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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 171003

Area Forecast Discussion
National Weather Service San Francisco Bay Area
303 AM PDT Mon Jun 17 2019

.SYNOPSIS...Warmer temperatures are forecast for today and Tuesday  
as high pressure strengthens over California, and onshore flow 
weakens. However, warming at coastal areas will be limited due to 
persistent onshore winds. Seasonable temperatures, along with 
areas of night and morning low clouds, are forecast through the 
end of the week and into next weekend.


.DISCUSSION...As of 3:00 AM PDT Monday...Low clouds have developed
well inland, and into most valleys, once again this morning as 
the marine layer remains relatively deep (2000 feet currently at 
Fort Ord). Patchy drizzle is occurring in coastal areas, but
drizzle is not as widespread, or generating as much measurable
precipitation, compared to early yesterday morning.

The marine layer is expected to compress today as an upper ridge
over the Eastern Pacific starts to build inland across northern
California. Also, surface onshore pressure gradients are trending
weaker. These developments are expected to result in earlier
inland clearing of low clouds today as well as warmer 
temperatures. Today's high temperatures are forecast to be 5 to 
10 degrees warmer than yesterday in most areas, which would bring 
our temps back to about seasonal averages. Persistent onshore flow
and only partial afternoon clearing near the ocean will mean 
warming near the coast today will only be about 5 degrees, or 
less. The most robust warming will occur in the hills and inland 
valleys. The National Blend of Models (NBM) has been too warm over
the past few days, and this warm bias by the NBM appears to be 
continuing. Therefore, maximum temperatures for today, and through
much of the week ahead, have been adjusted downward from the NBM 
by an average of about five degrees. 

Another area where the NBM appears to be deficient is in how it's
handling marine layer clouds. Specifically, low clouds are more
widespread at present than indicated by the NBM, and the NBM is
indicating a significant reduction in night and morning low
clouds as the week progresses, something that doesn't appear to be
rooted in reality given the persistent onshore flow depicted by 
the models. Therefore, cloud cover has been increased, 
particularly in coastal areas, over what is forecast by the NBM. 

Warmer temperatures are expected to persist into Tuesday. In 
fact, we will likely see a few additional degrees of warming on 
Tuesday as H5 heights increase to 588 dm and 850 mb temps warm to 
25 deg C. However, temperatures at the surface are not expected to
warm anywhere near the record-breaking heat of a weak ago when 
offshore flow developed. By contrast, onshore flow is expected to 
persist this week which will keep surface temperatures in check, 
particularly in coastal areas.

An upper trough currently in the Gulf of Alaska is forecast to
slide to the southeast and into the Pacific Northwest by midweek.
This will likely result in slight cooling on Wednesday, and a few
additional degrees of cooling on Thursday as the trough digs into
the northern Great Basin, forcing the ridge to retreat offshore.
Temperatures are then forecast to warm slightly at the end of the
week and into the early part of the weekend. But beyond that, both
the GFS and ECMWF ensemble means indicate that an upper trough 
will be centered along the West Coast, which will likely mean 
cooler than normal temperatures late in the weekend and into the 
first part of next week. 

&& of 10:45 PM PDT Sunday...The strong north-south 
component of the onshore gradient has kept stratus out of SFO so 
far this evening but satellite image shows it is expanding inside 
the bay. Therefore look for low clouds to spread over SFO Bay 
overnight. Ft Ord Profiler shows the marine layer is trending 
lower so the cloud bases will be in the IFR range. The clouds 
should also burn off earlier on Monday. As an upper level ridge 
builds into the Pacific Northwest Monday night the flow above the 
inversion will become more offshore. This will bring drier air 
down to the surface resulting in less clouds for Monday night.

Vicinity of KSFO...IFR cigs after 06Z with bases 800-1000 feet. 

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...IFR cigs clearing after 18Z at SNS and
19Z at MRY. 

&& of 10:40 PM PDT Sunday...High pressure off the
Pacific Northwest will keep the strongest  winds over the northern
outer waters through next week.


     .Tday...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 PM




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