Weather

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fxus66 klox 060525 
afdlox 


Southwest California area forecast discussion 
National Weather Service Los Angeles/Oxnard California 
925 PM PST Monday Jan 5 2009 


Update...winds are slower to develop over the area this 
evening...but models are still indicating best subsidence pushing 
over the area overnight. The threat of precipitation is 
diminishing tonight as the frontal boundary continues to fall 
apart as it moves south. 


NAM-WRF...UKMET...and GFS indicate best subsidence pushing over 
the area around 06z tonight...and continuing through Tuesday 
morning. NAM-WRF indicates surface gradients are still hovering 
near advisory criteria...but the NAM-WRF still initialized too 
weak with the surface gradients for the 00z run. NAM-WRF and GFS 
850 mb thermal gradients begin to tighten from 06z on tonight and 
into tuesdday morning...with the tightest 850 mb thermal 
gradients occurring between 12z and 18z Tuesday. Winds should 
start to pickup later this evening and intensify throughout the 
night. Surface gradients are particularly strong...but there is 
some concern with 850 mb thermal gradients approaching 10 degrees 
celsius in the NAM-WRF and 6 degrees celsius in the GFS that 
winds could approach High Wind Warning criteria in some areas. 
With models handling this system poorly...low confidence exists in 
the scenario unfolding at this time...but the next shift will be 
briefed. High confidence does exist that the advisory will need 
to be extended into at least Tuesday afternoon as both NAM-WRF 
and GFS solutions indicate the best winds occurring between 12z 
and 18z. 


Offshore flow weakens Tuesday night and the air mass should warm 
as it stagnates. A warming trend will develop for Wednesday and 
possibly into Thursday. With southwest flow aloft establishing on 
Thursday night into Friday...the marine layer should redevelop. 
NAM-WRF is indicating the possibility of marine in the boundary 
layer and 975 mb relative humidity pattern. 


&& 


Previous discussion... /issued 220 PM PST Monday Jan 5 2009/ 


Short term (today-thu)...dying cold front dragging through the 
central coast this afternoon. A few light showers being reported 
but little if any measurable rain showing up in the gauges. Main 
impact from this system will be to bring about an increase in 
north to northwest flow across the area tonight into Tuesday, with 
advisory level winds expected in the mountains and southern sba County. 
May even get some gusty winds into the northern valleys of la/vtu 
counties later tonight but will not include them in the advisory 
for now. Winds should start to taper off Tuesday morning but 
enough of a gradient should linger through Tuesday to produce some 
northerly winds through the day and into Tuesday night as well. In 
la County the strongest winds as usual will be through the 
Interstate 5 corridor. 


Behind the front skies should clear out under the northerly flow. 
Exceptions to this include the Grapevine area where upslope 
precipitation and clouds are possible through Tuesday morning, and 
eastern la County where low clouds may linger. There is some concern for 
a possible eddy circulation developing overnight, but at this time 
it is felt that the northwest flow will be strong enough to push it 
far enough south as to not have much impact locally. Thus, skies 
should be mostly sunny across most of the area Tuesday. Highs will 
be similar to today. 


High pressure will build into the area Tuesday night and Wednesday, 
resulting in several degrees of warming Wednesday afternoon. 


On Thu, models are now showing a very weak cold front approaching 
from the northwest. This will cause gradients to turn onshore, 
forcing an earlier sea breeze to develop and daytime highs to fall a 
few degrees most areas. May see a slight increase in cloud coverage 
north of pt Conception by afternoon but no precipitation is expected. 


Long term (fri-mon)...the last couple GFS runs threw a little 
wrench into our plans for a big warmup late in the week, instead 
cutting off an upper low from the trough moving into the Midwest and 
bringing in a lot of cold air with it. This solution was the only 
one of the ensemble members that did this, and the European model (ecmwf) is also not 
in agreement with this. However, the ec did show the trough hanging 
back west farther than earlier runs and for now have struck a 
compromise by lowering temperatures a few degrees on Friday as the trough moves 
through. However, the current forecast maintains a warm and windy 
forecast Sat-Mon, the latter day or two of which are still indicated 
by even the newer GFS runs. It's just the Friday/Sat timeframe that is 
in question at this point. It still appears a decent Santa Ana wind 
event will develop, but the strength and timing, as well as the 
associated warm up is still uncertain. All models still point to 
unseasonably warm temperatures across the area by next Monday, with 
widespread 80+ readings possible at lower elevations. 


&& 


Aviation...06/0000z. 
VFR conditions will likely persist at all terminals throughout the 
period...with a 40 percent chance of MVFR conditions developing at 
terminals north of Point Conception and over Los Angeles County 
terminals. Low-wind shear and turbulence issues will develop over 
ksba...kbur...and kvny through 12z. 


Klax...VFR conditions are expected to persist throughout the period. 
There is a 30 percent chance that 6000 foot ceilings could develop 
between 01z and 05z. After 05z...there is a 40 percent chance that 
IFR to MVFR conditions could develop and persist into 18z Tuesday. 


Kbur...VFR conditions are expected to continue with a 30 percent 
chance that a 6000 foot ceiling may develop between 01z and 05z. 
Between 06z and 11z...weak to moderate turbulence will develop with 
low-level wind shear issues. 


&& 


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


&& 


$$ 


Hall/Wofford/meier 


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