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NWS Technical Weather Discussion

FXUS66 KMTR 240355

Area Forecast Discussion
National Weather Service San Francisco Bay Area
855 PM PDT Fri Mar 23 2018

.SYNOPSIS...A cold front will spread rain from north to south
across much of the region tonight and Saturday morning. Additional
shower activity is forecast Saturday late afternoon and evening
with isolated showers possibly lingering into Sunday. Dry weather
is expected to return on Monday and continue through next week. A
warming trend is forecast for the first half of the week.


.DISCUSSION...As of 8:55 PM PDT Friday...Satellite, radar, and
observations indicate that the cold front approaching from the
northwest is currently near Point Arena. Based on the latest HRRR
model, expect the cold front to spread rain across the North Bay
between about 10 pm and 2 am, then south across the rest of the SF
Bay Area between 2 am and 5 am Saturday, and finally through the
Monterey Bay Area between 5 am and 8 am. Several lightning strikes
have been detected along the northern California coast north of
Point Arena this evening, so a slight chance of thunderstorms have
been added to the North Bay for later this evening.

All of the models indicate little or no precipitation following
the front on Saturday until the cold upper low approaches late in
the day. Thus, most of Saturday may end up being dry under partly
cloudy skies. The evening forecast update included a reduction in
shower chances for most of Saturday.

Shower chances will increase late Saturday afternoon and evening
as the cold upper trough approaches the northern California coast.
Based on the 00Z models, shower chances will spread into our
forecast area from late Saturday afternoon into Saturday night.
But rainfall then is not expected to be as widespread as with the
cold front moving through tonight. There is also a slight chance
of thunderstorms late Saturday afternoon and early evening, mainly
across the North Bay where instability will be greatest.

Isolated showers may linger into Sunday, mainly near the coast in
the morning, and then along the eastern portion of our forecast
area in the afternoon. But for the most part Sunday is expected to
be a dry and cool day.

This incoming system will not have a whole lot of moisture to work
with and so rain totals from tonight through the weekend are
forecast to generally be a half inch or less, with highest amounts
in the North Bay. Some southern inland areas may see little, if
any, rainfall.

Snow levels will start off around 3500 feet tonight, but then drop
as low as 2500 feet by Saturday and Saturday night. Since the bulk
of the precipitation is expected to fall with the cold front
tonight and prior to the arrival of the coldest air, only light
snowfall accumulation is expected on the higher peaks.

Models continue to indicate dry weather through next week, along
with a warming trend, as a ridge of high pressure develops
offshore and then slowly moves eastward.


.AVIATION...As of 04:39 PM PDT Friday...for 00Z TAFs. Currently
VFR conditions at all sites with scattered clouds and occasional
showers in the North Bay. A cold front is still expected to move
through late tonight bringing another round of steady showers. As
the front approaches expect cigs to begin to lower back to MVFR.
Shower chances will end early tomorrow morning once the front has
passed and VFR conditions should once again prevail.

Westerly winds will remain breezy through the evening at around 15
kt before taking a more southerly turn and somewhat decreasing.

Vicinity of KSFO...VFR anticipated through about 03z or so, with
FEW/SCT clouds between 3,000 to 5,000 ft AGL. Cigs expected to
lower to MVFR this evening, ahead of a cold front that is forecast
to pass through the terminal around 08-11z tonight. Models still
suggest a narrow band of rain as the cold front sweeps through.
Left a few hours of light rain in the tafs. Breezy southwesterly
winds expected to continue into tonight.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions through tonight. Cigs will
lower after midnight tonight, ahead of an approaching cold front
forecast to pass through early Saturday morning. Expect wet
runways from light rain as the front passes. Breezy westerly winds
will continue into the evening.


.MARINE...As of 8:16 PM PDT Friday...Lightning strikes have been
observed off the Northern California coast near Fort Bragg.
There`s a slight chances of thunderstorms across the northern
waters over the next few hours. Light to moderate west to
southwest winds will prevail as a cold front makes its way through
the coastal waters late tonight. Light rain is possible along the
front, and winds will turn northwesterly in its wake. A second
cold front is expected Saturday evening into Sunday morning.
Northwest swell will increase slightly overnight tonight.


     .Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...SF Bay from 9 PM until 3 AM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm from 9 PM until 3 AM




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Prepared by Weather at: Fri Mar 23 20:30:04 PDT 2018

From the National Weather Service

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Revised: 29 Mar 2008 09:53 -0800 GMT (Pacific)