Area forecast discussion 
National Weather Service San Francisco Bay area 
1101 PM PDT Thursday Oct 30 2014 


Synopsis...a cold front will spread rain across the entire region 
late tonight into Friday. Showers and possible thunderstorms are 
forecast from late Friday afternoon through Friday night. 
Scattered showers will continue into Saturday. Dry weather will 
return by Sunday and continue through much of next week as high 
pressure rebuilds. 


&& 


Discussion...as of 8:45 PM PDT Thursday...incoming Pacific 
weather system continues to look promising on satellite with a 
cold upper trough digging southeast offshore. Precipitable water values 
as high as 1.2 inches are feeding in from the SW along the cold- 
frontal rain band which is approaching the northern California coast this 
evening. Models have lately been trending wetter with this system 
and that seems like a good bet given relatively strong dynamics 
and a good moisture source. Another model trend has been to focus 
the heavier precipitation across the southern portion of our forecast 
area. 


Timing of precipitation onset remains consistent. Rain will likely begin 
in the North Bay late tonight and spread into San Francisco by 
around sunrise. Looks like much of the sf Bay area will have a wet 
morning commute...particularly the North Bay...San Francisco 
peninsula and East Bay. Rain probably won't reach the South Bay 
until late morning and then the Monterey Bay area around midday or 
early afternoon. The southeastward progression of the frontal rain 
band is expected to slow considerably around early afternoon as a 
130 knot upper jet digs into the base of the upper trough offshore. 
Also...the NAM forecasts a wave to develop along the front just 
west of Monterey Bay late Friday morning....which will slow the 
front down and enhance low level convergence and precipitation intensity. 
For these reasons it looks like the largest rainfall totals will 
be over areas from the Santa Cruz Mountains southward. In 
fact...the 00z NAM forecasts over 2 inches of rain in the Santa 
Cruz Mountains vs. Only about a tenth in the interior of Sonoma 
County. Other models show a more uniform quantitative precipitation forecast distribution...but 
all of the models agree that heavier amounts will be across the 
south. 


Precipitation will begin to taper off in the North Bay by midday...across 
the rest of the sf Bay area by middle afternoon...but not until middle 
evening from Monterey Bay south. 


The airmass will become increasing unstable during the late 
afternoon and evening hours of Friday as cold advection occurs 
behind the front. Isolated thunderstorms are possible from late 
afternoon Friday through Friday night. Best chances will be over 
the ocean and across the southern half of the County Warning Area. 


Shower activity is expected to gradually diminish Friday 
night...but then become more widespread by late Saturday morning 
as a shortwave drops in on the backside of the departing longwave 
trough. Showers will then taper off Saturday evening and end by 
late Saturday night as the upper trough shifts well to our east. 


Rain totals across the north will mostly range from a quarter to a 
half inch. Farther south rain totals are generally expected to range 
from a half inch to an inch...except from one to two inches in the 
Santa Cruz and Santa Lucia mountain ranges. 


This system is not expected to produce strong winds. Local wind 
gusts of up to about 25 miles per hour are possible...but probably not much 
higher. 


Temperatures will be much cooler over the weekend...with highs 
mainly in the 60s...as much as 20 degrees cooler than what we've 
seen lately. 


Dry weather is expected from Sunday through most of next week as 
an upper level ridge builds over California. Temperatures will gradually 
warm to levels above seasonal norms by midweek. A weak system may 
bring rain to the northern portion of our area late in the week. 


&& 


Aviation...as of 11:01 PM PDT Thursday...for 06z tafs. Doppler 
radar is beginning to pick up some returns off the coast as the 
front gradually moves our way. Ksts began reporting light rain 
over the terminal this hour...however Doppler radar is not 
picking up these showers just yet as the Showers Pass just under 
the radar beam. As the front moves closer to the coast showers 
will begin to spread across the region from north to south and 
winds will turn south to southeasterly. Mainly MVFR ceilings expected 
with the exception of ksts that will see IFR ceilings through much of 
the period. Rain is forecast to begin over the North Bay around 
12z tonight then is expected to spread southward early Friday 
morning. Bay area should anticipate rain to begin around 14z with 
Monterey area terminals holding out till around 18z. Wet runways 
and south plan expected for the morning rush Friday. 


Vicinity of ksfo...MVFR ceilings expected tonight as the front 
approaches. Southeast winds will develop overnight with light rain 
likely by 14z Friday. Would expect to start the day with 
southeast winds and wet runways. Periods of moderate to heavy 
downpours possible around 21z. Wind shift to the SW also anticipated 
around 21z. 


Ksfo bridge approach...similar to ksfo. 


Monterey Bay area terminals...MVFR ceilings expected tonight as the 
front approaches. Southeast winds will develop overnight with 
light rain likely by 18z Friday. Periods of moderate to heavy 
downpours possible around 02z. Wind shift to the SW anticipated 
in the afternoon. 


&& 


Marine...as of 8:58 PM PDT Thursday...light wind and seas are 
expected through late tonight. Winds will gradually increase early 
Friday morning as a strong cold front approaches the northern 
waters. Winds will briefly increase ahead and just behind the 
frontal passage as rain moves from north to south through the 
waters on Friday along with a slight chance of thunderstorms. 
Behind the front lighter northwest winds are forecast Friday evening. On 
Saturday stronger northwest winds and building seas to around 10 feet 
will develop creating some hazardous seas for the start of crab 
season. 


&& 


Mtr watches/warnings/advisories... 
... Pt to pt pinos 0-10 nm from 6 am 
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm from 3 am 
Small Craft Advisory...pt Arena to pt Reyes 10-60 nm from 3 am 
Small Craft Advisory...pt Reyes to Pigeon pt 10-60 nm from 3 am 
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm from 6 am 
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm from 6 am 


&& 


$$ 


Public forecast: dykema 
aviation/marine: CW 


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