Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 261008
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
308 AM PDT TUE JUL 26 2016
Slightly above normal temperatures will continue this week, with
some warming for inland areas as high pressure strengthens. A
shallow marine layer will likely continue to result in night
through morning low clouds and fog for some coastal areas.
Monsoonal moisture may bring isolated thunderstorms to some
mountain and desert locations this upcoming weekend.
A very tricky forecast pattern for the next four days. Tricky
because the overall large scale pattern will be quite static from
now until Friday. A large warm upper high will remain anchored
over Las Vegas for the duration. 500 mb hgts will be about 594 DM
each day. At the sfc There will be moderate onshore flow both to
the east and to the north all four days. The fact that there are
no changes in the larger scale means that the small scale (and
harder to forecast) features will make for the day to day changes.
On Monday...for example...the eddy formed a little more to the
west than usual and LA county was quite a bit warmer than the day
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 0845z at KLAX...The marine layer was 800 feet deep. The top
of the inversion was located near 2800 feet with a temperature
around 26 degrees Celsius.
Overall... Moderate confidence in the 06z TAFs. The reduced
confidence is primarily due to the uncertainty in the timing...
location... and intensity of the marine intrusion. LIFR/IFR
conditions have already moved into many coastal locations
especially along the Central Coast. Low clouds and fog should get
into most coastal locations by 12z then diminish in the 16z-18z
time frame. There is a twenty percent chance of the marine
intrusion affecting KBUR and/or KVNY. There is a forty percent
chance of IFR/MVFR conditions due to smoke in the forecast period
at KPMD and/or KWJF. Otherwise and elsewhere VFR conditions will
KLAX... Moderate confidence in the 06z TAF. The reduced confidence
is primarily due to the uncertainty in the timing and intensity
of the marine intrusion. LIFR/IFR conditions will diminish in the
16z-18z time frame. Otherwise VFR conditions will prevail.
KBUR... Moderate confidence in the 06z TAF. The reduced
confidence is primarily due to the uncertainty in the timing and
intensity of the marine intrusion. There is a twenty percent
chance of the marine intrusion affecting KBUR. Smoke from the Sand
Fire will likely remain north of the airport. Otherwise VFR
conditions will prevail.
Conditions across the coastal waters will remain below Small
Craft Advisory criteria through at least the next couple of days.
Patchy dense fog of 1 NM visibility or less will affect much of
the coastal waters this morning.
Widespread relative humidities currently in the teens with
locally gusty onshore winds have led to some locations across the
LA Mountains into the Antelope Valley including the Sand Fire to
experience several hours of critical fire weather conditions.
However, the duration criteria for red flag conditions is not
expected to be met. The elevated fire weather conditions across
these areas as well as other interior mountains and valleys will
persist each afternoon and early evening through Thursday across
due to low relative humidities and locally gusty southwest 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 an higher relative humidities at the surface beginning
Friday. The increasing moisture will bring the slight chance of
thunderstorms over the Los Angeles and Ventura county mountains
and Antelope Valley each afternoon Friday through Sunday.