Scientific Forecaster Discussion
fxus66 kmtr 211036
Area forecast discussion
National Weather Service San Francisco Bay area
336 am PDT Wed Mar 21 2018
Synopsis...periods of moderate to locally heavy rainfall will
continue today through early Thursday afternoon as an atmospheric
river takes aim at the California coast. Breezy to locally gusty
southerly winds will also be possible ahead of a cold front that
will sweep through the region Thursday morning. After a brief break
in rainfall late Thursday, additional showers and cooler
temperatures will be possible from Friday into the weekend before
dry conditions return next week.
Discussion...as of 03:31 am PDT Wednesday...while the widely
scattered showers that moved across the region late last night
into this morning have diminished, a very moist southwesterly flow
persist over the region. As a result, light to occasionally
moderate rainfall continues over the coastal ranges and locally
inland. Expect this to continue through the morning as moisture
associated with the atmospheric river aimed at Southern California
advects inland and northward through the day.
The latest thinking is that precipitation will increase once again
in coverage and intensity this afternoon and evening in advance of an
approaching mid/upper level disturbance. An associated surface low
pressure system will open up and lift into northern California late
tonight into Thursday morning as well. Thus, southerly winds will
increase and become breezy to locally gusty at times along the coast
and in the ridges. However, latest guidance shows the low opening up
a bit farther offshore which should keep wind speeds from reaching
Wind Advisory criteria. With this said, local gusts of 40 to 45 miles per hour
cannot be ruled out. Atmospheric conditions will also become more
unstable as this system approaches the coast and will result in a
slight chance of thunderstorms later in the day. Some convection is
already showing early this morning offshore of Southern California.
Will need to monitor this to see if the thunderstorm potential will
come in sooner than currently expected.
A more defined frontal system is then forecast to push across the
region from north to south Thursday morning when the mid/upper level
trough axis rotates inland to our north. This boundary will be
another focus for more widespead moderate to locally heavy rainfall
as it sweeps through the region and interacts with the deep moisture
plume advecting into the central and Southern California coast. With
forcing from the frontal passage, there will again be a slight
chance of thunderstorms Thursday morning into early afternoon.
Conditions will then briefly clear from north to south Thursday
afternoon and evening as drier air filters in behind the cold front.
Rainfall totals through Thursday afternoon remain on track to range
from as much as 4.00" to 6.00" in the coastal ranges of the Santa
Lucia mountains with isolated amounts approaching 8.00". An
additional 2.50" to 3.50" can be expected in the wettest locations
of the Santa Cruz and North Bay mountains. Most urban areas can
expect between 0.50" and 1.25" with the lesser amounts in the Santa
Clara Valley and greatest totals near the coast and where any
convective showers develop.
Additional showers will be possible Friday into potentially Sunday
morning as the core of the mid/upper level system rotates over the
Gulf of Alaska. Temperatures will also turn cooler for late in the
week as the atmospheric river shifts well to the south and colder
air advects in from the north. Conditions appear to dry out late
Sunday into the start of next week as the large scale mid/upper
level trough axis finally shifts inland.
Aviation...as of 10:45 PM PDT Tuesday...scattered showers still
moving into the sfo Bay area this evening with MVFR cigs. Models
indicate shower activity decreasing after 14z with cigs rising
into VFR. Daytime heating will provide a little better chance of
showers and even possibly a thunderstorm during the day Wednesday.
Nothing organized is expected until the cold front arrives late
East to southeast winds will through Thursday. Surface winds have
been 10 kt or less except for gusty winds to 30 kt in the Salinas
valley and 25 kt in the Santa Clara Valley that will affect sjc
and sns. Winds up to 35 kt above 925 mb could produce low level wind shear
through early Wednesday morning.
Vicinity of ksfo...MVFR conditions expected through Wednesday as
showers continue to move through. Shower activity decreasing after
15z with cigs rising to 4000-5000 ft. Confidence is low as cigs
could remain lower. Stronger winds above the surface may cause
low level wind shear through 15z. Daytime heating will allow more cumulus clouds
to form in the afternoon with a slightly higher chance for
showers. Thunderstorms possible but most likely will be east of
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR for cigs remaining above 5000 ft.
Low clouds could form along the coast overnight and bring MVFR
cigs to mry and sns but confidence is low on this happening.
Shower activity should be limited to the Big Sur coast and the sfo
Marine...as of 03:31 am PDT Wednesday...moderate southerly winds
will persist over the coastal waters through the day ahead of an
approaching frontal system. These winds will generate southerly
swell which will mix with a northwest swell to create rough and
hazardous seas through at least Thursday. Winds will become
northwest on Thursday after the front passes. There is a slight
chance of thunderstorms Wednesday and Thursday afternoon mainly
... Arena to pt Reyes 0-10 nm
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...mry Bay from 11 am
Small Craft Advisory...sf Bay from 3 PM
Public forecast: rgass
aviation: west pi
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