NWS Los Angeles, CA wrote its last area forecast discussion before the first event at Sep 6, 10:16 AM PST, 2 hours 59 minutes earlier.
Area forecast discussion, Sep 6, 10:16 AM PST, 2 hours 59 minutes before
Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1116 AM PDT Sun Sep 6 2020
.SYNOPSIS...06/917 AM.
Record breaking temperatures are expected again today.
Temperatures will begin to gradually cool by Tuesday but will
remain above normal for the week. Fire danger will remain elevated
through the middle of the week as areas of gusty northerly winds
today and Monday become northeast on Tuesday and Wednesday.
&&
.SHORT TERM (TDY-TUE)...06/912 AM.
Exceptionally warm temps to start the day across much of the
area, especially in the foothills and lower mtns where lows never
fell below the 90s. One site in the Santa Monica Mtns (SCE
Encinal Canyon Rd) never even fell below 100! Morning soundings
corroborate this warm layer, with the 12Z VBG sounding showing
96-97 deg in a layer between 1600-3200 ft, and LAX ACARS sounding
at 13Z showing the temps at the top of the inversion of 102 deg at
2200 ft. LAX-DAG gradient at 15Z was -3.7 mb while the LAX-BFL
gradient was -4.1 mb, with some gusty NE winds across the
foothills and mtns this morning. 1000-500 mb thicknesses are
projected to be 590-592 dm across the area this afternoon, which
are probably some of the highest ever for the region. All
parameters are in place for an epic heat event today across swrn
CA, with many all-time high temps in jeopardy of being tied or
broken. For example, the all-time hi of 115 deg at Paso Robles
will almost certainly be broken, and the all-time hi of 119 deg at
Woodland Hills may be tied or broken. Even closer to the coast at
UCLA, with temps already up to 105 deg this morning, the all-
time hi of 109 deg may be tied or broken. Overall, hi temps today
will be in the 110-120 degree range for the valleys and lower
mtns, and generally in the 100-110 deg range for the inland
coastal areas. These expected highs are 15-30 deg above normal.
Offshore pressure gradients will trend onshore thru Mon. Lows
tonight will not be quite as warm as this morning, with the
foothills falling into the 80s, and much of the valleys and inland
coastal plain dipping into the 70s. Highs should be 5-10 deg cooler
on Mon for most areas, except minor if any cooling for most of SLo
County. Even with some cooling, it will remain excessively hot for
many areas away from the immediate coast. Widespread highs Mon will
be in the 100-112 degree range for many inland areas, except up to
115 degrees in the SLO County interior Vlys.
It cannot be stressed enough that is is a very dangerous and life-
threatening heat wave, and among the hottest that has ever occurred
in swrn CA. People are urged to use common sense, keep well-hydrated
and stay out of the heat and in air-conditioned locations as much as
possible. Hiking in the mtns could be deadly. The Excessive Heat
Warning continues thru Mon across the region. Please see the latest
Non-Precipitation Weather Message (LAXNPWLOX) for further details.
A super-strong H5 high over NV today (600 dm) will move into CA on
Mon and weaken to around 592 dm in the afternoon. For Mon night and
Tue, upper level ridging will extend along the W coast into swrn CA
while a large upper level trof moves into the inter-mountain West.
Southwestern CA will be under the southern and swrn portion of the
upper level high thru Mon, with H5 heights of around 597-599 dm for
today, then fall to about 592 dm by late Mon afternoon. H5 heights
should lower further to 587-589 dm for Tue. A broad E-SE flow
aloft will prevail thru Sun then turn more northerly for Mon and
Tue.
Some mid level moisture should move into the area today with
afternoon cu development over the mtns, but overall any monsoonal
moisture will be minimal. Otherwise, mostly clear skies will cover
the region thru Tue except for some marine layer low clouds later
Mon night and Tue morning for the L.A./VTU County coast.
In addition, gusty north sundowner winds can be expected each
evening through Mon for southern SBA County. These winds combined
with 950 mb temps 36-39 deg C will bring hot temps to the area this
evening, possibly to 100 degrees even down to the coast. Gusty NW
winds will also be possible in the L.A/VTU County mtns each evening
thru Mon, with very warm temps and low humidities. The gusty winds,
warm temps and low humidities will likely bring critical fire
weather conditions to the area at times. Please see the fire weather
discussion below for more details.
.LONG TERM (WED-SAT)...06/324 AM.
The EC and GFS both agree that the cold early season trof will
skirt the eastern portion of the Rockies before cutting off into
an upper level low early Wednesday morning. More importantly for
SoCal a strong SFC high builds into the wake of the trof and
rapidly switches the onshore flow to moderate offshore flow. By
dawn Wednesday there will be about 6 MB of offshore flow from the
east which represents a 9 MB reversal in 24 hours. This switch
along with a little cold air advection will bring the first Santa
Ana of the season to the area. It looks likely that there will be
advisory level NE winds developing in the mtns around midnight in
the mtns around the Santa Clarita Vly. These winds will then
spread to the SW reaching the southern half of the Oxnard plain
shortly after dawn. Typical of a Santa Ana there will be warming
across most of the coasts and vlys by the adiabatic compression
while the interior cools due to the cool air advection. Since the
airmass is no where near as warm as it is now Max temps while 10
to 12 degrees above normal will be not be as warm as today or
Monday. These winds will also greatly reduce humidities and
increase the fire weather danger.
