Area forecast discussion 
National Weather Service San Francisco Bay area 
904 PM PDT Monday Aug 29 2016 


Synopsis...an upper level system and associated cold front will 
push through the region overnight tonight into early Tuesday morning 
and result in breezy winds, slightly cooler temperatures and the 
slight possibility of dry thunderstorms over the North Bay. A broad 
troughing pattern over the West Coast will result dry weather 
conditions along with temperatures below seasonal averages through 
the remainder of the week. 


&& 


Discussion...as of 9:00 PM PDT Monday...an upper level low 
spinning approximately 450 nm west of San Francisco will approach 
the state tonight. The low and the associated cold front will 
bring increasing mid/upper level clouds across the region tonight 
into Tuesday morning. A slight chance of elevated convection will 
persist over the North Bay however models indicate the moisture 
associated with this system is moving in over the region between 
500-300 mb which is slightly higher than ideal 700-500 mb level 
for elevated convection over our area. Models also have pushed the 
treat further north then previous runs. While the chance for 
elevated convection and dry lightning over our area appear to be 
dwindling the forecast will maintain a slight chance across the 
North Bay through tonight. Winds will be another factor to contend 
with tonight into Tuesday morning. Strong and gusty west to 
northwest winds of 30 to 40 mph are expected behind the front 
however the latest ob from Altamont RAWS reported gusts to 39 
mph. Therefore anticipated gusty conditions through the rest of 
tonight and into Tuesday. 




From previous discussion...in wake of the frontal passage, the 
airmass will become more stable by Tuesday afternoon with the 
threat for convection diminishing and wind speeds relaxing late in 
the day. Temperatures will generally be a few to several degrees 
cooler compared this afternoon as well. Do expect some disruptions 
to the depth of the marine layer, however should see enough 
boundary layer moisture to allow for low clouds to return to the 
coast tonight and spread locally inland through Tuesday morning. 


A longwave trough is then forecast to remain stretched along much of 
the West Coast through late week. This will keep temperatures below 
seasonal averages, especially inland, with rounds of 
overnight/morning low clouds that will give way to mostly sunny 
conditions each afternoon. In addition, should see continued dry 
weather conditions outside of the potential for coastal drizzle late 
in the week. 


&& 


Aviation...as of 5:49 PM PDT Monday...drier air and increasing 
vertical mixing near the marine inversion rapidly eroded the 
stratus and fog since late morning, textbook example for our area 
of a warm-cool seasonal transition taking place in the nearest 
term. In general, looks like decent chances for VFR to hold area- 
wide this evening as mixing becomes increasingly focused and 
continues based on recent model output's handling of a deep 
layered trough and cold air advection pattern banking up against 
the sub-tropical ridge presently over California. Today's NAM run nearly 
in-line with present sfo-smx 3 mb gradient, which is forecast to 
hold this strong if not become a little stronger between 4 and 5 
mb tonight into Tuesday. Bottom line, areas gusty winds on coast 
with decent chances VFR persisting this evening, then low to very 
low confidence in stratus and fog redevelopment late tonight and 
Tuesday morning. 


Vicinity of ksfo...westerly winds with gusts to the 20 knot range 
this evening, fairly high confidence VFR persists this evening. 
Westerly winds may subside later this evening and overnight, but 
potential is there for it to be a slow process. As long as mixing 
prevails and high cloud cover with sub-tropical jet persist, surface 
winds will probably stay a bit gusty. Low confidence tempo MVFR 
cig 12z-16z Tuesday. Pretty decent chances VFR returns during the 
day Tuesday. 


Sfo bridge approach...similar to ksfo. 


Monterey Bay terminals...similar to the Bay area, VFR stands a 
pretty good chance of persisting this evening. Low confidence MVFR 
cigs 05z-09z followed by MVFR cigs into Tuesday morning. VFR returning 
late Tuesday morning, clearing could be quick process and thus possibly 
earlier than usual. 


&& 


Marine...as of 08:15 PM PDT Monday...high pressure off the coast 
of central California will cause a tightening of the pressure 
gradient over the coastal waters this afternoon through midweek. 
As a result...northerly winds and steep short period waves will 
build on top of a small southerly swell. Expect near gale to gale 
conditions in the Lee of Prominent Point such as Point Sur. 
Conditions are forecast to remain strong into the weekend...though 
an approaching upper trough may disrupt this pattern. 


&& 


Mtr watches/warnings/advisories... 
... Arena to pt Reyes 0-10 nm 
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm 
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm 
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm 
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm 
glw...pt pinos to pt Piedras Blancas 0-10 nm 
Small Craft Advisory...mry Bay until 9 PM 
Small Craft Advisory...sf Bay until 3 am 


&& 


$$ 


Public forecast: CW 
aviation: canepa 
marine: bfg 




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