Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 262108
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
208 PM PDT TUE JUL 26 2016
A stationary high will bring above normal temperatures inland this
week with the overnight marine layer keeping the coast below normal.
Partly cloudy today with a slight chance of thunderstorms in the
San Gabriel mountains. This weekend the monsoonal moisture could
bring possible showers and thunderstorms to the Ventura and Los
Angeles county mountains and deserts. Next week the high should
move East for a cooling trend.
Strong upper level high pressure system centered near Las Vegas
today with ridge extending westward over Southern California.
500 mb heights approaching 594 dm today with an east to southeast
flow aloft (above 10,000 feet). In addition, onshore pressure
gradients are somewhat weaker today as compared to yesterday.
This all adds up to even hotter temperatures today across many
inland areas. As of 1 pm, already seeing numerous triple digit
temperatures across our warmest valleys, lower mountains, and
Antelope Valley. With the weaker onshore flow, many valley areas
may see their warmest day of the week today, while mountain and
desert areas will see little change to their hot temperatures for
the next few days. While there is slightly more humidity in the
air today as compared to the last couple of days, heat index
values will generally remain below critical heat warning
thresholds. However, would not be surprised to see a few isolated
locations reach critical heat thresholds this afternoon. With
continued hot daytime temperatures and above normal minimum
temperatures (with some inland areas remaining in the 70s at
night), there will be some heat impacts with this current heat
wave this week. A shallow marine layer will keep low clouds and
fog mostly confined to coastal areas during the next few nights,
helping to moderate coastal temperatures.
Other item of note today is the increase in mid level moisture
across the mountains and deserts of Southern California, as
precipitable water values have risen to between 1.2 and 1.4 inches
across Los Angeles and Ventura counties. This added moisture,
combined with hotter and more unstable conditions resulting in
some cumulus cloud development over the local mountains. For our
forecast area, the eastern San Gabriel mountains will have a
20 percent chance of thunderstorms this afternoon and evening,
with a 10 percent chance for the remainder of the LA/Ventura
county mountains and Antelope Valley (including the Sand Fire
region). In addition, the hot and unstable conditions will make
conditions favorable for development of pyrocumulus clouds
at the Sand Fire burn location near Agua Dulce and Acton.
And if any storms develop to our east this afternoon and
evening, the easterly flow at the mid and upper levels could drift storm
activity into the eastern portions of the LA county mountains and
Antelope Valley. If any storms were to develop today, the main
threats would be isolated dry lightning strikes and gusty
downdraft winds. With southeast flow aloft prevailing the next few
days, there will be a 10 percent chance of thunderstorms across
the LA county mountains and Antelope Valley each afternoon and
evening through Friday.
The EC and GFS agree that the upper high will move to the east
over the weekend and then a little trof will ripple overhead for
Monday. The lower hgts will kick off a cooling trend. Max Temps
will fall to normal over the weekend.
The high will be in a slightly more favorable position for
monsoon moisture transport on Sat and Sun and there will be a
slight chc of afternoon convection over the VTA and LA mtns as
well as the Antelope Vly. Still the better chc of TSTMs will be
to the south and east of the area.
On Monday drier SW flow sets up as the upper trof rolls in to the
north. This will inhibit any monsoon flow and the slight chc of
convection will end.
At 18z at KLAX the marine layer was around 600 feet deep.
Moderate confidence in similar flight cats and winds for the next
24 hours as what occurred the previous 24 hours...with a few
exceptions. KLGB has a 60 percent chance of CIGS around 010
forming tonight. 50 percent chance of LIFR conditions tonight at
ksmx and ksbp...and a 30 percent chance at KSBA and KOXR. Best
chance would be from 09-15Z.
The Sand Fire looks less active this morning, but could intensify
this afternoon. KWJF and KPMD may have smoke issues again today as
a result with KPMD having a better chance than KWJF. The 21-01Z
period should be the most significant window.
KLAX...80 percent sure that clouds currently hugging the
coast will not affect the airport. Moderate confidence in VFR
conditions through 02Z as a result with typical winds. 10 percent
chance of cigs around 008 at 02Z increasing to 80 percent chance
by 10Z. Smoke from the Sand Fire should stay well north of the
KBUR...High confidence in VFR conditions through Wednesday. Smoke
from the Sand Fire should stay to the north of the airport.
Moderate confidence in winds and seas remaining below Small Craft
Advisory through tonight except for some local gusts up to 25 KT
near Point Conception. West to northwest winds will start
increasing some on Wednesday and Thursday with Small Craft
Advisory Likely between Point Conception and San Nicolas Island.
Moderate confidence in dense fog with visibility of 1 NM or less
will affect much of the coastal waters through Wednesday.
On track for another hot and dry day with similar conditions to
yesterday for interior areas...with locally gusty onshore winds
across southwest california. Relative humidities are currently in
the 10s to lower 20s across the antelope valley and portions of
the LA mountains. Temperatures are expected to be up 3-6 degrees
across many interior areas today. Areas with breezy to gusty
onshore winds mainly across the LA mountains into the Antelope
Valley will likely experience several hours of critical fire
weather conditions. However...the duration criteria for red flag
conditions is not expected to be met. There is the slight chance
of an isolated elevated thunderstorm across the far eastern San
Gabriel Mountains today. The additional heating of the atmosphere
combined with a more unstable air mass will produce a deep mixing
layer of 15000 feet or greater across the interior portions of
the Los Angeles county mountains, near the eastern flank of the
Sand Fire. This would support the potential development of a
significant pyrocumulus cloud in that region again today if the
Sand Fire remains active.
The elevated fire weather conditions across interior mountains and
valleys will persist each afternoon and early evening through
Thursday or Friday due to low relative humidities and locally gusty
onshore to northwest winds. Elevated fire weather concerns extend
into the upcoming week weekend due to the possibility of afternoon
thunderstorms. Monsoon moisture aloft is expected to gradually
deepen over the area through the week with increased moisture
leading to higher relative humidities at the surface beginning
Friday or Saturday. The increasing moisture will bring the slight
chance of thunderstorms over the Los Angeles and Ventura county
mountains and Antelope Valley each afternoon Saturday and Sunday.