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Area forecast discussion 
National Weather Service San Diego California 
200 PM PDT Friday Jul 18 2008 


Synopsis... 
high pressure temporarily building over the area will lower the 
marine layer tonight. However fog is likely to be more widespread 
and affect the coastal mesas and lower western valleys tonight 
through middle Saturday morning. Otherwise pleasant weather is expected 
for Saturday. However a strong surge of monsoon moisture Saturday 
night into Sunday will bring a return of thunderstorms over the 
mountains and deserts that will continue into Monday. Some of the 
showers and thunderstorms move west across the valleys and reach the 
coast Sunday evening. 


&& 


Discussion...for extreme southwestern California including Orange... 
San Diego...western Riverside and southwestern San Bernardino 
counties... 


Short term...tonight through Monday... 
three weather concerns for southwest California during this period 
are the depth of the marine layer and stratus burn off/return 
times...the renewed surge of monsoon moisture...and the building 
south swell. 


Marine layer increased slightly this morning to around 2200 feet. 
The high pressure ridge...whose axis is shifting southward across 
the area through tonight should cause the subsidence inversion to 
lower and strengthen a little more before weakening on Saturday. 
Based on nam12 model cross section trends after baselining to this 
mornings observations...marine layer depth expected to lower to 1500 
feet this evening and to around 1000 feet by Saturday morning. Cross 
sections show layer mean relative humidity 96-100 percent so thick stratus cloud 
layer likely with more of the lower valleys/coastal mesas impacted 
by dense fog late evening/overnight. For Saturday night/Sunday moist 
unstable air mass aloft expected to inhibit stratus formation...see 
below. 


The next weather problem to address is the return of monsoon 
moisture this weekend. The remnants of a large mesoscale convective 
complex are sitting over the southern Gulf of California poised to 
be drawn northwestward. With the upper trough approaching the 
northern/central California coast today and pushing slowly inland 
Saturday and Sunday...a cleft in the ridge is made for this moisture 
to be pulled up. Models all show southeast developing aloft on 
Saturday with strong southeast flow...25-40 kts...in the low levels 
up the Gulf of California. Strong surge of monsoon moisture moving 
across Salton Sea/Coachella Valley/Morongo Valley/Yucca Valley and 
into Johnson/Lucerne valleys expected Saturday evening. Only thing 
that might inhibit convection on Sunday would be extensive debris 
clouds from Saturday nights convection. Have tweaked the 
weather/pop/and sky cover grids to add chance of thundershowers for 
Saturday evening over Lower Desert areas and sprinkles for early 
Sunday morning over mountains/deserts for dissipating overnight 
convection...as well as upped probability of precipitation/quantitative precipitation forecast amounts Sunday evening over 
areas west of the mountains. 


Third problem...wamiii model is known for flattening amplitude and 
missing timing of arriving southern hemisphere swells. It usually 
gets the direction and the period correct though. The relatively 
steep approach angle...190 degrees...means Orange County and only a 
few parts of San Diego County will be affected. The long period 
indicates a significant amount of shoaling will occur with surf 
heights easily double the swell height on beaches with steep slope 
and directly facing the arriving swell. Relaying strictly on model 
guidance will likely result in underforecasting of surf heights at 
these beaches but probably overforecasting at the west facing 
beaches since most of the swell energy will pass by to the north and 
only a third to a quarter of it will be refracted towards these 
beaches. 


&& 


Long term Tuesday through Friday... 
backed off on threat of thunderstorms Tuesday and Wednesday over the 
mountains/deserts but left in for Thursday and beyond. GFS model for 
at least last four runs has been showing drying southwest flow 
returning Tuesday and Wednesday in the 850-700 mb layer. After which 
southeast flow returns as another tropical wave passes by to the 
south. A very shallow and disrupted marine layer will bring night 
and morning clouds and fog to coastal strip and lower western 
valleys but expect onset to be later and clearing times earlier. 


&& 


Aviation... 
181915z...mdcrs soundings continue to show the marine layer depth to 
be around 2000 feet. Expect little change in the marine layer depth 
through the weekend. A weak coastal eddy should help maintain the 
marine layer through the weekend. 


Stratus with bases around 1400 feet mean sea level along the immediate coast 
should spread back inland over the coastal valley and mesas between 
sunset and 04z and the western inland valley areas before midnight. 
Stratus should reach vicinity of kont between midnight and sunrise 
on Saturday. Breakup on Saturday should be between 16z and 18z. 


Above the marine layer...except for some afternoon cumulus/towering cumulus over the 
mountains during the afternoons...a mostly clear sky with 
unrestricted visibilities will continue through Saturday. Middle and 
high level moisture moving in from the southeast Saturday night and 
Sunday could bring thunderstorms to the mountains and deserts on Sunday. 


Maximum temperatures for kpsp and ktrm for Sat...42 to 44c. Horton 


&& 


Marine... 
a moderate south swell will begin to move into the coastal areas 
today...increasing through Sunday. The period will increase to 19 
seconds on Saturday with the peak of the swell expected on Sunday 
with a period dropping to 16 seconds by Sunday afternoon. Surf is 
forecast to be up to 3 to 6 feet on Saturday with sets to 9 feet or 
higher mainly in Orange County Sunday. A high surf advisory will be 
issued for the Orange County beaches for Saturday night through 
Monday night. There will be strong and dangerous rip currents during 
the weekend. 


&& 


Sgx watches/warnings/advisories... 
California...high surf advisory from 9 PM Saturday to 2 am PDT Tuesday for 
Orange County coastal areas. 


Quantitative precipitation forecast...qpssgx issued 122 PM PDT 
for rainfall estimates through Sunday evening 


Special Weather Statement...spssgx...issued at 116 PM PDT for 
rainfall and flash flood potential. Also refer to latest 
hazardous weather outlook. 




Pz...none. 


&& 


$$ 


Public...Balfour 
aviation/marine...Horton 


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