Thursday will still be offshore but much less than on Wednesday.
The interior will warm up a little but otherwise temps will not
change much.
Not much to talk about for Friday and Saturday. Weak ridging will
be over the area and there will be weak onshore sfc gradients.
which will bring cooling to the coasts and some vlys each day.
Still Saturday's max temps are fcst to be 4 to 8 degrees above
normal.
&&
.AVIATION...06/1814Z.
At 1730z at KLAX... The inversion was around 350 feet. The top of
the inversion was around 1250 feet with a temperature of about 38
degrees Celsius.
Overall... High confidence in the current TAFs. VFR conditions
will prevail throughout the forecast period.
KLAX... High confidence in the current TAF. VFR conditions will
prevail throughout the forecast period. There is a ten percent
chance of east winds greater than 8 kts during the 12z-18z period.
KBUR... High confidence in the current TAF. VFR conditions will
prevail throughout the forecast period.
&&
.MARINE...06/825 AM.
Overall... Winds across the outer waters will be Small Craft
Advisory (SCA) level through Tuesday night, primarily in the
afternoon and evening hours, with a thirty five percent chance of
gale force Monday evening. Across the nearshore waters from Point
Mugu to Santa Monica, there will be gusty northeast Santa Ana
winds on Wednesday with a fifteen percent chance of reaching SCA
level. Otherwise and elsewhere, conditions will remain below SCA
levels.
&&
.FIRE WEATHER...06/955 AM.
Strong high pressure and weak offshore flow during the morning
hours will continue to bring a dangerous and record breaking heat
wave with low humidities to Southwest California through Labor
Day. After exceptionally hot temperatures overnight (some foothill
areas remaining near 100 degrees all night), today's temperatures
are expected to be the hottest of the current heat wave with some
locations expected to reach ALL-TIME record high temperatures,
ranging from 100 to 120 degrees for areas away from the beaches.
Temperatures will once again remain very warm tonight, adding to
the heat stress and fire weather concerns, with many areas having
minimum temperatures in the 70s to lower 80s, except mid 80s to
upper 90s in the foothills and lower mountains. Minimum humidities
between 5 and 15 percent will be common across many areas away
from the beaches, with poor overnight recoveries, especially in
the foothills and mountains. The very hot and unstable conditions
will bring a significant threat of large plume dominated fires
with extreme fire behavior across the region through Labor Day,
resulting in elevated to brief critical fire weather conditions
across all of Southwest California.
Sundowner wind gusts between 25 and 45 mph are likely tonight
and Monday night across the Santa Ynez mountains and Santa Barbara
south coast, strongest in the foothills. Similar northerly winds
are also expected across the I-5 corridor tonight through Monday
night. With an extremely hot air mass in place, the Sundowner
winds will maintain very hot temperatures of 100 to 107 degrees
with humidities around 10 to 15 percent through much of the night
in the foothills and wind exposed canyons of the Santa Barbara
south coast. A Red Flag Warning is in effect for the mountains and
Santa Barbara south coast due to the combination of gusty
northerly winds, low humidities, record breaking very hot
temperatures, and critically dry fuels. Slight cooling is expected
across portions of the coast by Monday, with more widespread
cooling by Tuesday. There is increasing confidence that our first
Santa Ana wind event of the season will impact portions of Los
Angeles and Ventura counties Tuesday afternoon into Wednesday
night, bringing the potential for single digit humidities. As a
result, critical Red Flag fire weather conditions will be possible
across portions of Los Angeles and Ventura counties by late
Tuesday into Wednesday, and a Fire Weather Watch will be
considered later today. Still on track for a LAX-Daggett gradient
of around -6 mb or greater for Wednesday morning which is very
rare for early September.
&&
.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Warning in effect until 8 PM PDT Monday for
zones 34>38-44>46-51>54-59-88-547. (See LAXNPWLOX).
Excessive Heat Warning in effect until 8 PM PDT this evening
for zones 39>41-87. (See LAXNPWLOX).
Red Flag Warning in effect until 10 PM PDT Monday for zones
239-252. (See LAXRFWLOX).
Red Flag Warning in effect from 6 PM this evening to 10 PM
PDT Monday for zones 253-254. (See LAXRFWLOX).
PZ...Small Craft Advisory in effect until 3 AM PDT Monday for
zones 670-673-676. (See LAXMWWLOX).
&&
.HAZARD POTENTIAL OUTLOOK (TUE-SAT).
Strong and gusty Santa Ana winds will be possible Tuesday night
and Wednesday. These winds and very low humidities will bring the
threat of widespread critical fire weather conditions across
Ventura and Los Angeles Counties.
&&
PUBLIC...Sirard/Rorke
AVIATION...Kj
MARINE...Kj
FIRE...Gomberg
SYNOPSIS...TF/Sirard
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