeast. Locally gusty winds to 25 knots are possible into the
evening hours. North winds will return on Wednesday.

&&

Lox watches/warnings/advisories...
high rip current risk (see laxsrflox).

&&
$$

Public...kittell/rorke
aviation...kj
marine...kj
synopsis...seto

Www.Weather.Gov/losangeles

National Weather Service Glossary of Abbreviations