fxus66 kmtr 171724 

Area forecast discussion 
National Weather Service San Francisco Bay area 
issued by National Weather Service Los Angeles/Oxnard California 
910 am PST Wed Jan 17 2018 

Synopsis...areas of dense fog this morning will give way to 
mostly cloudy skies as high clouds increase ahead of the next 
Pacific front. Light rain will start in the North Bay Thursday 
morning and then spread southward through the Bay area and central 
coast by Thursday evening. Rain will quickly turn to showers 
Thursday night into Friday as much colder air settles over the 
region. Saturday morning will start out chilly but skies should be 
mostly clear and dry for the start of the weekend. The next front 
approaches later Sunday into Monday with the next chance of rain. 



Discussion...as of 09:10 am PST Wednesday...areas of fog and low 
clouds continue this morning, but will likely lift and dissipate 
for most areas later this morning. Otherwise the forecast in the 
near term is on track. Mid to high level clouds will continue to 
stream across the region, out ahead of an approaching cold front 
that was about 500 miles northwest of the Bay area. Similar high 
temperatures are expected for today as yesterday with highs 
generally near 60 to 70 degrees. Please see the previous forecast 
discussion below for additional details. 


Previous discussion...as of 3:25 am PST Wednesday...short term concern is 
dense fog being reported throughout the region. 3 am obs include 
dense fog at Napa, Concord, Novato, Monterey, Salinas and Half 
Moon Bay. Fog product is starting to become obscured as high 
clouds pass overhead but dense fog appears to be over the coastal 
waters and down the Salinas valley. Heads up for the early morning 
commuters across the region. 

Similar to the last few days once any morning fog clears skies 
will be partly to mostly cloudy today as mid and high clouds 
continue to come in off the Pacific ahead of the incoming trough 
and surface cold front. Upshot is that today remains dry with 
highs upper 50s to mid 60s most areas. 

Long anticipated cold front looks to arrive on Thursday. In terms 
of bringing significant rain, have to say the latest model trends 
have been drier. The system will have plenty of cold air 
associated with it and not much of a moisture tap (no talk of 
atmospheric rivers lately) as the main upper trough and cold air 
are dropping down from the Gulf of Alaska. 

Following the latest model runs we expect light rain in the North 
Bay Thursday morning then reaching the greater Bay area 
midday/afternoon before washing out over the central coast by 
Thursday night. The fairly fast moving nature of the surface front 
will preclude significant rainfall with totals in the 0.25-0.75 on 

Steady precip looks to shut off rapidly on the backside of the 
front with Post-frontal showers more favorable over the Sierra, 
our coastal hills and over the ocean due to the warmer SST. Anyway 
the forecast will keep shower chances going on Friday but there 
will be plenty of sunny breaks and a bite in the air as daytime 
highs struggle to reach the mid 50s. 

Friday night will feel cold as temps aloft will plummet leading to 
widespread 30s and lower 40s by sunrise Saturday morning. The 
start of the weekend looks dry but cool with highs in the mid 50s 
for Saturday with an upper trough aloft. 

Latest trends for the next system show rain moving into the North 
Bay Sunday night and then dropping through the Bay area and 
central coast for the Monday morning commute. Initial signals 
suggest another light to low end moderate rain event as the front 
will move through quickly. 

General consensus suggests one final system around midweek then 
high pressure looks to build heading into the last weekend of 


Aviation...as of 4:30 am PST Wednesday...metar observations show 
fog is limited to the coast and the inland valleys. Low cigs cover 
much of the sfo Bay area but so far vsbys have remained 5 miles or 
greater. Cloudcover should prevent dense fog from developing at 
sfo or sjc but the lack of clouds at Oak may allow vsby there to 
drop lower possibly down to 1-3 miles. Models indicate more drying 
in the afternoon so cigs should burn off after 20-21z. Warm front 
approaches late tonight with some pre-frontal rains beginning in 
the North Bay after 06z spreading south during the night. Winds 
will remain light. 

Vicinity of ksfo...MVFR through 21z with vsbys down to 3-5 miles 
at times. 

Sfo bridge approach...similar to ksfo. 

Monterey Bay terminals...some clear skies are mixed with fog 
around mry as the drainage wind struggles to develop. Looks like 
fog will be in and out of mry this morning so latest forecast 
calls for tempo dense fog through 16z. Dense fog could possibly 
linger on a little longer. For sns dense fog is expected through 
18z then lifting to IFR cigs. Clouds becoming sct after 20z as 
models forecast drier air to move in. 


Marine...as of 09:10 am PST Wednesday...high pressure to the 
west will shift south as a low pressure system approaches the 
Pacific northwest. A frontal system associated with the low will 
move through the area on Thursday. Gusty southerly will form 
through tonight, then shift quickly to westerly on Thursday, 
before becoming northwesterly by Friday. A large long-period swell 
will bring dangerous breaking waves to the coastline tonight 
through at least Friday. Dense fog with low visibility will impact 
some areas today. 


Mtr watches/warnings/advisories... 
... Arena to Pigeon pt 10-60 nm 
Small Craft Advisory...rough bar advisory for sf bar 



Public forecast: rm/rww 
aviation: west pi 
marine: rk 

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