Flash Flood Watch 7, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
957 PM PST Wed Dec 4 2019

.SYNOPSIS....Lingering showers will persist over portions of the
region through this evening before diminishing in coverage tonight.
After a brief break in wet weather conditions on Thursday, another
cold front will sweep through the region on Friday resulting in
widespread rainfall and gusty winds. Post-frontal showers and
potentially isolated thunderstorms will continue through Saturday
night before tapering off by Sunday.

&&

.DISCUSSION...as of 8:54 PM PST Wednesday...Strong thunderstorms
developed this late afternoon in northern Monterey County,
bringing heavy rain to portions of the Salinas Valley. This heavy
rain fell on saturated soils, and resulted in significant overland
flooding in and around the Chualar area. US Highway 101 was
underwater and closed going both directions for an extended period
of time this evening according to the California Highway Patrol.
The County of Monterey Office of Emergency Services has confirmed
multiple flooded homes in Chualar. While rain showers have ended
in this area, a Flood Advisory remains in effect for Chualar and
Gonzales through 10:30 pm PST because of the slow recession of the
overland flooding. Motorists with plans to travel in this area
overnight are encouraged to check with CalTrans for the latest
developments and information regarding road closures. If you
encounter flooded roadways: turn around, don't drown!

Across the rest of the region, rain showers have largely
dissipated in Santa Cruz and Santa Clara Counties since sunset.
Radar is still detecting a few light echos as of 8:45 pm around
Palo Alto and Mountain View, but additional rainfall will only
amount to a few one-hundredths of an inch.

No major forecast updates were made to the grids as this evening.
For additional forecast details, please refer to the previous
discussion section.

&&

.PREV DISCUSSION...as of 1:51 PM PST Wednesday...The upper level
system that brought widespread rainfall to the region overnight
into early this morning continues to push inland over the San
Francisco Bay Area. As the system pushes inland a bit further to
the north than previously expected, light rain continues over the
North Bay with isolated to scattered showers over the Central
Coast. Given the increased instability associated with this
system, isolated thunderstorms will be possible over the Central
Coast through the next several hours. Do expect the overall trend
is for precipitation to taper off later this afternoon and evening
as the core of the upper level system shifts inland. Thus, mostly
dry weather is likely for much of the region overnight tonight
into Thursday.

After a brief break in widespread rainfall on Thursday, southerly
winds will being to increase late in the day ahead of the next
approaching storm system. The latest forecast models are in better
agreement with light rain developing along the coastal ranges and in
the North Bay as deeper moisture advects inland. Rainfall will
become more widespread after sunrise and through mid-morning on
Friday for the North Bay and then across the remainder of the region
Friday afternoon and evening as the frontal boundary advances
southward. While this system is not as strong or forecast to be as
long of a duration as the one this past weekend, PWAT values are
forecast to exceed 1.00". Thus, moderate to locally heavy rainfall
will be possible along the frontal passage with the potential for
another narrow cold frontal rainband (NCFR) if dynamics align just
right. An NCFR would result in the potential for minor flooding
concerns and would be of greater concern for the Kincade Burn Area.
This is something that will be closely monitored in the coming days.

Post frontal showers and possible thunderstorms are expected from
Friday night into Saturday night as an unstable air mass moves
across the region. These showers will likely produce brief moderate
to locally heavy rainfall, gusty winds and the potential for small
hail. When all said and done, rainfall totals from Friday into early
Sunday range from 0.50"-1.25" in most urban areas, except 1.50-2.50"
in the North Bay Valleys. In the hills/mountains, rain totals are
forecast to range from 1.50"-4.00" in the North Bay Mountains, 1.25-
1.5" for the East Bay Hills, 1.50-3.00" in the Santa Cruz Mountains,
and 1.00-3.00" across Monterey and San Benito Co. Mountains. Lesser
amounts will be possible in the southern Salinas Valley.

Conditions will begin to dry out by Sunday and likely continue into
early next week. Of note, overnight conditions will also cool once
sky conditions clear into early next week. Medium range models do
show a few passing weather systems mid/late next week, yet are not
forecast to be robust and may dissipate before reaching the region.
Stay tuned.

&&

.AVIATION...as of 09:58 PM PST Wednesday...For 06Z TAFs. Very
light rain will be possible near KLVK through 08z, otherwise rain
has ended across the area. Low level moisture remains in place
this evening with SCT/BKN between 2500-4000 feet common. Where
clearing has taken place, some patchy fog has begun to form,
mainly in the North Bay and around the southern Santa Clara
Valley. Patchy fog may expand tonight where clearing takes place
for several hours. Otherwise, borderline MVFR/VFR cigs to continue
through the overnight hours and into Thursday. Mainly light east
to southeast winds tonight through early Thursday. Southeast winds
will begin to pick up some on Thursday with speeds approaching 10
kt at most terminals.

Vicinity of KSFO...Borderline MVFR/VFR cigs will continue
overnight tonight through Thursday. BKN with brief periods of SCT
cigs with bases 2000-3000 feet tonight. Clouds will persist
through Thursday morning with similar conditions. Light east winds
tonight become southeast 5-10 kt on Thursday.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Mainly VFR conditions prevailing
overnight but periods of MVFR cigs will develop. Some patchy fog
is possible especially if clearing continues much of the night.
Southeast winds will persist tonight at KSNS with speeds up to 10
kt. Southeast winds on Thursday up to 15 kt at KSNS with light
winds at KMRY.

&&

.MARINE...as of 8:40 PM PST Wednesday...Southerly winds will
develop tonight and become breezy on Thursday ahead of an area of
low pressure. Winds will strengthen further on Friday as a cold
front approaches and then moves over the coastal waters. Gale
force winds will be possible late Thursday night and into Friday.
Moderate northwest swell will continue through mid- week before
building to around 10 to 15 feet later this week and into the
weekend.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...NONE.

&&

PUBLIC FORECAST: Rowe/RGass
AVIATION: ST
MARINE: ST

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