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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 221754

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1054 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.

&& of 08:57 AM PDT Monday...Low clouds that
developed over the Monterey Bay overnight penetrated well inland
into the Salinas Valley early this morning. Meanwhile, the
remainder of the region remained under mainly clear sky
conditions. Stratus that did develop will continue to dissipate
through the morning as surface temperatures rise. Look for mainly
clear sky conditions region-wide this afternoon with a warming
trend beginning today as high pressure builds inland. With this,
no updates are anticipated for this morning. For additional 
information, please see the previous forecast discussion below. 


.PREV 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 10:54 AM PDT Monday...for 18Z TAFs. With the 
exception of some localized low clouds along the immediate coast 
from San Mateo County southward, VFR conditions are forecast to 
prevail throughout the region today as high pressure builds over 
the region. Light winds this morning will become onshore by the 
afternoon hours. Think that the San Francisco Bay Area terminals 
should remain VFR for Tuesday morning as the ridge axis is 
overhead, but the Monterey Bay terminals could see some MVFR/IFR 

Vicinity of KSFO...VFR conditions are forecast to prevail through
today and the evening hours. Light easterly winds this morning 
will become onshore by mid-day around 15-20 kt.

SFO Bridge Approach...Same as KSFO.

Monterey Bay Terminals...VFR conditions forecast through the day
with some localized low clouds along the immediate coast. Think
there will be some areas of lower ceilings around MRY and SNS 
overnight and into Tuesday morning. Models are hinting at a drier 
boundary layer compared today, so it's quite possible MRY and SNS
could even remain VFR. We'll continue to monitor the latest 
incoming guidance and adjust the TAFs accordingly.

&& of 08:57 AM PDT Monday...Moderate to locally strong 
northwest winds will prevail through the day as high pressure 
remains over the eastern Pacific. 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 Arena to Pigeon Pt 10-60 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm




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