000 
fxus66 kmtr 110627 
afdmtr 


Area forecast discussion 
National Weather Service San Francisco Bay area 
1027 PM PST Mon Dec 10 2018 


Synopsis...mostly clear and cool conditions are forecast for 
tonight, along with patchy fog. A weak system may bring a few 
showers to northern Sonoma County on Tuesday night. However, for 
the most part our forecast area is expected to remain dry and 
seasonably cool from tomorrow through Thursday. A chance of 
widespread rain will develop on Friday, with a wetter system 
possibly impacting our region by Sunday. 


&& 


Discussion...as of 9:00 PM PST Monday...the weak front that 
washed out over our area today didn't produce much precipitation, 
and also did not provide much in the way of mixing to scour out 
low level moisture in the valleys. As a result, patchy fog has 
already developed this evening in the north and East Bay valleys. 
The forecast was recently updated to add fog to these valleys 
overnight and into Tuesday morning. Farther south, areas of low 
clouds have developed from the Santa Clara Valley southward. These 
clouds will likely persist into Tuesday morning. 


After morning fog and low clouds clear, expect a mostly sunny day 
on Tuesday with high temperatures very close to normal. 


Current infrared satellite imagery shows a frontal system off the coast 
of the Pacific northwest. This system is forecast to move to the 
ESE, with the bulk of its precipitation expected to fall over the 
Pacific northwest and far northern California. However, the 00z 
NAM clips the far northern portion of Sonoma County with the 
trailing cold front and some light precipitation on Tuesday night. 
The 00z GFS keeps all rain to the north of our area, which is 
similar to the 12z European model (ecmwf). Current forecast looks reasonable with 
only slight chance probability of precipitation in the far northwestern corner of our 
forecast area on Tuesday night. 


A shortwave ridge is forecast to build over California by 
Wednesday and this ridge will provide our area with continued dry 
conditions and slightly warmer daytime temperatures on Wednesday 
and Thursday. Nights will remain seasonably cool and patchy night 
and morning valley fog is likely to persist. 


The potential for more widespread rainfall develops on Friday 
when a Pacific storm system sweeps in from the northwest. But 
there are some important model differences regarding the Friday 
system. The 00z GFS produces light to moderate rain across the 
North Bay on Friday, but then quickly washes out the front as it 
advances south of the Golden Gate late on Friday and brings no 
rain to areas south of San Mateo County. The 12z ECMWF, on the 
other hand, holds the Friday system together longer, bringing at 
least light rain south through most of our forecast area by late 
Friday. Not surprisingly, model quantitative precipitation forecast from these models are quite 
different, with the GFS forecasting a quarter inch or less in the 
North Bay, and little or nothing for the rest of our forecast 
area, while the European model (ecmwf) forecasts widespread rainfall amounts of a 
half inch, with locally more in the North Bay. If the 00z European model (ecmwf) 
trends toward the drier GFS solution, we will need to lower rain 
chances across the southern portion of our forecast area on 
Friday and Friday evening and lower quantitative precipitation forecast everywhere. 


The potential for more widespread and significant rainfall 
increases by late in the upcoming weekend. A stronger system with 
a decent moisture tap is forecast to sweep across our area from 
northwest to southeast on Sunday and Sunday evening. The models 
agree quite well on timing, but the European model (ecmwf) is forecasting greater 
rainfall amounts compared to the GFS. Preliminary rainfall 
estimates from the Sunday system range from less than a half inch 
in the southern interior portions of our area to 1.5 inches in the 
coastal mountains. The Sunday system could also produce at least 
a brief period of gusty winds. 


Longer range models indicate that we could see several days of 
dry weather next week as an upper ridge builds over central and 
Southern California. 


&& 


Aviation....as of 10:27 PM PST Monday...VFR except LIFR-vlifr in 
valley fog, locally dense tonight and Tuesday morning. Fog is very 
locally limiting the visibility to 1/4 mile or less in the North 
Bay this evening. 


Post cold frontal residual moist air became trapped beneath low 
level inversions today, it's much drier above the 800 mb level 
on the 00z Oakland sounding. Conditions are ideal for infrared 
cooling to space while residual moist air remains trapped under a 
stable air mass thus expect additional patchy fog, visibilities 
at times less than 1/4 mile. 


00z forecast soundings for ksfo, koak, ksjc also indicate IFR is 
a possibility tonight, no major adjustments made to these tafs, 
but will need to closely monitor for possible changes in sky cover 
and visibilities overnight into Tuesday morning. Clearing could 
be a slow going Tuesday morning for ksts based on the 00z forecast 
sounding. 


Vicinity of ksfo...sky is clear, light wind. Will go with VFR 
forecast tonight, but need to monitor possibility of IFR ceiling 
during the overnight hours. 


Sfo bridge approach...similar to ksfo. 


Monterey Bay terminals...lowering ceilings at times possible 
tonight into Tuesday morning, for now tafs continue to go with 
optimistic VFR forecast. 


&& 


Marine...as of 9:43 PM PST Monday...high pressure will build 
over the eastern Pacific and far northern California through mid- 
week. North to northwest winds will be gusty over the coastal 
waters tonight through Tuesday night. Wind speeds will then 
diminish briefly late Tuesday night before increasing again to 
moderate levels on Wednesday. Larger moderate period swell will be 
arriving mid to late week, followed by larger long period swell 
by early Sunday. 


&& 


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...pt pinos to pt Piedras Blancas 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 


&& 


$$ 


Public forecast: dykema 
aviation/marine: canepa 


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