876 
fxus66 klox 172054 
afdlox 


Southwest California area forecast discussion 
National Weather Service Los Angeles/Oxnard California 
155 PM PDT Wednesday Mar 17 2010 


Synopsis... 


Mostly sunny and warm today with a cooling trend into Friday as low 
pressure will bring night and morning low clouds and fog along the 
coast. High pressure will bring another warming trend for the weekend 
with weak offshore flow. 


&& 


Short term (today-sat)...gradients finally trending onshore this 
afternoon but not before temperatures shot up to pretty much the same as 
tuesday's levels, if not warmer in a few places. Considerable 
cooling expected Thursday as a shallow marine inversion develops tonight 
and weak onshore flow pushes this cooler air inland. Visible Sat 
imagery showing some low clouds developing well offshore this 
afternoon and we should see this expanding eastward overnight. Areas 
of dense fog possible along the coast of la County later tonight and 
would not be totally surprised to see clouds as far north as Ventura 
County. 


A weak system moving into the Pacific northwest tonight will drop south along 
the West Coast Thursday and Thursday night, passing through so cal 
as a dry inside slider Friday morning. Although the timing of this 
trough passage is similar to yesterday's models, the models have sped 
up the arrival of northeast winds by 12-18 hours, which makes Friday 
a very tricky forecast. There will be a battle between the 
increasing low level moisture under a pretty strong marine inversion 
and the developing offshore gradient behind the trough. More often 
than not the marine layer wins the battle, at least initially, and low 
clouds hang in there longer than expected. For this reason, the 
spread of possible high temperatures Friday is quite a bit larger 
than usual. For now decided not to make too many changes to the 
Friday forecast given the uncertainty, but MOS guidance certainly is 
favoring a rapid and early marine layer clearing with northeast winds 
arriving at the coast before sunrise Friday. As usual the NAM is 
stronger with the offshore winds and is calling for advisory level 
winds in the Ventura and la County mountains and valleys late 
Thursday night through early afternoon Friday. Will see if this 
remains the case with the next few model runs. 


A similarly tricky forecast Saturday. Surface gradients remain about 
the same as Friday, though upper support for gusty winds is 
significantly less. The airmass as a whole is warmer with the ridge 
axis almost directly overhead, however the NAM is still showing a 
shallow, but fairly widespread low level moisture field across the 
coastal areas with weak southeast flow south of pt Conception. If 
the marine layer is not completely wiped out from friday's offshore 
flow then clouds could easily linger into Saturday morning, 
resulting in a difficult forecast for highs Saturday. MOS guidance 
numbers trend most areas 3-6 degrees warmer, but this may be 
overdone, especially for the coast. The current forecast trends are 
a little more conservative given this uncertainty. 


Long term (sun-wed)...while yesterday's models strongly favored 
Sunday as being the warmest day, it now appears that most areas 
should start trending cooler Sunday as the ridge weakens and a trough 
approaches from the west. There still is weak offshore flow at the 
surface, and highs should still be well above normal, especially 
inland, but for now i've either gone with little change or slightly 
cooler Sunday. The approaching trough will definitely bring in much 
cooler days early next week, but still no precipitation. The coolest day 
will be Tuesday as a trough dives south through the Great Basin. Then 
warmer Wednesday with weak offshore flow developing behind the trough 
and building high pressure aloft. Long range models still not 
showing any precipitation through the end of the month. 


&& 


Aviation...17/1800z 


A weak and relatively dry middle/upper level trough of low pressure 
upstream will move over the area. Moderate southwest upper level 
winds will become moderate northwest while light south and southeast 
middle level winds become light north by 18/18z. A Catalina eddy is 
expected to develop overnight and light shallow south low level may 
become moist over the bight. A weak to moderate onshore pressure 
gradient will develop in the afternoon...otherwise a weak pressure 
gradient will exist. 


Klax...there is a chance 1/2sm fog will develop by 18/11z and linger 
through 18/18z...otherwise current sky/visibility conditions will 
continue. 


Kbur...very likely current sky/visibility conditions will persist. 


Virtually certain - 95-100% 
very likely - 80-95% 
likely - 60-80% 
chance - 30-60% 
very unlikely - 20% or less 




***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). 
High surf advisory (see laxcfwlox). 
High rip current risk (see laxsrflox). 


&& 


$$ 


Public...Wofford 
aviation...30 
synopsis...Hoffer 


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