Area forecast discussion 
National Weather Service San Francisco Bay area 
1051 PM PDT Monday Apr 27 2015 

Synopsis...a dry cold front will approach the coast resulting in 
increasing onshore flow and a cooling trend Tuesday and Wednesday. 
Dry and seasonable weather is then forecast from Wednesday through 
the upcoming weekend. 

&& of 8:55 PM PDT Monday...after a warm day that 
produced a record high at our downtown Oakland site of 82 degrees 
there is an impressive marine push under way. Late this afternoon 
the marine layer started to envelop the coast well ahead of a 
fairly strong surface cold front and upper trough. Earlier this 
afternoon the Sonoma County Airport reported a high of 89 degrees 
and its currently 55 degrees or 34 degrees of cooling in just a 
few hours! Even the city warmed to 76 degrees and is now down to a 
blustery 51 degrees with 24 miles per hour winds at the Golden Gate and up to 
30 miles per hour at ksfo. Its under these shallow marine layers (currently 
at about 1000 feet but deepening) and during the initial push of 
marine air that we often see some gusty winds along the coast and 
through the coastal gaps such as San Bruno. Cant help but notice 
with our SST temperatures back down in the upper 40s and lower 50s these 
marine pushes are behaving a little more like we used to observe 
and have some gusto as they push onshore. 

Anyway...this obviously signals the beginning of a cooling trend 
for Tuesday as a potent but dry front moves through northern 
California and ramps up the onshore gradients and associated winds 
off the ocean for Tuesday. Will have to see how deep the marine 
layer gets but there should be enough cooling aloft and mixing in 
the low-levels to preclude any all day gray inside the Bay for Tuesday. 

Conditions should moderate a little bit on Wednesday but still keep 
gusty onshore winds blowing Wednesday near the coast and inside 
the bays. 

The surface pattern will push a thermal trough towards the coast 
late Wednesday night into Thursday morning. This will allow dry 
northeast winds to develop in the hills. Expect a sharp upward 
climb in temperatures for Thursday with offshore flow and drying 
winds. The European model (ecmwf) MOS is picking up on this and shows a 90 for 
Santa Rosa on Thursday and 93 at Concord. Offshore pattern may 
persist into Friday but not as strong. 

An upper trough may try and develop over northern California this 
weekend keeping temperatures near normal with dry weather. 


Previous of 3:00 PM PDT Monday...sunny across almost all 
of our County Warning Area today with temperatures running 8 to 15 degrees ahead of 
yesterday at this time. Look at the latest readings show San 
Francisco at 73...San Rafael 79...Livermore 84...and San Jose plus 
Salinas at 80. The westerly surface gradient which had been nearly 
neutral has been slowly increasing through the day to weak 
westerly while at the same time the northerly gradient has been 
cut in half. Along with the inland heating...those factors have 
allowed clouds to move to many coastal spots along San 
Mateo...Marin...and Sonoma counties. Also received a report of 
less than 1 mile visibility at the farallones due to fog. 

For tonight...with the westerly gradient expected to increase and 
a large area of clouds upstream expect clouds along the coast plus 
adjacent valleys. Will likely get reports of fog during the 
morning commute particularly along Highway 1. At the same time an 
upper level trough will advance to the pacnw/British Columbia coast by tomorrow. 
Although all associated moisture with this feature will stay well 
to our will help to drop temperatures locally by more than 10 

Quiet weather will continue for the remainder of the week as the 
flow becomes weakly zonal with the main storm track well to the 
north. Temperatures will warm back close to the values we saw 
today by Friday. Still no chance of rain at least into the first 
part of next week. 
&& of 10:41 PM PDT Monday...the Bodega Bay and Fort 
Ord profilers show a well defined marine layer that has deepened 
to approx 1200-1300 feet. The middle-level ridge will gradually 
weaken through Tuesday night but in the interim should continue to 
hold the marine layer inversion together fairly well. Gusty west-northwest 
surface winds are temporarily subsiding late this evening after 
advecting the stratus and fog in a little farther than previously 
expected. Inland areas are still clear however. On Tuesday the 
diurnal land-ocean temperature difference will likely again set up for a 
windy day...probably more widespread than today...with the 
accompanying inland surge of stratus and fog. Gusty west-northwest winds 
are expected to return again Wednesday afternoon and evening. 

Vicinity of mentioned the stratus surged in a bit farther 
than expected by late this evening. Recent observation indicate broken IFR ceiling 
occasionally mixing back to scattered. Overall the forecast is for 
occasional IFR/MVFR ceilings overnight then a return of strong and gusty 
west winds Tuesday afternoon and evening. 

Sfo bridge approach...VFR except a few brief patches of stratus are 
possible Tuesday morning. 

Monterey Bay area terminals...IFR ceilings. Expecting the stratus clouds 
to mix out by late Tuesday morning but most likely poised to move back 
in early Wednesday evening. 

&& of 10:11 PM PDT Monday...high pressure will build over 
northern California through middle-week resulting in a period of gusty 
northwesterly winds across the coastal waters including the San 
Francisco Bay waters. In addition...northwest swell will build through 
midweek. Gusty northwest winds will continue over the northern outer 
waters late in the week. 


Mtr watches/warnings/advisories... 
... Arena to pt Reyes 0-10 nm 
Small Craft Reyes to Pigeon pt 0-10 nm 
Small Craft Arena to pt Reyes 10-60 nm 
Small Craft Reyes to Pigeon pt 10-60 nm 
Small Craft pinos to pt Piedras Blancas 10-60 nm 
Small Craft pinos to pt Piedras Blancas 0-10 nm 



Public forecast: rww 
aviation/marine: canepa 

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