Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 251830
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
1130 AM PDT MON JUL 25 2016
Updated Aviation and Fire Discussions
The high will bring above normal temperatures this week. Then a
combination of an overnight marine layer and moisture from the
south will bring partly cloudy skies and a inland cooling trend
by Tuesday. On Friday and into the weekend some moisture systems
could bring possible showers and thunderstorms to the Ventura and
Los Angeles county mountains and deserts.
Satellite imagery this morning showing low clouds and fog starting
to pull off the LA/Ventura county coastal areas but remains solid
along the Central Coast and SBA south coast. 12z Vandenberg
sounding showing marine layer depth around 1200 feet this morning
while ACARS sounding showing marine layer depth around 800 feet
across the LA Basin. The more solid marine layer clouds this
morning brought slight cooling and some humidity recovery across
coastal and valley areas, but mountain and desert areas not
feeling much impact from the shallow marine air.
The other notable item of interest this morning on satellite and
radar is the influx of monsoonal moisture associated with a weak
wave bringing some showers and isolated thunderstorms across
northern Baja extending into San Diego county. The NAM is showing
some southeast upper level flow between 10,000 and 20,000 feet
that will bring some mid and high level clouds across Los Angeles
county today. The better threat of showers and isolated
thunderstorms today will be south and east of our area, but have
included a very slight chance (10 percent probability) for eastern
LA county extending southward into the adjacent coastal
waters...with the main threats being dry lightning strikes and
gusty downdraft winds if storms were to develop.
Another main concern today will be the continuation of gusty
onshore winds across interior sections of the forecast area.
Already seeing a slight increase in onshore pressure gradients
this morning as compared to yesterday morning, with LAX-Daggett
gradient projected to be in the +6 to +7 mb range by late
afternoon. As a result, already seeing some gusty onshore
winds develop as of 9 am between Santa Clarita Valley and
Antelope Valley (Newhall Pass currently gusting at 23 mph and
Lake Palmdale at 38 mph). For this afternoon and early evening,
looking for south to southwest winds gusting up to 30 mph in the
Santa Clarita Valley, up to 35 mph in the mountains and up to 40
mph in the Antelope Valley (except isolated gusts to 50 mph in the
foothills of the Antelope Valley). These gusty onshore winds
will continue to bring elevated fire danger across interior
sections today, with brief critical fire weather conditions
possible across the Los Angeles county mountains and Antelope
Valley (where humidities will remain in the 8 to 20 percent
*** From previous discussion ***
Both the EC and GFS agree that the Tuesday to Thursday period will
be rather static with an upper high anchored over Las Vegas. This
position of the upper high is not at all conducive to monsoon
flow. Temps will fluctuate some each day but will really not
change much through Thursday. The coasts will continue to have
night through morning low clouds and fog and there might be some
intrusion into the lower coastal vlys as well.
The long term is looking more and more like the short term. Now
both the GFS and EC are keeping the upper high in a more westward
location rather than moving it back east which was the forecast
earlier. If this more west solution comes to fruition there will
be no chc of afternoon convection. For now will keep the 20
percent chc of afternoon TSTMs in the forecast but if the 12Z mdls
hold this pattern the TSTMs will have to be removed.
Otherwise slightly above normal temps will persist...esp inland.
The coastal night through morning cloud pattern will also
At 18z at KLAX the marine layer was 500 feet deep.
Moderate confidence in similar flight cats and winds for the next
24 hours as what occurred the previous 24 hours...except for KLAX
and KLGB where there is a 70 percent chance of CIGS around 010
forming tonight. 70 percent chance of LIFR conditions tonight at
ksmx and ksbp...and a 30 percent chance at KSBA and KOXR. The big
source for uncertainty is the mid-level clouds currently over
Orange County that will move to the north. If these clouds become
thicker than expected, low clouds will likely be less than
KWJF and KPMD will likely have smoke issues again (brief IFR cats
possible) today but it will be very hard to forecast how the Sand
Fire behaves. The 22-02Z period should be the hardest hit.
KLAX...High confidence in VFR conditions and typical winds through
01Z. 10 percent chance of cigs around 010 at 02Z increasing to 60
percent chance at 12Z. Smoke from the Sand Fire should stay well
north of the airport.
KBUR...Moderate confidence in VFR conditions through Tuesday.
Smoke from the Sand Fire should stay to the north of the airport.
High confidence in winds and seas remaining below Small Craft
Advisory through at least Wednesday. Moderate confidence in dense
fog with visibility of 1 NM or less will affect much of the
coastal waters through today and possibly through the middle of
this week. There is a 10 percent chance for thunderstorms over the
waters south of point mugu today.
.FIRE WEATHER...25/1050 AM.
Elevated fire weather conditions will persist each afternoon and
early evening through Thursday across the interior due to the
combination of gusty onshore winds and low relative humidities.
Temperature will gradually increase with a slight drop in relative
humidities through Thursday in response to high pressure shifting
westward into southern Nevada. Temperatures will remain generally
5-10 degrees above normal through the week. A moderate onshore
pressure gradient WILL provide breezy to gusty winds through the
favored interior valley and mountain locations including the
location of the Sand Fire each afternoon and evening. A MARINE
LAYER UP TO 1500 FT ASL will provide moderate overnight relative
humidity recovery for valley locations including the Santa Clarita
valley near the Sand Fire each night through the week. However,
above the marine layer or above about 2000 feet relative humidity
overnight recoveries will be poor.
Monsoon moisture is expected to gradually deepen over the area
through the week...although relative humidities at the surface may
decrease through mid week before gradually increasing heading into
the weekend. 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.