Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 221741

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
941 AM PST Wed Nov 22 2017

Strong upper level high pressure and offshore flow will bring
very warm and dry weather to the area through Thursday, with
record or near record temperatures expected. The high will weaken
Friday through the weekend with a cooling trend through early
next week. A weakening frontal system could bring light rain to
northern areas late in the weekend, followed by gusty north winds
and cool weather Monday. Warmer weather is expected Tuesday as
north winds transition to a Santa Ana wind event.



A very strong 595 dm upper high was located off the coast of
northern Baja CA early this morning, with a strongly amplified
ridge extending into southwest California. This unusually strong
upper level ridge combined with with a stronger offshore flow
this morning (LAX-Daggett peaking at -6.7 mb) will bring
widespread record breaking temperatures across the region
today. In fact, some locations could see records shattered
by as much as 5 to 10 degrees! Early morning profiler data over
LAX showing very impressive warming in the boundary layer (with
950 mb temperatures of 28 degrees celsius) which is up around 4
degrees celsius from this time yesterday. Based on the pressure
gradient and boundary layer trends, have increased temperatures
upward in some locations for today. The most notable increase
is across the Ventura county coastal plain which could see some
of the hottest temperatures today due to the downslope
compressional heating associated with the offshore winds, with
a projected high temperature of 97 degrees today at Camarillo!
Offshore winds will generally gust between 25 and 35 mph today
across typical passes and canyons of LA/Ventura counties this
morning, with isolated gusts as 40 mph in the mountains.

The combination of hot temperatures, low humidities, gusty
offshore winds, and very dry fuels will bring elevated to brief
critical fire weather conditions today, especially Los Angeles
and Ventura counties.

*** From previous discussion ***

The upper pattern will change little through Thu, although the
upper high may be nudged a bit southward during the day as a
broad trough moves into the eastern Pacific. Low level gradients
are still forecast to be about 4 mb offshore between KLAX and KDAG
in the morning, then become neutral in the afternoon. There may
be some locally gusty northeast winds in the mtns and foothills
tonight and early Thu, but far below advisory levels. It will be
another very warm night, with lows again in the 70s in the
foothills and in windy areas. Heights and thicknesses will fall
ever so slightly on Thu, as will temps at 950 mb. Expect a couple
of degrees of cooling on Thu, especially near the coast. However,
high temps should still reach or exceed 90 degrees in the
valleys, and possibly across the interior plain. The highest
temperature ever recorded on Thanksgiving in Downtown Los Angeles
was 90 degrees, set on November 26th, 1903. That record is in
jeopardy tomorrow.

The models show the upper high beginning to weaken on Fri, and
onshore flow in the afternoon will increase. Expect several
degrees of cooling on Fri, especially west of the mountains, but
temps should still be well above normal in most areas.


The upper high will continue to weaken and shift eastward over
the weekend as an upper trough approaches the West Coast, so the
cooling trend will continue. Temperatures should still be above
normal Saturday, then closer to normal by Sunday.

Latest models show a bit of sharpening with the trough as it moves
onshore, and it now appears that the southern end of a weakening
cold front could bring some light rain to areas north of Point
Conception late Sunday into Sunday night or very early Monday.

Strong north to south gradients will develop once the trough axis
moves to the east of the region on Monday. It looks as though
there may be some rather strong northerly winds in the mountains
and across the SBA south coast later Monday into Monday night,
followed by a possible strong Santa Ana wind event Tuesday. While
it is still very early, the GFS and ECMWF models are in good
agreement on the upper level pattern and potential for a strong
north-northeast wind event in the Monday night-Tuesday time period.



At 1000Z, there was a surface-based inversion at KLAX. The top of
the inversion was around 1400 feet with a temperature of 29
degrees Celsius.

High confidence in 12Z TAF package as weak offshore flow will keep
all sites VFR through the period.

KLAX...high confidence in 12Z TAF.

KBUR...high confidence in 12Z TAF.


.MARINE...22/130 AM...

For the Outer Waters, good confidence in current forecast. Winds
and seas are expected to remain below Small Craft Advisory (SCA)
levels through tonight. For Thursday/Friday, northwest winds are
likely to increase to SCA levels across PZZ670/673 and remain just
below SCA levels for PZZ676. The winds will subside on Saturday,
before strengthening to SCA levels again on Sunday.

For the Inner Waters, good confidence in current forecast. For the
waters north of Point Sal, winds and seas are expected to remain
below SCA levels through Sunday, although there is a 20% chance of
SCA level winds Thursday afternoon/evening. For the waters south
of Point Conception, winds and seas are expected to remain below
SCA levels through Sunday.




Warm and very dry conditions are expected Friday which
will continue to bring elevated fire danger. There is the
potential for greater fire weather impacts Monday night into
Tuesday. On Monday night, there will be the potential for
strong northerly winds and lowering humidities, followed
by the potential for a strong Santa Ana wind event and even
lower humidities on Tuesday.



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