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fxus66 kmtr 180515 

Area forecast discussion
National Weather Service San Francisco Bay area
915 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.

&& of 9:15 PM PST Wednesday...infrared 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.


Aviation...(issued by weather forecast office eureka) as of 2:00 PM PST Wednesday...all Bay
area terminals showing subtle improvement from this mornings fog and stratus
induced reduction of vsbys and cigs. However the North Bay and sfo still
plagued by mfvr conditions with no immediate relief expected but still an
improvement from what was observed earlier this morning. Some challenging
changes expected later this evening and overnight as a weak cold front slowly
migrates across the Bay area. Have slowed the timing of rainfall from earlier
forecasts to early Thursday morning for the North Bay to late Thursday morning
for Oak and sfo. While terminals farther south should not see any significant
rain till thurs afternoon. Challenge is whether low stratus and fog will
redevelop tonight since mid to high cloud cover should be a mitigating factor.
Also, some pre-frontal mixing should aid in maintaining somewhat higher cigs than
what was observed last night and early this morning. Thus have hedged the next
taf cycle in a more MVFR direction for all sites and only a slight reduction
of vsbys associated with the rainfall. Winds look to remain quite light.

Vicinity of ksfo...MVFR through 22z with vsbys down to 4-6 miles in haze.
At times.

Sfo bridge approach...similar to ksfo.

&& of 8:37 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.


Mtr watches/warnings/advisories...
... Surf warning...entire coast
Small Craft Advisory...mry Bay from 3 am
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm from 3 am
Small Craft Arena to Pigeon pt 10-60 nm from 10 PM
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm from 3 am
Small Craft Arena to pt Reyes 0-10 nm from 12 am
Small Craft Advisory...rough bar advisory for sf bar
Small Craft Reyes to Pigeon pt 0-10 nm from 3 am



Public forecast: dykema
aviation: west pi
marine: CW

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