Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
FXUS66 KMTR 211616

Area Forecast Discussion
National Weather Service San Francisco Bay Area
916 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.

&& of 09:14 AM PDT Wednesday...Very moist
southwesterly flow continues to take aim over CA, including the
Bay Area. Morning radar imagery around the Bay Area continues to
show scattered showers moving form SW to NE across the region.
Some of the showers are heavy at times given the amount of
moisture in the atmosphere. 12z KOAK sounding had PWATs of 0.96
inches, which is well above normal for this time of year.
Rainfall amounts over the last three hours have been highest over
the Santa Cruz and Big Sur Mts with a 0.25-0.6 inches. Other
locations have generally been less than 0.25".

For the rest of today - showers, locally heavy, will continue to
move across the Bay Area this morning into early this afternoon.
After noon, expect rain to increase in coverage and intensity as
another higher PWAT plume drifts northward and up the Central
Coast. Latest model guidance brings a plume of PWATs greater than
1 inch over the Bay Area. One of item of concern that could
translate to local impacts are thunderstorms. Forecast point
soundings and model guidance all show an unstable atmosphere with
CAPE a few hundred j/kg. In fact, the HRRR even has values
approaching 1,000 j/kg for the Central Valley, which is decent for
CA. One limiting factor could be lack of surface heating due to
cloud cover. Convective temps are forecast to be in the low 60 to
mid 60s, which is also the forecast max temps today as well. None
the less, current forecast will have a mention of thunder. SPC Day
1 Outlook morning update will likely include a large portion of
NorCal. Given the already wet ground any developed thunderstorms
could pose concerns for localized flooding. Will have to monitor
close this afternoon, especially North Bay burn scars.

Minor tweaks to the forecast this morning to better reflect
reality, but the overall message remains the same - moderate to
locally heavy rain possible through tomorrow with
afternoon/evening thunderstorms.


.PREV 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 MPH
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 4:51 AM PDT Wednesday for 12Z TAFs. A mixed bag
observed this early morning across the district with conditions ranging
from IFR to VFR. With scattered showers expected periodically
through the day, expect SCT/BKN cloud decks generally above 3,000
ft after 15 to 18Z this morning. Can`t ignore the chance of a few
thunderstorms in the district this afternoon/evening, but the odds
of any single thunderstorm impacting a terminal are too low to
include in the TAFs. Frontal passage will occur overnight
Wednesday and we`ll see a renewed chance of moderate rain with
deteoriating conditions to sub-VFR levels. Southerly winds will
be strong at times today at some terminals, including but not
limited to KSNS and KSJC.

Vicinity of KSFO...Bouncing back and forth between VFR and MVFR
as rain showers have impacted the terminal overnight. Think the
threat of LLWS will diminish as the morning continues. Skies will
be SCT/BKN through the day, likely between 3-5k ft AGL. Scattered
showers will periodically impact the terminal through the day and
may bring temporary lower ceilings. Winds will be out of the south
through the day. Steady rain chances return at the tail end of the

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Generally MVFR/VFR with scattered
showers. Locally strong SE winds expected again today at KSNS with
speeds exceeding 30 kt quite possible.

&& 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
near land.


     .Tday...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Mry Bay from 11 AM
             SCA...SF Bay from 3 PM




Visit us at

Follow us on Facebook, Twitter, and YouTube at: is the U.S. government's official web portal to all federal, state and local government web resources and services.