Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 180623

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1023 PM PST Wed Jan 17 2018

.SYNOPSIS...A weather system approaching form the northwest will
spread rain across the North Bay late tonight and Thursday morning
and then southward across the remainder of the forecast area by
Thursday evening. Showers are forecast for Thursday night and Friday
as much colder air settles over the region. A few showers may
linger into Saturday morning, but for the most part Saturday
should be a dry day. The next front will bring rain chances late
Sunday into Monday.


.DISCUSSION...As of 9:15 PM PST Wednesday...IR satellite imagery
shows a cold frontal boundary approaching the northern California
coast this evening. Latest HRRR and NAM models both forecast light
rain to begin spreading into northwest Sonoma County shortly after
midnight tonight, and then develop across much of the North Bay
by the morning commute. Rain is then forecast to develop in San
Francisco by late morning, across the rest of the SF Bay Area by
mid-afternoon, and then into the Monterey Bay Area by late
afternoon. Rain is expected to change to scattered showers by
Thursday evening. Both the 00Z NAM and 00Z GFS then forecast
little additional rainfall from late Thursday evening through
about sunrise Friday. Rainfall amounts through Thursday night are
expected to be light in most areas, generally less than a quarter
of and inch. The exception is northern Sonoma County which is
forecast to pick up between a half inch and an inch of rain,
according to the latest NAM.

Scattered showers are expected to redevelop on Friday morning as
the cold upper trough approaches from the northwest. Showers will
then taper off Friday afternoon and evening. The incoming system
will usher a much colder airmass into our area from late tomorrow
through Friday. 850 mb temperatures are forecast to drop from
about 13 deg C at the present time to -2 deg C by Friday morning.
Snow levels will drop as low as 3000 on Friday, but precipitation
will be light and scattered by then, so no more than a dusting of
snow is expected on higher peaks. Isolated thunder is possible
late Thursday night and Friday, mainly over our northern coastal

Isolated showers may linger into Saturday morning, but for the
most part Saturday should be a dry and cool day as the upper
trough moves off to our southeast and weak shortwave ridging
develops along the California Coast. Based on latest model data,
most of Sunday should be dry as well.

The next system arriving from the northwest is expected to spread
rain across our region from late Sunday through early Monday. This
system will move through quickly and is also expected to generate
only light rainfall amounts, except locally moderate in the North

Yet another fast-moving system is forecast to arrive from the
northwest around the middle of next week.

&& of 8:50 PM PST Wednesday...for 06z TAFs...Cigs are
quickly filling in across area terminals this evening resulting in
decreased vsbys. Low cigs and vsbys are anticipated through the
night with light winds prevailing. An approaching cold front will
introduce rain to the region beginning in the North Bay early
Thursday morning and spreading south across the entire Bay Area
through the afternoon. Showers are anticipated to linger through
the evening. Expect wet runways through the morning and afternoon

Vicinity of KSFO...LIFR to VLIFR conditions will prevail through
17Z Thursday with vsbys down to 1/2SM-2SM and fog. Drizzle
possible as early as 13z with -RA beginning around 17z with MVFR
cigs expected. Light winds.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions will lower to LIFR/VLIFR
overnight. Patchy fog expected. Rain will begin around 22z-23z
Thursday afternoon. Light winds.

&& of 09:52 PM PST Wednesday...A large long period
westerly swell generated by a strong low  pressure system
currently off the Pacific Northwest coast will  impact the coastal
waters beginning tonight and continuing through the end of the
week. This swell will bring hazardous seas to the  region as well
as large breaking waves to the coastline through  Friday.


     .Tngt...High Surf Warning...Entire coast
             SCA...Mry Bay from 3 AM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 3 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm from 10 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 3 AM
             SCA...Pt Arena to Pt Reyes 0-10 nm from 12 AM
             SCA...Rough Bar Advisory for SF Bar
             SCA...Pt Reyes to Pigeon Pt 0-10 nm from 3 AM




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