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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 190534

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1034 PM PDT Thu Apr 18 2019

.SYNOPSIS...A cooling trend will begin Friday, though with inland
temperatures still well above seasonal norms. More significant 
cooling is then expected over the weekend as onshore flow 
deepens and strengthens and an upper level low pressure system 
passes through. Can't rule out a chance of some sprinkles or even
light rain, especially Saturday night into Sunday morning coast 
and coastal hills south of the Golden Gate. A renewed warming 
trend appears in store for the first part of next week.

&& of 09:30 PM PDT Thursday...Latest fog product 
satellite imagery show marine stratus now blanketing the coast and
starting to work its way inland around Monterey Bay. Webcam 
imagery from the East Bay hills earlier this evening showed a 
shallow narrow strip of stratus extending in through the Golden 
Gate and across to the North Berkeley/Albany area, but no 
signature in present 1 km resolution satellite images. Marine 
layer depth is pretty restricted. Fort Ord profiler data put the 
inversion lid at about 800 ft, and it's even lower than that at 
both Bodega Bay and Pt Sur. Otherwise skies are generally partly 
cloudy, reflecting a long plume of high cloudiness which will 
continue to pass through overnight.

Although not reaching record levels, many inland locations did
achieve their warmest temperatures thus far in 2019. It's also 
the first time this year that we've hit the 90 degree mark in our 
CWA, albeit just barely and at only a few locations in the 
southern interior. Both the Bradley and Arroyo Seco RAWS sites 
reported highs of 90 deg, and King City just missed at 89. Along 
portions of the coast however, temperatures remained cool -- as a 
shallow marine layer persisted in the absence of any appreciable 
offshore flow. Today's high of 57 at Half Moon Bay Airport was 
exactly the same as on Wednesday.

The return of widspread coastal stratus marks the start of a
cooling trend, though with inland temps still well above seasonal
norms. The cooling trend will continue into the weekend as an
upper level trough drops down the coast from the Pacific NW. 
Latest model output is in good agreement in moving the associated 
vort max through the northern portion of our district in a late 
Saturday afternoon time frame. 850 mb temps then look to bottom 
out around 5 deg C, about 12 deg C cooler than this afternoon! So
even the aforementioned hot spots in our southern interior today 
are then likely to be confined to no more than the lower 70s. 

Some of the new model runs are also developing a sufficiently deep
and moist boundary layer, especially coastal areas south of 
the Golden Gate Saturday evening into Sunday morning, to wring 
out some precip along the coast and over the coastal slopes. This 
may need to be added in to the next forecast package.

In the extended...renewed upper level ridge development along the
coast will lead to mostly clear skies and a warming trend for the
first part of next week.

&& of 10:35 PM PDT Thursday...Stratus has expanded 
along the coast and into the MRY Bay Area. The marine layer is 
very shallow at around 1000 feet so vsbys will drop to under 3 
miles at times. Right now the marine layer is too shallow to 
spread into the SFO Bay Area but a weak upper low off the southern
California coast will trigger a deepening of the marine layer on 
Friday. Even if the deepening occurred earlier than expected 
overnight a south to southwest flow above the surface will act as 
a deterrent to stratus getting into SFO and the approach. The 
latest forecast keeps VFR conditions in the Bay Area tonight. The 
North Bay Valleys could see some light fog. 

Vicinity of KSFO...VFR through Friday. 

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...IFR cigs with vsbys dropping below 3
miles after 09Z. Clearing after 16Z. 

&& of 08:55 PM PDT Thursday...High pressure developing 
off the Pacific Northwest and northern California coast will 
cause increasing northwest winds over the northern waters on 
Friday. Stronger winds will also be possible along coastal jets 
along the Big Sur coast. 






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