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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 220347

Area Forecast Discussion
National Weather Service San Francisco Bay Area
847 PM PDT Thu Mar 21 2019

.SYNOPSIS...Dry conditions are expected tonight ahead of the next
frontal system that will bring widespread rainfall to the region 
on Friday and Friday night. Mainly dry conditions are expected 
over the upcoming weekend, with unsettled weather then likely 
throughout at least the first half of next week.


.DISCUSSION...As of 8:45 PM PDT Thursday...Dry conditions
prevailed across our region today, except for an isolated shower
that developed over the Santa Cruz Mountains late this afternoon.
Afternoon highs were mostly in the lower to mid 60s.

Evening satellite imagery shows a relatively robust frontal 
system approaching the northern California coast. The moisture
plume along the frontal boundary is respectable with precipitable
water values as high as 1.2 inches. Models agree quite well on 
rainfall timing: Light warm advection precipitation is forecast 
to begin spreading onto the North Bay Coast just prior to sunrise 
Friday, with light rain then developing across the entire North 
Bay during the morning hours, and as far south as San Mateo and 
Alameda Counties by midday. For tomorrow morning's commute, wet 
roadways are most likely in the North Bay, while locations south 
of the Golden Gate will probably remain dry through the bulk of 
the morning commute. 

Rain rates are expected to increase in the North Bay during the 
early afternoon hours as the cold frontal rain band pushes 
onshore. Moderate to brief heavy rain rates with the cold front 
are then expected to push south and east through the remainder of 
the SF Bay Area during the late afternoon and early evening hours,
and finally south through most of the rest of our forecast area 
during the late evening hours and overnight Friday night. 

Latest HRRR model rainfall totals through late Friday night range
from a half inch to an inch in most locations, except a quarter
inch or less across the far southern portion of our area where the
front is forecast to wash out late Friday night. Up to 1.5 inches
are expected in the North Bay Mountains, Santa Cruz Mountains, and
locally in the Santa Lucia Mountains. The HRRR's rainfall forecast
with this system is consistent with other model output.  

Winds are not expected to be a significant factor with this
incoming system. The local WRF model forecasts locally gusty winds
along the coast from San Mateo County northward, as well as
locally in the hills, on Friday and Friday evening. But wind gusts
in these areas are forecast to be no more than about 30 mph.

The models agree that there won't be much in the way of post-
frontal shower activity with this system. So, after a few
lingering showers on Saturday morning, mainly across the southern
portion of our area, we can expect dry conditions for the
remainder of the upcoming weekend. Weekend high temperatures are 
forecast to be about five degrees cooler than seasonal averages on
Saturday, but then warm a few degrees on Sunday.

After a brief dry break over the weekend, the models agree that 
an active weather pattern will return by early next week, with a
system forecast to spread rain across much of our area from Sunday
night into Monday and another set to bring more rain from late
Tuesday into Wednesday. Details regarding timing and exact 
location of heaviest rainfall with these systems next week remains
rather hard to pin down given model differences. One item of
note...Yesterday's ECMWF indicated the potential for prolonged 
rainfall in the North Bay from Sunday night through late Tuesday,
with very little break between the two systems, but today's ECMWF
forecasts about a 24 hour dry break in between.


.AVIATION...As of 11:00 AM PDT Thursday...A few cumulus clouds
over the higher terrain are not impacting the approach or the 
terminals today. VFR expected tonight as mid and high clouds
increase and gradually lower ahead of the next frontal system. 
Initially a warm front will bring cigs down to MVFR in the Bay
Area with an initial round of light rain Friday morning. There 
may be a short break in the rain between the warm front passage 
and the arrival of the cold front but cigs likely remaining MVFR. 
Vsbys will lower and light to moderate rain will return in the 

Onshore winds will diminish tonight then switch to southeast on
Friday. Winds should really pick up in the afternoon in the Bay
Area with gusts to around 25 kt.

Vicinity of KSFO...VFR through tonight. Mid and high clouds will
move in tonight with cigs gradually lowering overnight. MVFR cigs
after 15Z continuing through the day. Light rain could begin as
early as 10-12Z as the warm front moves through. There may be a
short break in the rain late in the morning but rain will return 
after 21Z along with reduced vsbys to 3-5 miles. Onshore winds
to 15 kt diminishing overnight then becoming light southeast
Friday morning. Wind speeds increasing after 21Z with gusts to
25 kt. 

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR tonight. Increasing clouds Friday
morning with cigs remaining in the VFR category. Rain and lower
cigs late in the afternoon. Winds becoming southeast overnight 
with stronger gusts through the Salinas Valley. Gusts to 15 kt at
SNS Friday morning increasing to 25 kt after after around 16Z. 

&& of 04:29 PM PDT Thursday...Weak transitory ridge over
the area through the remainder of the day will result in light to
locally breezy west to northwest  winds. Winds turn southerly and
increase by sunrise Friday as a  frontal system moves in. A
moderate west swell will dissipate through the next 24 hours but a
different, longer period west  swell has already begun to build
into the waters this afternoon.


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




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