Weather

950 
fxus66 klox 080319 
afdlox 


Southwest California area forecast discussion 
National Weather Service Los Angeles/Oxnard California 
819 PM PDT sun Sep 7 2008 


Update...marine layer making fairly aggressive return tonight. 
Majority of coastal waters are already covered with stratus 
clouds. Latest acars soundings place marine layer inversion near 
1100 feet this evening and the marine layer should deepen slightly 
throughout tonight as a developing weak trough continues to dig 
south...and interacts with the weak eddy circulation in place 
south of Point Conception. NAM-WRF time height sections indicate 
slight lifting of the marine layer for areas south of Point 
Conception throughout tonight. The marine layer should push into 
the Los Angeles and Ventura County valley areas later tonight. 
Along the central coast...lower confidence exists in the lifting 
although gradients are starting to indicate the possibility for 
dense fog not forming over the area. Current package leaves out 
the mention of dense fog...although GFS and Eta model MOS guidance for 
ksmx and ksbp continues to suggest it. For now...any updates will 
delayed for further clarity with arriving marine layer. 


Some concern is developing for an isolated shower or thunderstorm 
over the San Gabriel range and the Antelope Valley along the Los 
Angeles-San Bernardino border on Monday. Though moisture pattern 
is not impressive...850-700 mb mixing ratio around 6 g/kg develop 
over the San Gabriel range and the eastern portion of the Antelope 
Valley which could be enough for possible precipitation. 700-500 
mb relative humidities increase and NAM-WRF solutions develop some 
middle-level cape over the area for Monday afternoon and evening. In 
addition...a fair amount of 500 mb divergence is present in a weak 
southeast flow pattern. GFS has similar signatures in the 18z 
solutions but pushes the area of instability over the area about 3 
to 6 hours earlier in the day on Monday. With good instability but 
a lack of moisture...for now...an update will be performed to 
increase cloud cover over the mountains and desert for Monday 
afternoon and evening and the next shift will be briefed about 
precipitation possibilities. 


The trough digging south into northern California will stall out 
over northern California on Monday. Then...a secondary wave drops 
in behind and kicks the trough south for midweek. Better model 
agreement is developing for midweek with better cooling possible 
for Tuesday and Wednesday. Cooling trend exists in the current 
package...but temperatures could be slightly cooler over the 
interior than what the current package suggests...especially the 
mountains. NAM-WRF and GFS 850 mb temperatures cool through 
Wednesday afternoon...while the package provide similar 
temperatures for Tuesday and Wednesday...so additional cooling 
may be needed in the package. 


&& 


Previous discussion... /issued 115 PM PDT sun Sep 7 2008/ 


Short term (today-wed)...a few afternoon clouds developing over the 
vtu/la County mountains today but so far they are showing very 
little vertical development and I don't expect any precipitation from them. 


Little change to the forecast through midweek. Temperatures fell a few 
degrees today in the la/vtu County valleys and this trend should 
continue and become more widespread at least through Tuesday. 12z 
GFS shows the trough digging a little more than previous runs and we 
could actually see the cooling trend lasting another day or two. The 
marine lyr, which has been confined mostly to the coastal and lower 
coastal valley zones the last few days will push farther inland each 
of the next few days. Although the forecast currently does not have 
clouds reaching the southern Salinas valley or the Santa Clarita 
valley would not be surprised to see this happen by Tuesday morning. 


Long term (thu-sun)...with the GFS digging the trough farther south 
it may take an extra day or two before temperatures start to rebound. 
However, the European model (ecmwf) has a deeper low but also is faster moving the 
trough east. This solution would result in an earlier warming trend. 
For now have trended the forecast closer to the cooler GFS as this 
solution seems a little more plausible. 


&& 


Aviation...08/0003z. 
IFR conditions will likely return to most coastal terminals around 
02z...except for klgb and ksbp. Klgb and ksbp will likely see IFR 
conditions develop between 04z and 08z tonight...with confidence 
lower in this timing. The central coastal terminals have a 80 
percent chance of LIFR conditions developing between 06z and 08z and 
persist into around 15z. South of Point Conception...there is a 40 
percent chance that IFR conditions could improve to MVFR after 06z. 
IFR conditions will likely develop at kbur and kvny after 11z 
tonight. 


Klax...IFR conditions will redevelop over the klax area by 02z...or 
03z at the latest. IFR conditions will likely persist with a 40 
percent chance of IFR conditions improving to MVFR after 06z. If 
there is a most likely time frame for ceilings to lift into the MVFR 
category...the ceiling should lift between 11z and 15z. VFR 
conditions should develop between 17z and 19z on Monday. 


Kbur...VFR conditions will persist through at least 06z...with IFR 
conditions spreading into the area after 06z. Low confidence exists 
in the timing of the ceilings. A more likely time period of 11z to 
16z was given...but there is a 40 percent chance that IFR ceilings 
could develop between 06z and 11z. VFR conditions will redevelop 
between 15z and 17z on Monday. 


&& 


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


&& 


$$ 


Public...Hall/Wofford 
aviation...Hall 


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