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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 221012

Area Forecast Discussion
National Weather Service San Francisco Bay Area
312 AM PDT Mon Apr 22 2019

.SYNOPSIS...High pressure building over the region will result in 
a warming and drying trend through midweek. Temperatures will warm 
well above seasonal averages by Tuesday when light offshore flow
develops. Temperatures will then moderate and cool slightly later
in the week as the ridge of high pressure weakens.


.DISCUSSION...As of 3:10 AM PDT Monday...Areas of low clouds and
fog persist early this morning, mainly along the San Mateo County
Coast and near Monterey Bay. Very little increase in low cloud 
coverage has occurred since last evening. The latest ACV to SFO 
surface pressure gradient is at nearly 7 mb, so it's unlikely we 
will see much inland development of low clouds this morning, 
especially in the SF Bay Area. Patchy low clouds may yet develop 
inland from Monterey Bay by sunrise. Increasing dry northerly 
flow near the top of the boundary layer should hasten clearing of 
low clouds this morning, leaving mostly clear skies across our 
region for the remainder of the day. Also, onshore flow will be 
weaker today due to a building surface ridge to our north. 
Meanwhile, at the upper levels, a ridge currently centered just 
offshore along 130W is forecast to build eastward and over 
California by the end of the day. These developments will result 
in warmer temperatures today compared to Sunday, with about 5 
degrees of warming expected near the coast and up to 10 degrees of
warming inland.

The warming trend will continue as we move into Tuesday. The 
upper ridge is forecast to continue strengthening over California 
and light offshore flow is expected to develop by Tuesday 
morning. Patchy fog and low clouds are still possible tonight and 
Tuesday morning, but only close to the ocean. Up to an additional 
10 degrees of warming is likely region-wide on Tuesday. Forecast 
highs on Tuesday are mostly in the 70s near the ocean, with 80s 
and lower 90s inland. Temperatures on Tuesday are expected to be 
similar to what our region experienced this past Thursday, except 
possibly a bit warmer for inland areas.

The upper ridge will remain firmly in control of our weather into
Wednesday. However, the ridge axis is forecast to shift to our
east on Wednesday and so we will likely see a return of light 
onshore flow by afternoon. This will result in slight cooling at
the coast and in some coastal valleys, but the inland valleys and
hills are expected to be as warm as Tuesday, if not slightly 
warmer. During this upcoming warm-up, overnight temperatures are
expected to cool off enough to keep Heat Impacts mostly in the low
risk category. Only some of the warmest inland areas are 
projected to see Heat Risks rise into the moderate category on 
Tuesday and Wednesday.

Onshore flow is expected to increase sufficiently by Thursday to 
result in modest cooling across our entire region. Little change 
is then expected going into Friday. Even with the forecast cooling
late in the work week, temperatures are expected remain at least 
slightly warmer than normal.

The forecast becomes less straightforward as we move through next
weekend. Both the GFS and ECMWF indicate that an upper low will 
approach the southern California coast during the weekend and move
inland south of Point Conception by late Sunday or Sunday night.
The 00Z deterministic GFS develops scattered light precipitation
across the far southern portion of our forecast area from late
Saturday night through Sunday night while the deterministic ECMWF
keeps all precipitation to our south and southeast. Some ensemble
members of both the GFS and ECMWF develop very light amounts of
precipitation in our area by late in the weekend, mainly across
the south. Currently, more than half of these models' ensemble 
members develop no rain or just trace amounts through the weekend,
so will keep the mention of rain out of the forecast for now.
Temperatures over the weekend are expected to be near or slightly
warmer than seasonal averages.

Longer range models indicate increasing rain chances next week as
a cold upper trough drops in from the north. 


.AVIATION...As of 10:51 PM PDT Sunday...Evening satellite shows a
strong intrusion of low clouds along the San Mateo coast and the 
Monterey Peninsula. As high pressure builds over the area tonight,
the inward extent of the low clouds should remain with limited 
inland penetration. Therefore, expect most terminals to remain 
generally VFR. If clouds form, they will form early and dissipate 
early morning or shortly after sunrise. That said, where there are
terrain gaps, low clouds will have access to fill in to that 
valley and possibly spill into other portions of the Bay Area. 
Regardless, by the time the sun comes up the main air terminals 
should be VFR. Coastal low clouds will develop again Monday night.

Vicinity of KSFO...VFR. Low clouds are trying to squeak into SFO,
but the building high pressure and developing offshore flow are
expected to keep these limited. If clouds do thicken up, they will
dissipate late in the night with conditions expected to go VFR
right around sunrise. Monday night will likely be very similar to

SFO Bridge Approach...VFR is expected through the night as the low
level clouds should not be able to make it around the corner far
enough to impact SMB.

Monterey Bay Terminals...IFR. There is an outside shot that with
the building ridge the marine layer becomes compressed and fog
becomes an issue. This is not expected at this time as a departing
upper level low over Southern Cal should keep the low level
atmosphere mixed enough to lift the marine layer off the surface.
Thus look for low clouds tonight, VFR Monday, and some fog Monday


.MARINE...As of 3:10 AM PDT Monday...High pressure off the 
California coast will bring moderate to strong northwest winds 
through late tonight. As the high shifts north on Tuesday the 
winds will remain strong over the northern waters but will 
decrease over the central and southern waters. 


     .Tday...SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Mry Bay until 3 AM




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