Scientific Forecaster Discussion
fxus66 klox 201756
Area forecast discussion
National Weather Service Los Angeles/Oxnard California
956 am PST Mon Nov 20 2017
a ridge of high pressure building into the region and weak
offshore flow will likely bring record heat to portions of
southwest California sometime Tuesday through Thanksgiving day.
The ridge will gradually break down as a trough of low pressure
approaches the West Coast, supporting a cooling trend heading into
Plenty of high level clouds were moving into the forecast area this
morning, with some mid level clouds also noted north of Point
Conception. Filtered sunshine thru the high clouds will prevail
today over much of the area, altho for slo/sba counties, mostly
cloudy skies at times cannot be ruled out. Fairly decent offshore
gradients this morning (e.G., At 16z -2.0 mb lax-dag, -2.8 mb lax-
bfl, and -3.6 mb sba-bfl) was supporting locally gusty north winds over
the Santa Ynez mtns and l.A. County mtns. The winds are expected to
continue into the afternoon, and may expand some over these areas,
with gusty northwest winds possible along the I-5 corridor by late today.
Temps are forecast to be a bit warmer today and several degrees
above normal, with highs in the warmest vlys and inland coastal
areas expected to top out in the mid 70s to low 80s.
Generally flat and dirty upper level ridging over the forecast area
today, with 500 mb heights around 582-584 dm, will gradually build
tonight thru Wed. By Wed afternoon, 500 mb heights will increase to
590-592 dm across the region.
The mid and hi level moisture over the region will linger into
tonight then clear out by early Tue for mostly clear skies Tue
Northerly gradients will increase some thru tonight which will
support gusty North Canyon winds for the sba County S coast and mtns,
with some gusts to advisory levels possible. Gusty northwest winds can also
be expected in the l.A./Vtu mtns along the I-5 corridor this
evening, then transition to north to NE and expand S into the favored
foothills and vlys of vtu/l.A. Counties later tonight into Tue
morning. Locally gusty NE winds will be possible over these areas of
vtu/l.A. Counties again Tue night into Wed morning. Otherwise, there
will generally be weak offshore low level flow over swrn California
tonight thru Wed, especially during the night and morning hours.
The rising 500 mb heights, warming boundary layer and 950 mb temps,
and offshore flow, will all help to bring much warmer than normal
temps to the forecast area Tue and Wed. Near record to record high
temps will be possible for several climate stations, with temps on
Wed increasing to 15 to 20 deg above normal for many areas. Temps in
the warmest vlys and inland coastal areas should reach the mid 80s
to low 90s on Tue, and low to mid 90s on Wed. Even overnight temps
should be very mild in the breezy foothill and lower mtns Tue night
and Wed night where lows in the low to mid 70s will be possible.
***From previous discussion***
The upper high will begin to be forced eastward on Thanksgiving
day as a large trough moves into the eastern Pacific. However,
latest computer models show the upper high remaining firmly
entrenched across the region through Thu morning, and low level
gradients remaining offshore. While there may be a few degrees of
cooling, especially near the coast, it will likely be another very
warm to hot day in most coastal and valley areas Thu, after a
warm Wed night. Max temps could still reach or exceed 90 degrees
on Thu, possibly even across interior sections of the coastal plain.
The highest temperature ever recorded at downtown Los Angeles on
Thanksgiving day since 1877 (on which the date varies from year to
year of course) was 90 degrees on November 26th, 1903. This record
has the potential to be tied or broken this upcoming Thanksgiving
The upper high will continue to be suppressed southward and will
weaken on Fri as a large trough slowly approaches the West Coast.
There will be several degrees of cooling across the entire
region on Fri, but still, temps will be at least 10 degrees above
normal in many areas. The cooling trend is expected to continue
through the weekend as heights gradually fall and onshore flow
increases, but temps will likely remain a bit above normal even
through Sunday. Any rain with the approaching trough is likely to
stay to the north of the region.
At 1722z, there was a suggestion of a marine layer 400 feet deep
with an inversion top of 1700 feet with a temperature of 19 degrees
c. However, marine clouds are not expected through the period.
High confidence in 18z taf package as weak offshore flow will keep
all sites VFR through the period.
Klax...high confidence in 18z taf.
Kbur...high confidence in 18z taf.
For the outer waters, moderate confidence in current forecast.
A Small Craft Advisory has been issued for zones 673 and 676, although the winds
are only expected to gust to about 25 knots through 05z this
evening. Winds will stay below Small Craft Advisory speeds Tuesday and Wednesday.
For Thursday and Friday, Small Craft Advisory level winds are likely.
For the inner waters, good confidence in current forecast. For the
waters north of Point Sal, winds and seas are expected to remain
below Small Craft Advisory levels through Friday. For the waters south of Point
Conception, winds and seas are expected to remain below Small Craft Advisory levels
Pz...Small Craft Advisory in effect until 9 PM PST this evening
for zones 673-676. (See laxmwwlox).
Hazard potential outlook...(wed-sun)
hot and very dry conditions are expected Wednesday and Thursday,
with warm and dry conditions Wednesday night. There will be
gusty winds at times across portions of Los Angeles and Ventura
counties. There will likely be elevated fire danger across
portions of Los Angeles and Ventura counties during this time.