Area forecast discussion 
National Weather Service San Francisco Bay area 
937 am PST Thursday Nov 27 2014 


..periods of rain forecast from late Friday through the middle of 
next week... 


..potential heavy rain...Hydro concerns...and strong and gusty 
winds next week... 


Synopsis...high pressure will maintain dry and mild weather 
through today. Rain will develop in the North Bay by 
late Friday and press south through most of the rest of the region 
Friday night into Saturday. Periods of rain are expected to continue 
into Sunday. More rain...possibly heavy...is then likely around 
the middle of next week. 


&& 


Discussion...as of 08:04 am PST Thursday...latest observations 
show a very moist boundary layer this morning. The ample low level 
moisture and cool temperatures led to patchy fog around the Bay 
area this morning. Visibility restrictions have been greatest 
around sf Bay...North Bay and East Bay ranging 3 to 6 miles. A few spots 
of dense fog have also been reported across the North Bay. A quick 
update to the forecast was made to include patchy fog through this 
morning. 


Otherwise...high pressure west of Baja California and low pressure 
headed toward the Pacific northwest will keep a moist westerly flow over the 
Bay area today. As a result...filtered sunshine can be expected 
this afternoon as ample middle-high level clouds stream over the 
region. Regardless of the high clouds...temperatures this 
afternoon will be pleasant for the Holiday. In fact...much of the 
region will be a few degrees above normal and in the 60s to lower 
70s. A few spots in interior Monterey and San Benito counties may 
even hit 80. 


The rest of the forecast will focus on a very active pattern 
shaping up Friday through next week. Fore specific details see 
previous discussion. Will fine tune forecast after all the 12z 
model suite comes in. 


&& 


Previous discussion...as of 3:00 am PST Thursday...after another 
clear and seasonably cool night overnight, the much anticipated 
rain event will begin across our area starting Friday evening as 
moisture associated with a cold front to our west makes its way 
down our County Warning Area along with a shortwave trough aloft. Models have been 
in excellent agreement over the past couple of runs and remain so 
this morning with the start time. Rain will begin over the North 
Bay Friday evening and progress down to sf Bay area Friday night 
into Saturday morning while the Monterey Bay region will see 
precipitation start close to noon on Saturday. In many respects the 
timing is a near Carbon copy of last weekend! Rainfall amounts 
with the first push are expected to be 1-2" over the North Bay 
with generally 1/4"-3/4" for most urban areas. 


A second round of rain will then move back to our area as another 
shortwave moves across. Model solutions are greatly diverging with 
where the focus of the rain will be. GFS is trending to the North 
Bay, European model (ecmwf) to the Monterey Bay region, the NAM is virtually dry, 
while the Gem has widespread rain with the highest amounts from 
Monterey Bay up to San Francisco Bay. Decided to trend toward the 
European model (ecmwf)/Gem solutions with the NAM entirely discounted. That line of 
thinking brings 1/2-1" for many urban spots with 1.5-2.5" around the 
Santa Cruz Mountains plus coastal ranges in Monterey County. Rain 
will end by Sunday evening. Locally gusty southerly winds can be 
expected through the weekend although they should remain under 
advisory levels. 


Beyond Sunday models start to have more disagreement on the timing 
of the moisture as the parent low progresses toward our area. The 
GFS remains the most bullish and progressive with the low and has 
it advancing to our area by Tuesday night and well to the east by 
Wednesday. In that scenario, the break from Sunday is short and is 
followed by at least two heavier bands of rainfall. The slower European model (ecmwf) 
has slowed even more from previous runs and now has the low staying 
nearly stationary and well to our west on Monday and Tuesday 
before bringing back rain Wednesday through possibly Saturday (and 
maybe beyond that). European model (ecmwf) ensembles were not available for the 
shift, so just going off the GFS ensembles shows the mean very 
close to the operational GFS suggesting that there are two very 
distinct solutions without a clear path to choose. Therefore, 
confidence in the forecast remains high for substantial rain next 
week, however the timing of the rain continues to have a low 
confidence. 


By the middle of next week rainfall totals are expected to range 
from 4-7" over the North Bay plus coastal ranges of Monterey and 
Santa Cruz counties with generally 1.5"-3.5" for many urban and 
valley locations. Locally higher amounts are possible in both 
cases. Model guidance is also all over the place for wind speeds 
due to vast differences in the timing of any systems. Will 
continue to advertise the potential for gusty winds at some point 
next week. 


Finally, although it is past the scope of the forecast package, 
the 240 hour European model (ecmwf) panel brings a sharp longwave trough toward our 
area with a long fetch of moist air from the SW in advance of it. 
If the European model (ecmwf) verifies, next weekend (especially sunday) could be 
very wet as well. 
&& 


Aviation...as of 9:37 am PST Thursday...dry airmass continues 
over the area for another day of VFR conditions. Hazy conditions 
around the San Francisco Bay area may impact sfo and Oak. 


Vicinity of ksfo...VFR. Hazy conditions today with light winds. 


Ksfo bridge approach...similar to ksfo. 


Monterey Bay area terminals...VFR. 


&& 


Marine...as of 09:32 am PST Thursday...weak high pressure off the 
Southern California coast will persist today with light and 
variable winds expected across the coastal waters. Southerly winds 
will develop Friday as a frontal system approaches the area. These 
southerly winds will increase Sunday or Monday as another storm 
system approaches California. 


&& 


Mtr watches/warnings/advisories... 
... 


$$ 


Public forecast: mm/Bell 
aviation/marine: Sims 


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