Weather

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Area forecast discussion 
National Weather Service San Diego California 
245 PM PDT Friday Aug 29 2008 


Synopsis... 
increasing monsoon moisture and an upper level trough will bring a 
chance of thunderstorms through Saturday night for all areas of 
southwest California. Some of these may produce flash flooding over 
the mountains and deserts. A trough of low pressure will bring 
drier...cooler weather Sunday and Labor Day. Temperatures will 
slowly warm next week but remain near seasonal averages. 


&& 


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


Short term...tonight through Labor Day..... 
Forecast still on track except was about two hours earlier on 
development of thunderstorms to the south/east and eventually over 
the deserts/mountains this evening. Bumped probability of precipitation and quantitative precipitation forecast up some more 
for tonight through Saturday night....ended up with probability of precipitation close to 60 
percent for most areas during some or all of this period and quantitative precipitation forecast 
amounts ranging from less than two tenths west of the mountains to a 
third to one inch over the mountains and a third to half inch over 
the deserts. These are areal amounts and slow moving or repeat cells 
could easily add up to two to three inches due to high precipitable 
water amounts. Flash flood potential is high for mountains and 
deserts from San Bernardino/Riverside County line southward. Was 
late on coordinating with neighboring offices so have agreed to 
defer decision on issuing a Flash Flood Watch to next and/or 
midnight shift to allow sufficient coordination time. For 
Sunday/Monday a strong drying trend is indicated with the troughs 
passage to the north. This has been incorporated into grids. 


Long term... 
Monday night/Tuesday surface high pressure building over Great Basin 
in wake of departing trough sets up weak offshore flow event that 
lasts into Wednesday. Tweaked temperature/dewpoint/rh/and wind grids to 
reflect this. From midnight shift discussion...Wednesday through 
next Friday a strong trough over western Canada drops southward into 
the northern rockies bringing a early taste of fall there. This 
feature is too far north to affect US...so fair weather and 
seasonable temperatures for the end of next week. 


&& 


Aviation... 
292030z...the marine layer cloud deck...based around 1200 feet mean sea level 
and topped out around 1700 feet deep...was somewhat patchy and 
disorganized due to subtropical activity earlier today. Continued 
moisture and cloudiness streaming in through Sunday will result in 
reduced confidence in the behavior of the low clouds. The current 
plan for tonight is for the low clouds to return around 03z this 
evening along the South Coast before pushing north along the coast 
and locally into the coastal valleys. This may change to bring in 
the low clouds later this evening on the next forecast update. 


Above the marine layer...mainly scattered locally broken clouds 
8000-10000 feet mean sea level through Saturday and possibly on Sunday. 
Isolated to widely scattered thunderstorms are possible in the 
mountain and desert areas in the afternoons. Thunderstorm tops 
should be mainly around 35000-45000 feet mean sea level. 


&& 


Fire weather...threat of dry lightning is rapidly being eliminated 
as monsoon moisture continues to stream in from the southeast and 
fill in the remaining dry holes. Lightning accompanied by wetting 
rains expected to begin late afternoon to the south and east and 
move north and northwest across the lower deserts and mountain areas 
of southwest California tonight. Wetting rains and thunderstorms all 
areas Saturday morning and over the mountains and deserts Saturday 
afternon and night. Thunderstorms may linger/reform over deserts and 
higher mountains Sunday morning but by Sunday afternon strong drying 
trend and increasing southwest winds begin to spread over southwest 
California from strong trough moving in to the north. For the upper 
deserts this translates into 3 to 8 hours of relative humidity below 
15 percent and sustained southwest winds +25 miles per hour with gusts +45 
miles per hour...Sunday afternoon through late Sunday evening...thus after 
coordinating with weather forecast office Las Vegas...issued a Fire Weather Watch for 
fire weather zone caz260...Apple Valley...Lucerne Valley...Yucca 
Valley...and Morongo Valley. Weak offshore flow event continues to 
be advertised for Monday night/Tuesday and extending into Wednesday 
by models. Relative humidity will drop below 15 percent over 
mountains/valleys but northeast wind gusts should remain below 25 
miles per hour even in wind prone areas. 




&& 


Sgx watches/warnings/advisories... 
California...Fire Weather Watch from Sunday afternoon through late Sunday 
night for Victor valley-Apple Valley-Lucerne Valley-Yucca 
Valley-Morongo Valley. 


Still strongly considering Flash Flood Watch 
for mountains and deserts for 0600-2200 PDT Saturday. 


Pz...none. 


&& 


$$ 


Public/fire weather...Balfour 
aviation/marine...small 


National Weather Service on the web at weather.Gov/sandiego