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

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FXUS66 KMTR 192103

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

.SYNOPSIS...Look for dry weather conditions and slightly warmer
temperatures Monday. A system will then arrive on Tuesday bringing
widespread rainfall to our region through Thursday with periods
of heavy rain and locally strong southerly winds. Showers then
linger into Friday and Saturday before drying conditions return
late in the forecast period.

&& of 01:50 PM PDT Monday...Nice March day as
temperatures warmed into the 50s and 60s around the Bay Area.

All eyes now focus on a robust storm system currently spinning
over the Eastern Pacific near 140.5W 33N. This storm system will
begin to pump subtropical moisture to CA as an Atmospheric
River. Source region is near HI so this AR can be described as a
Pineapple Express. Latest satellite imagery has PWATs 1.5-2" or
about +200%  of normal.

Models have remained rather consistent bringing the first moisture
push to the region early Tuesday. Light to moderate rain is on
track to develop Tuesday morning along the Big Sur Coast and then
spread northward through the day. As light to moderate rain
spreads north, moderate to heavy rain is expected to develop over
the Big Sur Coast/Santa Lucias Tuesday afternoon as low level
increases and higher PWATs slam into 5k ft peaks. The moderate to
heavy rain will likely cause some minor hydro issues along the Big
Sur Coast with creek rises, ponding and possible low end flooding
at this time. The lower confidence part of the forecast is where
this intense plume of moisture sets up. Current forecast/model
guidance keeps it at Big Sur, but if it shifts just 30-50 miles
north it could be a different ballgame and Santa Cruz Mts could
get dumped on. That solution is less likely, but still wanted to
mention it. Winds will also increase out of the south Tuesday into
Wednesday. Winds will be strongest along the coast and higher
coastal peaks with gusts 30-40 mph.

The main moisture plume begins to shift slowly southward Tuesday
night as the low pressure begins to move eastward. Rain intensity
will decrease across the Bay Are as it shifts toward SoCal, but
light to moderate rain will continue Tuesday night into Wednesday.

Rainfall will increase again on Wednesday as another impulse comes
off the Pacific. One change to the forecast on Wednesday was to
include a slight chance for thunder during the afternoon/evening
as the airmass becomes slightly unstable. SFC CAPE is weak, but
soundings show a little instability to warrant at least a
mention. Best chance will be across the East/South Bay and
locations southward. Moderate to heavy rain will be possible
Wednesday night into early Thursday as a cold front begins to
move into the Bay Area. Thunderstorm chances return again on
Thursday with the passage of the cold front. Latest guidance
shows even more instability than Wednesday, especially the
Central Valley (500 j/kg CAPE for the Bay Area, which is pretty
solid). Given how moist the atmosphere is any developed
thunderstorm will be capable of producing localized heavy rain.
Thunderstorms combined with an already wet ground could result in
flooding. Moderate to heavy rain rain will finally begin to
diminish from NW to SE late in the day Thursday. Lingering showers
will be possible Friday and even into Saturday as the core of the
mid/upper level system pushes inland into the Pacific Northwest.

Heaviest rainfall is still slated for Tuesday morning through
Thursday morning with pulses of moisture. Only minor tweaks to
previous rainfall, but Big Sur Coast will see the most. Rainfall
amounts are anticipated to range from 0.75" to 1.50" in most urban
areas with 1.00" to 3.00" in the North Bay and Santa Cruz
Mountains and 2.00" to 5.00" in the Santa Lucia Mountains above
the Big Sur Coast. Could see isolated amounts upwards of
8.00-10.00" along the southwest facing coastal slopes of Monterey
County (Mining Ridge/Chalk Peak).

Simply put this event will be interesting to watch. It will be an
AR orographic event initially and then transition to a convective
event with the frontal boundary. Please pay attention to the
weather the next few days as there is potential for hazardous

Drier conditions appear to develop Saturday into Sunday region-wide
as a weak ridge of high pressure builds over the eastern Pacific. At
this time, dry conditions with temperatures near seasonal averages
look likely for the final week of March.


.AVIATION...As of 10:44 AM PDT Monday...For 18z Tafs. High
confidence VFR through the rest of the day, with light onshore
winds for most coastal terminals and light offshore winds for
inland terminals this afternoon.

Conditions begin to deteriorate Tuesday morning as a storm system
approaches from the west. Cigs are forecast to lower below 5000
ft, with rain affecting many of the TAF sites by end of the
forecast period (by 18z Tuesday). Rain may become moderate to
heavy at times Tuesday afternoon. Some uncertainty remains exactly
how low and how quickly cig heights will lower Tuesday morning.

Overall forecast confidence: Moderate to High

Vicinity of KSFO...VFR today. Generally light offshore winds will
continue this morning, with light onshore winds expected briefly
by late afternoon. Light rain and lower cigs possible by late
Tuesday morning.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. SE winds will turn W/NW this
afternoon, then returning to SE direction overnight. MVFR cigs
possible by early Tuesday morning with rain developing soon

&& of 8:50 AM PDT Monday...Light winds and seas will
prevail today as high pressure continues over the region.
Increasing south to southeast winds develop over the coastal
waters and bays Tuesday and Tuesday evening as a strong warm front
arrives from the southwest later Tuesday. Rain arrives Tuesday
and will be heavy at times into Thursday as an eastern Pacific low
pressure system taps deep subtropical moisture from Hawaii and
lifts it northeast toward California. Freshly steepening mixed
seas develops Tuesday through Thursday.






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