FXUS66 KMTR 200547

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1047 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 10:30 PM PDT Monday...Conditions will remain
VFR through tonight with high clouds. Models continue to trend
slower with the arrival of deeper moisture so timing of rain and
lower cigs has been pushed back to 21Z for the MRY Bay Area and
01-02Z for the SFO Bay Area. A break in the action is expected
later Tuesday evening. Winds over land remain light during the
day then increase in the evening as an upper level shortwave

Vicinity of KSFO...VFR. Cigs lower after 18Z but bases still
above 5-6 thousand feet. Cigs lowering to around 4000 ft with
light rain after 01Z. Southeast winds around 10 kt.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. Light rain after 20Z 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.


.MARINE...as of 10:40 PM PDT Monday...A 998 mb low centered 1100
miles west of Monterey will move east  tonight and Tuesday then
turn northeast on Wednesday. This will  bring increasing southeast
winds to the central waters tonight and the northern waters
Tuesday. As the upper level trough passes thrugh on Thursday there
will be a slight chance of afternoon thunderstorms. Large mixed
swells will develop Tuesday through Thursday resulting in rough
and erratic seas.






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NWS MTR Office Area Forecast Discussion