701 
fxus66 klox 181936 
afdlox 


Southwest California area forecast discussion 
National Weather Service Los Angeles/Oxnard California 
1235 PM PDT Thursday Mar 18 2010 


..new aviation discussion... 


Synopsis... 


Expect cooler temperatures today and Friday as a trough moves 
through Southern California. Gusty winds will also affect the area 
tonight and Friday morning...especially through the mountains. 
Warmer temperatures will return for the weekend with weak offshore 
flow...followed by another likely dry and cool trough early next 
week. 


&& 


Short term (today-sat)... 
the upper level trough has finally made a swing onshore this 
morning...ushering in some patchy dense fog along the central coast. 
Some decent cold air advection and -3mb gradients both from lax-bfl 
and sba-smx are combining this morning to bring local gusts to 40 
miles per hour to area mountains. The patchy dense fog should diminish through 
the morning as northwest flow behind the trough increases over the 
area...making for a breezy afternoon along the central coast. 
Onshore gradients have become neutral this morning and are expected 
to turn a few mb onshore by the afternoon...which should help to 
usher in some cooling across the area...most noticeably along the 
coast. 


As the trough moves inland tonight...a shortwave inside-slider-like 
feature rotates through the area this evening. This should bring 
some pretty decent north winds to the mountains starting this 
evening. Winds shift to the northeast after midnight...and should 
expand locally into the valleys and coastal plain. At this time...it 
looks like advisory level winds are possible at least for the 
mountains... and will let the day shift take a look at another model 
run to make the final call. 


Winds should persist into the early morning on Friday...creating a 
stratus conundrum. Both the 00z and 06z NAM-WRF indicate a pretty 
good eddy spinning up tonight as a result of the increased northwest 
flow...which should bring low clouds and fog to the la and Ventura 
coasts as well as into the San Gabriel and locally into the San 
Fernando valleys. Expect that the offshore push overnight will erode 
this away fairly early...and many locations may even be clear by the 
time the sun rises. Expect little change in temperatures from 
Thursday...with the residual offshore flow battling some cooling at 
950mb. 


On Saturday...expect low clouds and fog again...but only across the 
Los Angeles coastal plain. Another ridge builds into the area with 
the axis overhead Saturday afternoon...and temperatures should climb 
a few degrees...back about 10 degrees above climatology. Saturday should 
be the warmest day of the weekend. 


Long term (sun-wed)... 
on Sunday...the ridge starts to shift eastward...which should bring 
a few degrees of cooling to the area...just in time for the Los 
Angeles Marathon. The GFS and European model (ecmwf) diverge starting Monday...each 
handling a trough/cutoff low feature a bit differently. The 06z GFS 
swings a 567dm cutoff low about 500nm offshore of the Southern California coast 
on Monday...then pulls it inland over the Baja California peninsula Tuesday. 
This solution keeps Southern California dry but much colder Monday and Tuesday... 
with simply some clouds expected to stream over the area. The 00z 
European model (ecmwf)...however...swings a trough through on Monday...then pulls a 
moist inside slider over the area on Tuesday. This should also cool 
things down and bring clouds...but adds a few tenths of an inch of 
precipitation to the area Tuesday. For now...have left out any probability of precipitation...but 
cooled temperatures through Wednesday and increased cloud cover. 
Will wait to see the models come into line with this feature before 
adding in any late March rain. 


&& 


Aviation...18/1925z. 
A fairly low confidence forecast for tonight with an expected return 
of low clouds and fog. Expected depth of marine layer tonight is 
around 1 kft south of Point Conception and 500 feet north of Point 
Conception...but that is just a best guess. The latest satellite 
pictures are now indicating that the low clouds may move into klax 
and klgb (and maybe koxr) much sooner than indicated in the 18z 
tafs. If the predicted marine layer depths are correct...klax will 
have MVFR visibility under an IFR ceiling. A bank of fog may sit 
just south of kbur with MVFR visibility in haze by early Friday 
morning. 


Klax...low confidence in timing and depth of impending marine 
layer...however some confidence in the existence of a marine layer 
by as early as this evening. 


Kbur...there is a 30 percent chance of no ceiling and MVFR 
visibility in haze...30 percent chance of no ceiling and no 
visibility restrictions...and a 30 percent chance of IFR ceilings 
and visibility. A very low confidence forecast. 


***Note*** 
the ASOS equipment at kbur...kvny...khhr...kcma...and koxr has a 
software error that may lead to incorrect cloud heights when there 
are two or more cloud layers present. Observations at kbur will be 
quality controlled 24 hours a day...observations at kvny will be 
quality controlled from 13z-07z...observations at koxr will be 
quality controlled from 15z-05z. Tafs for kvny and koxr will not be 
amended for ceilings during the time the observations are not monitored 
and there are two or more cloud layers present. 


&& 


Lox watches/warnings/advisories... 
Small Craft Advisory (see laxmwwlox). 


&& 


$$ 


Public...meier 
aviation...Sweet 
synopsis...meier 


Www.Weather.Gov/losangeles