Scientific Forecaster Discussion
Southwest California area forecast discussion
National Weather Service Los Angeles/Oxnard California
500 am PST Friday Dec 13 2013
A fast moving upper low will pass through the area on
Friday...bringing cooler daytime temperatures. A surface high
pressure will build into the Great Basin on Saturday resulting in
gusty offshore winds for parts of the area and warmer temperatures.
Warm weather will continue into early next week.
Short term (today-sun)...
dry trough moving overhead had just enough oomph let in it to mix up
stratus over l.A. County. Updated forecast to make the l.A. County coast
sunny. Low clouds still hanging tough over Catalina.
Little pop up ridge embedded in northwest flow is overhead at the moment
(but it wont be for long) more importantly gradients have turned
onshore both to kdag and kbfl. Patchy marine layer clouds have
formed over the l.A. County coast...the central coast and the Santa
Ynez Valley. Not as much as an eddy as forecast and the low clouds are not
as extensive as thought ydy for l.A. County. The effects of the
onshore flow will more than match the height rises of the ridge and
today will be cooler than ydy across the csts and valleys while the
interior will likely warm a degree.
The only thing the higher heights have done is to squish the central
coast marine layer low enough to allow for some dense fog
And then its back to the offshore flow as a weak little inside
slider ripples down the California/Nevada border and high pressure builds into
Nevada. Gradients snap offshore and Santa Ana winds will develop. A
Wind Advisory has been issued for most of vta and la County from
tonight through Sunday morning. There will be the typical diurnal
lull later Saturday afternoon and evening. The mountains will be
hardest hit with frequent gusts to 55 miles per hour likely and a few 65 miles per hour
gusts across the windiest ridgetops. Santa Ana winds mean dry air
and this is no exception the very low humidities will combine with
the winds to make for a potentially dangerous fire weather environment. A
Fire Weather Watch is in effect for the weekend.
Maximum temperatures will warm Saturday but will be tempered some by the cooler
air coming in the with winds. But further warming Sunday will drive
temperatures up to 10 degrees above normal.
Long term (mon-thu)...
two glorious days on tap for Monday and Tuesday courtesy of a 586 dm upper
high and attendant offshore flow. Not enough upper support for wind
advisories but there will likely be 15 to 25 miles per hour winds below the
usual passes and canyons. Maximum temperatures will be 10 to 15 degrees above
normal across the csts and valleys under sunny afternoon skies. The
very dry air and very long nights will allow wind sheltered areas to
have very cool lows and there will be some non mountain areas with min
temperatures at or below freezing.
The models are not in complete agreement but there is a weak consensus
that some sort of trough will move through the area Wednesday and Thursday.
Certainly enough to add some clouds to the forecast and dump temperatures down
as offshore flow ends. There is a non zero chance of some rain on
Thursday but will not mention it for now and wait for a little
better agreement before going thumbs up or down.
Overall...moderate to high confidence in 12z taf package. Haphazard
stratus development across the central coast and Los Angeles coastal
plain will result in low confidence taf forecasts for ksmx/ksbp/klax
and klgb through this morning as ceiling/visibilities categories could vary
between VFR and LIFR. Otherwise...high confidence in VFR conds for
other taf sites this morning.
For this afternoon through tonight...high confidence in VFR conds
for all sites as offshore develops across the area. There could be
some low level wind shear/turbulence issues through/below passes and canyons late
tonight into Saturday morning.
Klax...overall...moderate confidence in 12z taf. Low confidence in
forecast through 18z due to haphazard stratus pattern. High
confidence in VFR conds from this afternoon through tonight.
Kbur...high confidence in 12z taf. There is the potential for some
low level wind shear/turbulence issues late tonight as offshore flow develops.
A period of moderate offshore flow will return later Friday night
and continue through Sunday. Peak lax-Daggett pressure gradients
during this time are projected to range between -6 and -7 mb. Best
upper level wind support is expected on Saturday when offshore winds
are expected to peak. During this time...wind gusts between 45 and
55 miles per hour are expected across the mountains...and between 35 and 45 miles per hour
across coastal and valley areas. Initially... there will likely be
some moisture embedded in the northeast flow which is projected to
keep humidities mostly above 15 percent Friday night into much of
Saturday. However...humidities are expected to lower to around 15
percent on Saturday afternoon. Further drying is expected on
Saturday night and Sunday...when minimum humidities will likely
range between 8 and 15 percent. This offshore wind event will be
significantly warmer than our event this past Monday...with
temperatures climbing to around 80 degrees on Sunday and Monday. A
Fire Weather Watch has been issued for much of Los Angeles and
Ventura counties from Saturday afternoon through Sunday afternoon
due to the combination of moderate offshore winds...low
humidities...and dry fuels. Offshore winds are not expected to be as
strong across San Luis obsipo and Santa Barbara counties...however
wind gusts between 25 and 35 miles per hour with warm and dry conditions will
bring elevated fire danger to these areas.
Offshore breezes will continue to bring warm and dry conditions
on Monday and Tuesday...maintaining elevated fire danger across
much of Southern California.
Fire Weather Watch (see laxrfwlox).
Wind Advisory (see laxnpwlox).
Small Craft Advisory (see laxmwwlox).