Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 261821
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
1121 AM PDT TUE JUL 26 2016
Updated Fire Weather Discussion
A stationary high will bring above normal temperatures inland this
week with the overnight marine layer keeping the coast below normal.
Partly cloudy skies this afternoon with a slight chance of
thunderstorms in the San Gabriel mountains. On Friday and into the
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 at 9 am are weaker this morning as compared to
yesterday at this time (LAX-Daggett gradient at +1.8 mb compared
to +3.5 mb yesterday). This all adds up to even hotter
temperatures today across many inland areas today. In morning
update, have adjusted a number of temperatures upward, with many
triple digit readings across the valleys, lower mountains, and
Antelope Valley. Locations near the Sand Fire can expect
temperatures ranging between 100 and 104 degrees. With the
weaker onshore flow today, 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 today.
However, would not be surprised to see a few isolated locations
reach critical thresholds today, especially near Acton and the San
Satellite imagery showing low clouds and fog have retreated to
the beach locations this morning, with a shallow marine layer
depth of around 800 feet. As mentioned above, the onshore winds
will be slower to develop and not quite as strong as yesterday
across interior sections. 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. Not much
in the way of convection this morning, but there is an area of mid
level clouds across the eastern deserts of Southern California
that may get pulled into our forecast area later this afternoon
and tonight due to the easterly flow aloft. Still not much in the
way of lower level moisture or a significant trigger mechanism,
but if storms develop to the east of the forecast area, there is a
20 percent chance of those storms drifting into the eastern San
Gabriel mountains, and about a 10 percent chance reaching the Sand
Fire as well as the Antelope Valley.
*** From previous discussion ***
The combination of high hgts and moderate onshore flow will result
in well above normal temps inland and only slightly warmer than
normal temps closer to the beaches. The high peaks on Thursday and
this should be the warmest day away form the coasts.
The onshore push to KDAG will be 6 or 7 mb each afternoon and this
will be enough to produce 35 to 40 mph winds in the mtns and the
Antelope Vly. These winds combined with the hot dry conditions
will produce elevated fire danger through the period.
The marine layer will be the most difficult forecast parameter. It
will likely form in a different way each night depending on
if...when and where the eddy sets up. The very high hgts will keep
the marine layer depth shallow enough (unless a stronger than
normal eddy spins up) to keep the stratus out of the vlys. A few
beaches (esp along the central coast) will likely stay cloudy in
the afternoon due to the strong inversion and moderate onshore
The NAM shows a little PVA moving over the area this afternoon.
Even though there is very limited moisture this lift along with
the afternoon mtn instability should be enough to warrant a 20
percent chc of a TSTM over the eastern San Gabriels this
afternoon. Otherwise the location of the upper high over Las Vegas
is not very conducive to monsoon moisture transport. The chc of
afternoon TSTMs is not zero but is not high enough to include
Thunder in the fcst.
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 and then perhaps below normal
The high will be in a 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 much 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 to 22 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.