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fxus66 klox 070527 
afdlox 


Southwest California area forecast discussion 
National Weather Service Los Angeles/Oxnard California 
1030 PM PDT Monday Oct 6 2008 


..updated aviation discussion... 


Update...weak offshore flow pattern keeping the marine layer 
absent from the area this evening. 00z knkx sounding indicates 
warming in the lowest levels of the atmosphere with 950 mb 
temperatures warming by ten degrees celsius over the last 24 
hours...while current water vapor imagery illustrates the upper- 
level ridge of high pressure building over the area. 


A warmer day seems to be developing for Tuesday as the ridge 
continues to build over the area and surface gradients continue 
to trend offshore. Models seem to be throwing in some uncertainty 
though. NAM-WRF model initialization seems about two to three 
degrees celsius too warm at 950 mb based on upper-air data from 
knkx and koak sites. 18z GFS 950 mb pattern seems to be doing a 
better job...with only slight differences at 950 mb. Overall...the 
current package appears to be reasonable for warming for Tuesday 
based on current trends...and best approach is to wait for 
evening GFS solution before making any changes to the package. 


Ridge starts to weaken for midweek and into the latter part of the 
week as strong westerly flow over the Pacific northwest weakens 
the ridge of high pressure...and trough digs south along the 
Pacific northwest coast. Good agreement exists between the models 
indicating the ridge weakening some...surface gradients trending 
onshore...and 950 mb cooling for Wednesday and Thursday. This 
could be an altering point from the current package as package has 
Wednesday being as warm as Tuesday afternoon...but model suggest 
otherwise. Next shift will be briefed on this issue. 


Model solutions start diverging as the trough approaches the West 
Coast. NAM-WRF...GFS...and European model (ecmwf) solutions are in pretty good 
agreement with the upper-level pattern through Wednesday 
night...but 500 mb heights seem to be rather muddled through late 
week. GFS ensemble solutions offer little confidence on which way 
to trend. With NAM-WRF solutions following more closely to the 
European model (ecmwf) solutions...higher confidence should be placed in the NAM- 
WRF and European model (ecmwf) solutions in the middle to long term. Later packages 
may lean toward the European model (ecmwf) solution of a stronger inside-slider 
trough moving through the Great Basin. This could introduce probability of precipitation 
and rain chances into the forecast area...and possibly a moderate 
offshore event for the area over the weekend. A cooling and more 
moist trend should be expected for late week with marine layer 
enhancement ahead of the trough. A drying trend should occur over 
the weekend...with a possible warming trend for early next week. 


&& 


Previous discussion... /issued 200 PM PDT Monday Oct 6 2008/ 


Short term (today-thursday)... 
with lax-dag peaking near -2 mb and smx-bfl near -3 mb this 
morning...offshore grads have brought some light to moderate 
northeast flow to the mountains and favored valleys. With this flow also 
keeping the stratus at Bay while a 592 dm ridge builds in from the 
southwest...temperatures are up across the board. Valleys and mountains are 
trending up about 10 degrees from yesterday...with coastal spots 
trending just under that clip. 


Temperatures look to continue the warm up Tuesday and Wednesday...as offshore grads 
increase a bit more and the ridge continues to nudge deeper into the 
southwest US. In addition...min temperatures will see a little boost as 
well...giving US a bit of a break from the fall like morning temperatures 
we have been feeling. East winds should reform each morning through 
those passes and canyons...as well as a few mountain peaks...but should 
remain well below advisory strength. Central coast will continue to 
be the trickiest to forecast...being heavily dependent on how much 
wind surfaces and if it continues into the afternoon. Current 
forecast reflects middle of the range of possible outcomes. 


Things should begin to cool on Thursday...as a trough amplifies to the 
north and digs southward to our east. This will rapidly turn grads 
onshore and drop thicks...resulting in cooling everywhere but 
especially in the la/vtu valleys and mountains did introduce some dense 
fog to the Thursday morning forecast for la/vtu coast due to near 
neutral morning grads and a weak eddy possibly spinning up. 
Although this will quiet the offshore flow...some breezy northwest winds 
will likely be felt along the central coast and some of the mountains 
and deserts. 


Long term (friday-monday)... 
as the previously mentioned inside sliding trough digs further to 
the south and into Utah Friday night...temperatures should continue to cool 
Friday and Sat...especially in the mountains it could cool even more than 
the current forecast reflects if the 12z European model (ecmwf) solution materializes... 
as it keeps the low further west on Friday night than the GFS. Current 
package is weighted more on the 12z GFS and the previous 00z 
European model (ecmwf)...but nudged a bit in the 12z European model (ecmwf) direction. 


As with all inside sliders...this will continue to be a developing 
story...and the best forecast is that the models will continue to 
waver each run. The one story that has been consistent over the last 
couple of days and continues to look favorable is that we could be 
in store for our first Santa Ana event of the Young season. Starting 
Sat or sun and continuing into Monday...the cold air associated with 
this system will create a healthy surface high in the Great Basin. 
Although the European model (ecmwf) and the GFS differ in timing and intensity...this 
should be a significant event. There is a good chance of multiple 
wind advisories and possible warnings first in the mountains on Sat for 
northerly flow...then into the valleys sun as the flow shifts 
northeast. Did temper the warming in the valleys and coastal areas over 
the weekend...as the cold air associated with the low may linger 
around ab it later. 


&& 


Aviation...07/0525z. 
VFR conditions will likely occur at most terminals throughout the 
period...except for 20 percent chance of MVFR visibilities expected 
at ksmx between 10-13z. 


Klax and kbur...VFR conditions are expected throughout the period 
with no wind issues. 


&& 


Lox watches/warnings/advisories... 
none. 


&& 


$$ 


Public...Hall/kittell 
aviation...Kaplan 


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