Weather Service Report

FXUS66 KMTR 200357

Area Forecast Discussion
National Weather Service San Francisco Bay Area
857 PM PDT Mon Mar 19 2018

.SYNOPSIS...A storm system will produce widespread rainfall 
across our region from Tuesday through Thursday. Periods of heavy
rain are possible, mainly along the Big Sur Coast, as well as locally
strong southerly winds and thunderstorms. Showers will linger 
into Friday and Saturday before dry conditions return on Sunday.


.DISCUSSION...As of 8:55 PM PDT Monday...More sun today, along 
with a warming airmass under a shortwave ridge, allowed high 
temperatures to climb into the 60s, which are close seasonal 
averages. Some locations in the Salinas Valley reached into the 
lower 70s. 

Evening satellite shows that an upper low continues to spin well
offshore, about 1100 miles west of San Francisco. A plume of
subtropical moisture flowing south of this low contains 
precipitable water values of between 1.5 and 2.0 inches. This 
moisture plume is taking aim at central California just as models 
have been forecasting. KMUX radar is already picking up some weak 
returns offshore, but this is mostly mid level moisture. The deep 
moisture plume is due to arrive along the Big Sur coast by sunrise
Tuesday and then spread north and east across most of the rest of
our area by Tuesday afternoon. Rain may become heavy along the 
Big Sur Coast and in the Santa Lucia Mountains by Tuesday 
afternoon as warm advection cranks up. Elsewhere, rain rates 
tomorrow are expected to be mostly light to moderate. Southerly 
winds will increase by Tuesday afternoon and evening, particularly
along the Big Sur Coast and across the higher hills of Monterey 
and San Benito Counties. Gusts up to 35 mph are possible in these 

The models generally agree that there will be a decrease in
precipitation from late Tuesday evening through much of Wednesday.
Periods of rain will continue during this time, but rainfall 
won't be as widespread or intense. Then, by late Wednesday the
upper low will begin to approach the coast, ramping up dynamic
forcing as well as orographic lift as southerly winds increase. 
Precipitation rates could again be heavy at times from late 
Wednesday through early Thursday. Again, the heaviest rain is
expected to be in the Santa Lucias and Big Sur region, but
localized heavy rain could occur elsewhere too. Strongest
southerly winds this week are expected from late Wednesday
afternoon through mid morning on Thursday when local gusts of up 
to 50 mph are possible along the coast and in the hills. 

Thunderstorms are possible starting late Wednesday afternoon and
early evening, mainly across the eastern portion of our forecast
area where the NAM forecasts surface CAPE values as high as 400
J/kg. Thunderstorm potential will also exist on Thursday as the
cold front sweeps through, but it looks like the most unstable
airmass will be located over the Central Valley on Thursday. 

Rainfall totals from Tuesday through Thursday are forecast to
range from 0.75-1.50" for most urban areas...1-3 inches for Bay 
Area mountains (except locally up to 4 inches or more in the Santa
Cruz Mountains if the moisture plume shifts slightly farther 
north)...and 3 to 5 inches in the Santa Lucia Mountains with 
locally up to 8 or even 10 inches on some of the higher southwest-
facing slopes or the Santa Lucias. Thus, the highest potential for
small stream flooding and mudslides/debris flows will be along 
the Big Sur coast.

Showers are expected to taper off by late Thursday and mostly end
by Thursday night. But then a system approaching from the north
will bring renewed shower chances from late Friday through early
Saturday. Dry weather is expected to return to all areas by
Sunday. Longer range models indicate most of next week will remain


.AVIATION...As of 5:30 PM PDT Monday...Conditions will remain VFR
through tonight with high clouds. Deeper moisture arrives Tuesday
morning. Models have been trending slower on the timing and it 
now looks like the rain and lower cigs do not arrive until after 
18Z in the MRY Bay Area and 20Z for the SFO Bay Area.

Southeast winds increase but strongest winds remain along the
coast and over the waters.  

Vicinity of KSFO...VFR tonight. Cigs lower after 15Z but bases
still above 5-6 thousand feet. Cigs lowering to MVFR with light
rain after 21Z. Southeast winds increase to 10-15 kt Tuesday
morning after sunrise.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. Light rain after 18Z with cigs
lowering to 4000 ft. Winds becoming se and increasing to 15-18 kt
with gusts to 22-24 kt in the Salinas Valley. 

&& of 04:04 PM PDT Monday...Winds will increase from the
south to southeast over the coastal  waters and bays starting
Tuesday morning as a strong warm front  arrives from the
southwest. Showers and possibly thunderstorms will affect the
coastal waters and bays Tuesday through Saturday  morning. Freshly
steepening mixed seas develops Tuesday through  Thursday, creating
rough and erratic seas.






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