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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 170534

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1034 PM PDT Sat Mar 16 2019

.SYNOPSIS...High pressure will keep dry and mild conditions in
place through the remainder of the weekend and through at least
midday Tuesday. A Pacific frontal system will then bring rain 
chances back to the area by late Tuesday and Wednesday along with 
cooler temperatures for the second half of the week. Another 
trough with additional rainfall is likely late next week. 


.DISCUSSION...As of 8:55 PM PDT Saturday...The gradual warming 
trend that began across our area at midweek continued today as a 
ridge of high pressure strengthen along the West Coast. High 
temperatures today mostly ranged from the upper 60s to mid 70s and
were from 5 to 10 degrees warmer than normal. Downtown San 
Francisco recorded a high temperature of 71 today. This was the 
first time since November 8 that San Francisco was 70 degrees or 
warmer (128 days). King City was the warmest location in our 
forecast area today with a high of 81.

No significant changes are forecast to occur through Monday. The
upper ridge will continue to amplify along the West Coast,
resulting in continued dry and mild weather conditions. Skies will
be mostly clear with only occasional high clouds. We may also see
some patchy night and morning low clouds near the ocean. Model 
guidance forecasts a continued gradual warming trend through 

By late Tuesday the upper ridge is forecast to shift far enough to
the east to allow a negatively-tilted upper trough to reach
California. The models have come into better agreement with this
system and the consensus now is that the upper trough will
approach the south-central California coast by Tuesday evening 
and then develop into a closed low by the time it reaches the 
southern San Joaquin Valley by Wednesday afternoon. Showers may 
begin in our southern coastal areas by late Tuesday afternoon or
evening, but it appears the best chance of shower activity will 
be from late Tuesday night through Wednesday evening as the low 
slowly moves inland. Scattered showers will linger through 
Thursday morning, and perhaps into Thursday afternoon when our 
area will be under a deformation zone to the northwest of the 
departing upper low. 

Rainfall amounts with the midweek system (Tuesday night through
Thursday) are expected to be mostly light, with perhaps moderate 
amounts across the southern portion of our area. A tenth to a half
inch of rain is expected across the San Francisco Bay Area and 
Santa Cruz County, with a half inch to as much as an inch across 
Monterey and San Benito Counties. The deterministic GFS forecasts 
as much as 1.75 inches of rain through Thursday afternoon across 
southern Monterey County, although it should be noted that most of
the GFS ensemble members forecast considerably less than that.

Dry weather is expected to return to all areas by Thursday night as
a shortwave ridge develops briefly over California. But then the 
next system is forecast to quickly arrive from the west and bring 
renewed rain chances Friday and Friday night. The system due in 
late next week is expected to bring more widespread rainfall, as
well as heavier rain across the northern part of our area 
compared to the midweek system. But the Friday/Friday night system 
will move through quickly and so overall rainfall amounts look to 
be light to moderate once again. 

The longer range models maintain an unsettled pattern into next
weekend and beyond, but disagree on timing of the next system
which may arrive as early as Saturday evening (GFS) or not until 
Monday (ECMWF).


.AVIATION...As of 10:34 PM PDT Saturday...VFR through the period.

Vicinity of KSFO...VFR. Wind west to northwest becoming light E-NE
late tonight into Sunday morning. Onshore wind resumes by mid Sunday

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. Wind light E-SE tonight, up to near 10
knots in the Salinas Valley by early Sunday morning. Onshore winds
resume Sunday late morning and afternoon.


.MARINE...As of 8:44 PM PDT Saturday...Eastern Pacific high 
pressure will remain nearly stationary and slowly weaken through 
the weekend. A long period northwest swell will arrive on Sunday 
followed by another one Tuesday increasing shoaling on bars and 
harbor entrances.





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