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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 181610

Area Forecast Discussion
National Weather Service San Francisco Bay Area
910 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 4:34 AM PDT Tuesday...For 12z TAFs. A robust
5 MB northerly gradient coupled with a 1800 ft marine layer has 
keep low clouds from intruding as far inland over the Bay Area as
it has the last few nights. On the other hand the northerly flow 
has pushed stratus far down the Salinas Valley resulting in what 
will likely be a late burn off over KSNS and to a lesser extent 
over KMRY. Light to locally variable winds will prevail through
the morning then ramp up in the afternoon becoming onshore around
10 to 15 kt.

Vicinity of KSFO...MVFR cigs expected to return around 12-13z 
this morning. Clearing is anticipated around 16-17z. Light 
onshore flow will prevail followed by a short period of offshore 
flow is expected between 15z-21z over KSFO. Winds are expected to
ramp up again in the afternoon to around 15 kt.

SFO Bridge Approach...Low clouds will fill in over the approach
this morning but will scatter out around 16z as offshore flow and
a shallower marine layer allow for a quick burn off.

Monterey Bay Terminals...IFR to LIFR cigs will prevail over KSNS 
with KMRY through the morning hours. Clearing is anticipated 
between 19z-22z Tuesday. Light to locally variable winds will 
prevail through the morning then ramp this afternoon becoming 
onshore around 10 kt.

&& 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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