fxus66 kmtr 230535 

Area forecast discussion 
National Weather Service San Francisco Bay area 
1035 PM PDT Sat Sep 22 2018 

Synopsis...the marine layer will remain entrenched along the 
coast tonight and Sunday as a weak upper level trough approaches. 
Then as it passes through, high pressure will begin building 
inland Sunday night. This will bring offshore flow and a noted 
warming trend, along with fire weather concerns for the North Bay 
hills early in the week. Temperatures may cool again by about 
Friday as the next upper level trough approaches. 


Discussion...as of 9:15 PM PDT Saturday...latest fog product 
satellite imagery shows stratus blanketing the coast from around 
the Golden Gate southward, and already extending well down the 
Salinas valley. Afternoon highs around the bays were generally 4 
to 8 degrees cooler than on Friday. A bit of additional cooling is 
expected for Sunday, as the tail end of a weak upper level trough 
approaches from the northwest. Current forecasts appear on track 
and no updates are presently anticipated. 


Previous discussion...as of 11:50 am PDT Saturday...Fire Weather 
Watch was issued earlier this morning for the North Bay mountains 
to be in effect from 11 PM Sunday through Tuesday. In the short 
term the marine layer remains around 1400 feet along the coast 
with lunch time trends showing most inland areas running about 10 
degrees cooler than yesterday while smaller cooling trends are 
noted at the coast. Look for mostly sunny and pleasant weather 
inland this afternoon as the sun works its magic with afternoon 
readings back into the 70s to near 80. 

Incoming upper trough over Oregon will keep the marine layer in 
place overnight and allow it to potentially deepen some as it 
spreads into the bays overnight. Slightly deeper marine layer 
should mean less fog Sunday morning but perhaps more drizzle 
along the coastal slopes. Skies may be slightly slower to clear 
on Sunday with a deeper marine layer but still expect mostly sunny 
and seasonable temps for the second half of the weekend with most 
inland areas back into the 70s. 

Main forecast item of impact will be developing north winds after 
sunset Sunday night. Looks like initial shot of winds will reach 
northern Napa County before midnight. Impacts will be fire weather 
related and for now the Fire Weather Watch has been hoisted. As 
north winds continue into early Monday morning over Napa County 
the humidity values will decrease. Wind speeds look only moderate 
over the hills with sustained 10-20 mph and gusts in the 35 mph 
range. So its a marginal event but warranted given the dry state 
of fuels, time of year and duration of the event. Winds will ease 
on Monday but humidity will continue to lower as temperatures 
warm into the 80s and 90s under a classic fall offshore wind 
pattern. Latest NAM solutions show a second burst of winds late 
Monday night into Tuesday morning with humidity values showing no 
recovery and likely hovering around 15% for much of Monday 
night/early Tuesday morning. Winds will ease on Tuesday but the 
watch remains in place through the daytime burn period as hot 
temps and low humidity will continue. 

Public forecast will show seasonably warm/hot temps Tuesday and 
weds with thermal trough along the coast and light offshore flow. 
Coastal highs in the 70s with lots of 80s and lower 90s inland. 
The pattern may break down late Thursday or Friday as an upper 
trough approaches the coast. 


Aviation...as of 10:35 PM PDT Saturday...for 06z tafs. The 
marine layer remains at around 1400 ft per The Fort Ord profiler 
while satellite imagery shows stratus along much of the coast. A 
strong northerly gradient has eroded some of the stratus, making 
for an uncertain overnight forecast in terms of timing of IFR cigs 
around the Bay area and North Bay taf sites. Any cigs that do 
develop are expected to remain in place through 16z-18z tomorrow. 
Generally light winds overnight and tomorrow morning before 
onshore winds increase in the afternoon to 10-15 kt. 

Vicinity of ksfo...VFR through tonight. IFR cigs by around 10z- 
11z, although confidence is low on timing. Onshore winds tomorrow 
afternoon around 15 kt. 

Sfo bridge approach...same as terminal. 

Monterey Bay terminals...LIFR cigs overnight with possible patchy 
fog in the early morning. 


Fire weather...as of 9:00 PM PDT Saturday...Fire Weather Watch 
remains in effect for the North Bay mountains from 11 PM Sunday 
night through Tuesday afternoon. North to northeast winds reach 
Napa County Sunday night and drive humidity lower during the pre- 
dawn hours of Monday. Event looks moderate especially in terms 
of wind strength, but confidence is pretty high on pattern 
developing. Fuels remain critically dry, so any new starts will be 
susceptible to the winds, low humidities and dry fuels. Biggest 
threat area looks to be Napa County hills, northeast Sonoma County 
and around Mt tam. Winds will ease during the day Monday as 
temperatures warm up into the upper 80s and 90s inland. 

Offshore winds then look to redevelop of similar magnitude Monday 
night into Tuesday, but with an even drier air mass then moving 
in. Overnight humidity recoveries could then well be confined to 
around 20% or even less. Continued warm/hot day inland on Tuesday 
with winds easing. Pattern remains warm and dry for much of next 


Marine...as of 08:15 PM PDT Saturday...northwest winds will 
gradually increase across the coastal waters through tomorrow. The 
strongest winds are expected north of Point Reyes. Winds will 
begin to subside into the start of the work week and remain 
relatively light through the forecast period. Mixed swell will 
continue as well, with a shorter period moderate height northwest 
swell and a weak, longer period southerly swell. 


Mtr watches/warnings/advisories... 
... Pt to pt pinos 0-10 nm from 9 am 
Small Craft Advisory...pt Arena to pt Reyes 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 
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm from 9 am 
Small Craft Advisory...mry Bay from 2 PM 
Small Craft Advisory...sf Bay until 11 PM 



Public forecast: blier/rww 
aviation: as 
marine: as 
fire weather: blier/rww 

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