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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 181805

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1105 AM PDT Tue Jun 18 2019

.SYNOPSIS...Strengthening high pressure will continue a warming 
trend today. However, onshore surface winds will persist, keeping
temperatures seasonably cool at the coast. Seasonable 
temperatures, along with areas of night and morning low clouds, 
are forecast through the end of the week and into the upcoming 

&& of 09:06 AM PDT Tuesday...No changes needed to
the ongoing forecast this morning. Stratus continues to quickly
dissipate over inland areas as surface temperatures begin to 
warm. Strong and gusty winds mentioned in the previous forecast 
discussion have also diminished except for in the highest peaks of
the North Bay and East Bay where gust of 30 to 50 mph continue to
be reported. For full details, please see the previous forecast 
discussion below. 


.PREV DISCUSSION...As of 3:05 AM PDT Tuesday...The marine layer has
compressed slightly since yesterday as an upper level ridge over 
the Eastern Pacific builds inland across Northern California. 
Meanwhile, surface high pressure to our north over Oregon and far 
northern California is maintaining a moderate north-to-south 
pressure gradient of nearly 5 mb from ACV to SFO. The result has 
been locally gusty north winds across the higher hills of the 
North Bay. These gusty winds, in conjunction will rather poor 
overnight humidity recoveries in the hills above 2000 feet, is 
somewhat concerning from a fire weather perspective. However, 
winds in the North Bay Hills are forecast to subside by mid 
morning, and are therefore not expected to have a prolonged or 
significant impact. 

The combination of a shallower marine layer and reduced onshore
flow has resulted in less inland development of low clouds 
overnight compared to the past few nights, especially across the 
North and East Bay. Therefore, many inland areas are expected to 
experience more prolonged sunshine today. In addition, subsidence
under the building upper level ridge has warmed the airmass aloft
a few additional degrees C since yesterday. These factors suggest
that afternoon highs today will be warmer then yesterday, 
especially across inland areas. However, persistent onshore flow 
at the surface will limit the amount of warming to about 3 to 5 
degrees in most areas, and also prevent some coastal areas from 
clearing today.

Today will likely be the warmest day of the week. Slightly cooler
temperatures are forecast for Wednesday as an upper trough settles
into the Pacific Northwest and the ridge over California begins to
weaken. Further cooling is forecast for Thursday as the trough 
digs into the northern Great Basin and the ridge retreats 
offshore. There will also likely be an increase in inland 
development of night and morning low clouds over the next few 

Slight warming is expected on Friday and Saturday as the upper 
ridge builds back towards the coast. Despite the changes in 
temperatures anticipated during the forecast period, temperatures
are not forecast to vary all that much from seasonal averages 
through the week and into next weekend. 

In the longer range, the GFS Ensemble Mean and ECMWF Ensemble 
Mean both forecast an upper trough to be centered along the West
Coast next Monday and Tuesday. It therefore appears likely that
cooler than normal conditions will dominate the first half of next

&& of 11:05 AM PDT Tuesday...For 18z TAFs. Northerly 
gradient remains strong but has weakened slightly to around 4 mb 
from SFO to ACV. Marine layer about 1500-1800 ft combined with 
strong northerly gradient kept stratus out of inland locations 
such as KSTS, KLVK, and KSJC while stratus developed along the 
coast and into the Salinas Valley, and over the SF Bay for a few 
hours this morning. Satellite and surface obs reveal stratus has 
retreated to the coastline in most locations with all TAF sites 
expected to remain in VFR this afternoon. Winds this afternoon 
will become onshore and increase to around 10-15 kt. Stratus will 
expand this evening and overnight once again. A slight increase in
southerly winds along the coast will favor a return to cigs at 
STS overnight tonight. 

Vicinity of KSFO...Clearing occurred between 16-17z with VFR
expected to prevail this afternoon. Light northerly winds will
turn west-northwest this afternoon and increase to around 15kt.
Occasional gusts to 20 kt are possible. VFR expected through the
evening with MVFR late tonight through Wednesday morning. 

SFO Bridge Approach...Similar to SFO. 

Monterey Bay Terminals...Clearing has occurred at the terminals
this morning with VFR conditions through the afternoon. Winds will
become onshore and increase to around 10-15 kt this afternoon. IFR
to LIFR cigs will develop this evening and overnight tonight. May
see a slightly later return to IFR at KMRY if southerly winds keep
stratus out until closer to 06z. 

&& of 09:06 AM PDT Tuesday...Moderate and gusty 
northerly winds will continue over the northern outer waters 
through late week producing steep fresh swells. A thermal trough 
along the coast will maintain light southerly winds and seas over 
the near shore waters. Winds will increase each afternoon over the
bays as onshore flow increases.


     .Tday...SCA...Pt Arena to Pigeon Pt 10-60 nm




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